TUNNEL ENGINEERING

20 

Lars Christian F. Ingerslev, Arthur G. Bendelius 

Parsons Brinckerhoff 

New York, New York 

TUNNEL ENGINEERING 

Tunnel engineering makes possible many 

vital underwater and underground 

facilities. Unique design and construction 

techniques are involved because of 

the necessity of protecting the constructors and 

users of these facilities from alien environments. 

These facilities must be built to exclude the 

materials through which they pass, including 

water. Often, they have to withstand high 

pressures. And when used for transportation or 

human occupancy, tunnels must provide adequate 

lighting and a safe atmosphere, with means for 

removing pollutants. 

Tunnels are constructed using many methods, 

depending upon the kind of soil and/or rock 

through which they will pass, their size, how deep 

they need to be, and the obstructions that may be 

encountered along the route. These methods 

include cut-and-cover construction, drill and blast, 

tunnel boring machine (TBM), immersion of 

prefabricated tunnels, and sequential excavation 

methods (SEM). More specialized methods, such as 

ground freezing and tunnel jacking, are used less 

frequently and often under very difficult conditions. 

Compressed air working has become 

uneconomical because of working hour restrictions, 

time for decompression that results from 

high working pressures (over 40 psi is not 

unusual), union labor agreements for work under 

compressed air, and high workmen’s compensation 

and health benefit rates. Occasional entry 

under compressed air may still be required, such as 

to clear obstructions ahead of a tunnel boring 

machine, or to perform essential maintenance on 

parts of such a machine. 

The design approach to underground and 

underwater structures differs from that of most 

other structures. Internal space, design life, and 

Source: Standard Handbook for Civil Engineers 

other requirements for the tunnel must first be 

defined. Geological and environmental data must 

then be collected. Critical design loading conditions 

must then be established, including 

acceptable conditions of the tunnel following 

extreme events (for example, how long before the 

tunnel is reusable). Appropriate construction 

methods are then evaluated to determine the most 

appropriate to meet the established criteria, 

conditions, and cost. The methods under consideration 

should include both temporary and permanent 

excavation support systems as well as the 

structures itself. Design standards and codes of 

practice apply primarily to above-ground structures, 

so that care should be used in their 

application to underground and underwater 

structures. 

20.1 Glossary 

Adit. A short, transverse tunnel between parallel 

tunnels or to the face of the slope in a sidehill 

tunnel. 

Air Lock. A compartment in which air pressure 

can be varied between that of the compressed air 

used in shield tunneling and that of the outside air, 

to permit passage of workers or material. 

Bench. Top of part of a tunnel section, with 

horizontal or nearly horizontal upper surface, 

temporarily left unexcavated. 

Blowout. A sudden loss of a large amount of 

compressed air at the top of a tunnel shield. 

Breast Boards. Timber planks to hold the face of 

tunnel excavation in loose soil. 

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20.2 n Section Twenty 

Dry Packing. Filling a void with a stiff mortar, 

placed in small increments, each rammed into 

place. 

Evasé Stack. An air-exhaust stack with a cross 

section increasing in the direction of air flow at a 

rate to regain pressure. 

Face. The surface at the head of a tunnel 

excavation. A mixed face is a condition with more 

than one type of material, such as clay, sand, gravel, 

cobbles or rock. 

Grommet. A ring of compressible material 

inserted under the head and nut of a bolt 

connecting tunnel liners to seal the bolt hole. 

Heading. A small tunnel, or tunnels, excavated 

within a large tunnel cross section which will be 

enlarged to the full section. 

Jumbo. A frame that rolls on tracks or rubber 

wheels and carries drills for excavation of rock 

tunnels. 

Lagging. Timber planks or steel plates inserted 

above tunnel-supporting ribs to hold back rocks 

or soil. 

Liner Plate. A steel segment to support the 

interior of a tunnel excavation. 

Lining. A temporary or permanent structure 

made of concrete or other materials to secure and 

finish the tunnel interior or to support an 

excavation 

Mucking. Removal of excavated or blasted 

material from face of tunnel. 

Pilot Tunnel. A small tunnel excavated over part 

or the entire length to explore geological conditions 

and assist in final excavation. 

Pioneer Bore. (See Pilot Tunnel.) 

Poling Boards. Timber planks driven into soft 

soil, over timber supports, to hold back material 

during excavation. 

Scaling. Removal of loose rocks from tunnel 

surface after blasting. 

Shield. A steel cylinder of diameter equal to that 

of the tunnel, for excavation of tunnels in soft 

material to provide support at the face of the tunnel, 

to provide space for erecting supports, and to 

protect workers excavating and erecting supports. 

Spiling. (See Poling Boards.) 


20.2 Clearances for Tunnels 

Clearance in a tunnel is the least distance between 

the inner surfaces of the tunnel necessary to 

provide space between the closest approach of 

vehicles or their cargo or pedestrian traffic and 

those surfaces. Minimum tunnel dimensions are 

determined by adding the minimum clearances 

established for a tunnel to the dimensions selected 

for the type of traffic to be accommodated in the 

tunnel and the space needed for other requirements, 

such as ventilation ducts and pipelines. 

Clearances for Railroad Tunnels n Individual 

railroads have different standards to suit 

their equipment. But on tangent tracks, clearances 

for single- and double-track tunnels should not be 

less than those shown in Fig. 20.1. (Clearances 

shown are those in the “AREMA Manual” 

American Railway Engineering and Maintenanceof-Way 

Association, 8201 Corporate Drive, Suite 

1125, Landover, MD 20785, (www.AREMA.org). 

In rail tunnels, clearances for personnel are 

required on both sides where niches are not 

provided. These clearances should be at least 6 ft 

8 in or 2 m high and 30 in wide each side of the 

vehicle clearance diagram, although a 24-in 

minimum is permitted on some lines. In highway 

tunnels, a 3 ft or 0.9 m clearance from face of curb 

is used where walkways are provided. In both 

road and rail tunnels, it is common practice to 

provide a walkway along the common wall 

between adjacent ducts to facilitate emergency 

evacuation between ducts and to prevent people 

from emerging directly into the path of oncoming 

traffic. 

On curved tracks, the clearances should be 

increased to allow for overhang and tilting of an 85ft-long 

car, 60 ft c to c of trucks, and a height of 15 ft 

1 in above top of rail. (Distance from top of rails to 

top of ties should be taken as 8 in.) 

The track should be superelevated at curves 

according to AREMA standards. 

Clearances for pantograph, third-rail, or catenary 

construction should conform to diagrams 

published by the Electrical Section, Engineering 

Division of the Association of American Railroads. 

The latest clearance standards of AREMA 

should be checked for new construction. Local 

legal requirements should govern if they exceed 

these standards. 




Circular tunnels should be fitted to the clearance 

diagrams, with such modifications as may be 

permissible. 

Clearances for Rapid-Transit Tunnels n 

There are no general standards for clearances in 


Tunnel Engineering n 20.3 

Fig. 20.1 Clearances specified by AREMA for railway tunnels on a tangent. 

rapid-transit tunnels. Requirements vary with size 

of rolling stock used in the system. 

Figure 20.2 shows the normal-clearance diagram 

of the New York City BMT and IND Division 

267 ft cars. Figure 20.3 gives the clearances 

established for the San Francisco Bay Area Rapid 





Fig. 20.2 Clearance diagram for 67 0 car (BMT & IND Divisions). New York City Subway System. 

Transit System, which has cars 10 ft wide and 75 ft 

long on a 5-ft 6-in gage track. The clearances allow 

not only for overhang of cars, tilting due to 

superelevation, and sway, but for a broken spring 

or defective car suspension. 

Clearances for Highway Tunnels n The 

American Association of State Highway and 

Transportation Officials (AASHTO) has established 

standard horizontal and vertical clearances for 

various classes of highways. These have been 

modified and expanded for the Interstate Highway 

System under the jurisdiction of the Federal 

Highway Authority (FHWA) (Fig. 20.4). 

For rural and most urban parts of the Interstate 

Highway System, a 16-ft vertical clearance is 

required. 


Since construction costs of tunnels are high, 

clearance requirements are usually somewhat 

reduced. Although some older 2-lane tunnels have 

used roadway widths of 21 ft between curbs for 

unidirectional traffic and 23 ft for bi-directional 

traffic, usually with speed restrictions, these widths 

no longer meet current standards for 12 ft or 3.6 m 

lanes. Full width shoulders are rarely provided due 

to cost, but at least an additional 1 ft is provided 

adjacent to each curb. Wider shoulders or sight 

shelves may be required around horizontal curves 

to comply with sight distance requirements. A 

minimum distance between walls of 30 ft is a 

common requirement. Resurfacing within tunnels 

is rarely permitted without first removing the old 

surfacing, so no allowance for resurfacing is 

required for overhead clearance. It is usual in 




Fig. 20.3 Clearance diagram for San Francisco Bay Area Rapid Transit System. 

tunnels to provide overhead lane signals to show 

which lanes are open to traffic in the direction of 

travel, so extra overhead allowance is required for 

these, and when appropriate also for lighting, 

Fig. 20.4 Clearance diagram for interstate highway 

tunnels. 



overhead signs, jet fans for ventilation, and any 

other ceiling-mounted items. Minimum overhead 

traffic clearances depend upon which alternative 

routes are available for over-height vehicles and 

the classification of the highway, but accepted 

values usually lie between 14 ft and 5.1 m. 

Additional height may be required on vertical 

curves to allow for long trucks. Additional space 

may be required for ventilation, ventilation equipment, 

and ventilation ducts. 

20.3 Alignment and Grades 

for Tunnels 

Alignment of a tunnel, both horizontal and vertical, 

generally consists of straight lines connected by 

curves. Minimum grades are established to ensure 

adequate drainage. Maximum grades depend on 





the purpose of the tunnel. Construction of a tunnel 

in the upgrade direction is preferred whenever 

possible, since this permits water to drain away 

from the face under construction. 

Alignment and Grades for Railroad 

Tunnels n Straight alignments and grades as low 

as possible, yet providing good drainage, are 

desirable for train operation. But overall construction 

costs must be taken into account. 

Grades in curved tunnels should be compensated 

for curvature, as is done for open lines. In 

general, maximum grades in tunnels should not 

exceed about 75% of the ruling grade of the line. 

This grade should be extended about 3000 ft below 

and 1000 ft above the tunnel. 

Short (under 2500 ft), unventilated tunnels 

should have a constant grade throughout. Long, 

ventilated tunnels may require a high point near 

the center for better drainage during construction if 

work starts from two headings. 

Radii of curves and superelevation of tracks are 

governed by maximum train speeds (Art. 19.9). 

Alignment and Grades for Rapid-Transit 

Tunnels n Radii of curvature and limiting grades 

are governed by operating requirements. The 

New York City IND Subway has a 350-ft minimum 

radius, with transition curves for radii below 

2300 ft. Maximum grades for this system are 3% 

between stations and 1.5% for turnouts and 

crossovers. The San Francisco BART system is 

designed for train speeds of 80 mi/h. Relation of 

speed to radius and superelevation of track for 

horizontal curves is determined by 

E ¼ 4:65V2 

R 

where E ¼ superelevation, in 

R ¼ radius, ft 

V ¼ train speed, mi/h 

U (20:1) 

U ¼ unbalanced superelevation, which 

should not exceed 2 3 ⁄4 in optimum or 

4 in as an absolute maximum 

For 80 mi/h design speed, the radius with an 

optimum superelevation would be 5000 ft. For a 

maximum permissible superelevation of 8 1 ⁄4 in, a 

minimum radius of 3600 ft would be required. The 

absolute minimum radius for yards and turnouts is 


500 ft. Maximum line grade is 3.0% and 1.0% in 

stations. To ensure good drainage, grade should 

preferably be not less than 0.50%. 

Alignment and Grades for Highway 

Tunnels n For tunnels under navigable water 

carrying heavy traffic, upgrades are generally 

limited to 3.5%; downgrades of 4% are acceptable. 

For lighter traffic volumes, grades up to 5% have 

been used for economy’s sake. Between governing 

navigation clearances, grades are reduced to a 

minimum adequate for drainage, preferably not 

less than 0.25% longitudinally and a cross slope of 

1.0%. For long rock tunnels with two-way traffic, a 

maximum grade of 3% is desirable to maintain 

reasonable truck speeds. Additional climbing lanes 

for slower traffic may be required when grades 

exceed 4%. 

Radii of curvature should match tunnel design 

speeds. Short radii require superelevation and 

some widening of roadway to provide for 

overhang and sight distance. 

20.4 Pavements and 

Equipment for 

Highway Tunnels 

Roadway base is a reinforced concrete slab; on this 

is placed a renewable pavement. Well-designed 

bitumastic concrete has given good service and has 

good riding qualities. 

Average daily traffic capacity of a two-lane twodirectional 

tunnel is about 20,000 vehicles with a 

maximum of 1200 to 1500 vehicles per lane per 

hour. For single-direction traffic in both lanes, 

capacities are 10 to 15% higher. 

Red-amber-green traffic lights are installed at 

about 1000-ft intervals, or at such spacing that the 

driver always sees at least one light. Telephones are 

placed in recesses about 500 ft apart for service and 

emergency calls. 

Most tunnels, particularly those under water, 

are equipped with fire mains and hose outlets 

every 300 ft. Booster pumps in ventilation buildings 

raise supply pressure to 120 psi for use of 

foam. Fire extinguishers are mounted in recesses of 

hose outlets. Fire-alarm stations and phones are at 

the same locations. Emergency trucks with heavy 

hoists, fire hose, foam equipment, and emergency 

tools are kept in readiness at each portal. 




20.5 Preliminary 

Investigations 

Surveys should be made to establish all topographical 

features and locate all surface and subsurface 

structures that may be affected by the tunnel 

construction. For underwater tunnels, soundings 

should be made to plot the bed levels. 

Knowledge of geological conditions is essential 

for all tunnel construction but is of primary 

importance for rock tunnels. Explorations by 

borings and seismic reflection for soft ground and 

underwater tunnels are readily made to the extent 

necessary. For rock tunnels, particularly long ones, 

however, possibilities for borings are often limited. 

A thorough investigation should be made by a 

geologist familiar with the area. This study should 

be based on a careful surface investigation and 

examination of all available records, including 

records of other construction in the vicinity, such as 

previous tunnels, mines, quarries, open cuts, 

shafts, and borings. The geologist should prepare 

a comprehensive report for the guidance of 

designers and contractors. 

For soft ground and underwater tunnels, 

borings should be made at regular intervals. They 

should be spaced 500 to 1000 ft apart, depending 

on local conditions. Closer spacing should be used 

in areas of special construction, such as ventilation 

buildings, portals, and cut-and-cover sections. 

Spoon samples should be taken for soil classification, 

and undisturbed samples, where possible, 

for laboratory testing. Samples not needed in the 

laboratory, boring logs, and laboratory reports 

should be preserved for inspection by contractors. 

Density, shear and compressive strength, and 

plasticity of soils are of special interest. 

All borings should be carried below tunnel 

invert. For pressure face tunnels, borings should be 

located outside the tunnel cross section. 

For rock tunnels, as many borings as practicable 

should be made. Holes may be inclined, to cut as 

many layers as possible. Holes should be carried 

below the invert and may be staggered on either 

side of the center line, but preferably outside the 

tunnel cross section to prevent annoying water 

leaks. Where formations striking across the tunnel 

have steep dips, horizontal borings may give more 

information; borings 2000 ft in length are not 

uncommon. All cores should be carefully cataloged 

and preserved for future inspection. The ratio of 

core recovery to core length, called the rock quality 



designation (RQD), is an indicator of rock problems 

to be encountered. 

Groundwater levels should be logged in all 

borings. Presence of any noxious, explosive, or 

other gases should be noted. 

Where lowering of groundwater may be 

employed during construction of cut-and-cover or 

bored tunnels on land, the permeability of the 

ground should be tested by pumping tests in deep 

wells at selected locations. Rate of pumping and 

drawdown checked in observation wells at various 

distances should be recorded; as well as recovery of 

the water level after stopping the pumps. 

Geophysical exploration to determine 

elevations of distinctive layers of soil or rock 

surfaces, density, and elastic constants of soil may 

be used for preliminary investigations. The findings 

should be verified by a complete boring 

program before final design and construction. 

20.6 Tunnel Ventilation 

Tunnels will be required to be ventilated to dilute 

or remove contaminants, control temperature, 

improve visibility and to control smoke and heated 

gases in the event of a fire in the tunnel. 

20.6.1 Ventilation Requirements 

for Construction 

Occupational Safety and Health Administration 

(OSHA) establishes standards, regulations, and 

procedures necessary to maintain safe, sanitary 

conditions for all workers on construction sites. 

Employers are required to initiate and maintain 

programs that will prevent accidents. Also, 

employers are advised to avail themselves of safety 

and health programs provided by OSHA and are 

required to instruct and train employees to 

recognize and avoid unsafe, unsanitary conditions, 

including prevention and spread of fires. OSHA 

requirements also cover underground construction. 

Following are some of the requirements 

applicable to ventilation. 

Fresh air should be supplied to all underground 

work areas in sufficient quantities to prevent 

dangerous or harmful accumulation of dusts, 

fumes, mists, vapors, or gases. Unless natural 

ventilation meets this requirement, mechanical 

ventilation should be supplied. At least, 200 ft 3 of 





fresh air should be provided for each employee 

underground. The air flow should be at least 30 ft/ 

min where blasting or rock drilling is conducted or 

where polluted air is likely to be present or 

developed. The direction of air flow should be 

reversible. After blasting, smoke and fumes should 

be immediately exhausted to outdoors before work 

is resumed in affected areas. 

Underground operations are classified as gassy 

if air monitoring discloses for three consecutive 

days 10% or more of the lower explosive limit for 

methane or other flammable gases, measured 

about 12 in from work-area enclosure surfaces. 

Where such conditions occur, operations ether 

than those necessary for correcting the conditions 

should be discontinued. Ventilation systems should 

be made of fire-resistant materials. Controls 

for reversing air flow should be located above 

ground. 

At normal atmospheric pressure underground, 

the air should contain at least 19.5% but not more 

than 22% oxygen. Test should be made frequently 

first for oxygen, then for carbon monoxide, 

nitrogen dioxide, hydrogen sulfide, and other 

pollutants. If hydrogen sulfide concentration 

reaches 20 ppm or 20% or more of the lower 

explosive limit for flammable gases is detected, 

precautions should be taken to protect or evacuate 

personnel. 

Mobile diesel-powered equipment used underground 

in atmospheres other than gassy operations 

must either be approved by MSHA or the employer 

must demonstrate that it is fully equivalent to such 

MSHA-approved equipment. (30 CFR Part 32 

MSHA). 

For construction in compressed air, see 

Art. 20.16. 

[“Construction Industry: OSHA Standards for 

the Construction Industry (29 CFR 1926/1910),” 

Superintendent of Documents, Government Printing 

Office, Washington, DC 20402 (www.gpo.gov)]. 

20.6.2 Ventilation for Railroad 

and Rapid-Transit Tunnels 

Short tunnels generally have no forced ventilation. 

Longer tunnels for diesel trains may need 

ventilation to purge smoke and exhaust gases. 

Tunnels for electric traction are adequately selfventilated 

by piston action but may require 

emergency ventilation. 


A ventilation system dilutes and purges smoke 

and combustion and exhaust gases. Its capacity 

must be adequate to prevent irritating smoke or gas 

concentrations while a train passes through and to 

clear the air between train passages. Diesel content 

of nitrogen oxides may form corrosive acids in 

lungs when inhaled for long periods. The following 

systems are used by American railroads: 

Injecting a stream of air at high velocity in the 

direction of train movement to keep smoke ahead 

of the train. 

Injecting a high-speed high-volume air stream 

from the opposite end against the train motion to 

dilute smoke and clear the tunnel. 

Addition of portal doors with the first injection 

system, to increase efficiency and prevent backflow 

in case of a stalled train. Doors are interlocked with 

signal systems (Moffat Tunnel). 

Because of absence of smoke or exhaust gas 

when electric traction is used, ventilation by 

piston action of trains is adequate for tunnels for 

electric trains except under emergency conditions. 

Auxiliary exhaust fans should be installed to 

remove smoke in case of fire and to draw fresh air 

into the tunnel from the stations or portals. Fans 

may be installed in exhaust shafts between 

stations or in separate ventilation buildings in 

long underwater tunnels equipped with exhaust 

ducts. High-speed rapid-transit tunnels require 

air-relief shafts ahead of stations to prevent air 

blasts from entering the stations. In hot climates, 

heat dissipation in tunnels and stations requires 

special ventilation capacity and air conditioning. 

A computer program, the Subway Environment 

Simulation (SES), for system design has been 

developed. (“Subway Environmental Design 

Handbook,” Urban Transportation Administration, 

Washington, DC 20590.) 

20.6.3 Emission Contaminants 

in Road Tunnels 

Exhaust gases of gasoline internal combustion 

engines contain deadly carbon monoxide and 

irritating smoke and oil vapors. Diesel engines 

will also produce dangerous nitrogen oxides and 

aldehydes. The components of exhaust gases vary 

over a wide range. 




The ventilation system also must be capable of 

controlling smoke and hot gases in case of fire (see 

Ventilation Systems for Road Tunnels following). 

The Federal government or health authorities of 

states place restrictions on permissible carbon 

monoxide (CO) content. With new standards 

limiting contaminants in vehicle exhaust gases, 

however, it may eventually be possible to meet the 

CO limitations without extensive increase in 

ventilation. Engineers should check current rules 

at time of design. 

Haze from vehicle exhaust gases, particularly 

from Diesel engined vehicles, does reduce visibility 

in the tunnel. In practice, when the CO level within 

the tunnel is maintained at levels as proposed in 

Table 20.1, adequate dilution of the irritating parts 

of exhaust gases and adequate visibility is assured. 

New road tunnels built in the United States 

must comply with the time-weighted limits for 

concentration of CO established by the U.S. 

Environmental Protection Agency and the Federal 

Highway Administration. These limits are listed in 

Table 20.1. 

Other countries may set other standards for 

carbon monoxide (CO) concentrations within their 

tunnels. The World Road Association (PIARC) 

publications provide documentation on the 

subject. 

For tunnels in which traffic may incorporate a 

high percentage (10% or more) of diesel vehicles, 

the ventilation requirements for dilution of NO x 

particles of nitrogen and particulates (smoke) 

become significant. The NOx emitted by vehicles 

consists mainly of nitric oxide (NO), which 

oxidizes in the atmosphere to form nitrogen 

dioxide (NO 2). Based on exposure limits recommended 

by the American Conference of 

Governmental Industrial Hygienists and a typical 

4-to-1 ratio for NO to NO2, the maximum 

permissible concentration of NO x is about 10 ppm. 

Table 20.1 Limits on CO in Road Tunnels 

Exposure time, 

min 

Maximum CO 

concentration, ppm 

0–15 120 

16–30 65 

31–41 45 

46–60 35 



Carbon Monoxide (CO) has been proven to be 

the umbrella pollutant in most road tunnels. That 

is, when the CO level in any given road tunnel is 

maintained at or below the levels shown in Table 

20.1, all of the other vehicle pollutants will be 

within appropriate levels. The only exception to 

this is the case of particulate matter emitted by 

Diesel engined vehicles when the tunnel traffic 

stream contains on an average more than 15% 

Diesel engined vehicles. 

The current method of determining the vehicle 

emissions to be considered for a road tunnel 

ventilation system design is to apply the United 

Stated Environmental Protection Agency’s MOBIL 

series of computer programs. Mobil5B is the 

current version in use today. 

20.6.4 Ventilation Systems for 

Road Tunnels 

In straight tunnels up to about 1000 ft in length, 

natural air flow is usually sufficient, particularly 

with traffic in one direction. If a tunnel is exposed 

to heavy traffic congestion at times, installation of 

exhaust fans in a shaft or adit near the center for 

emergency ventilation is advisable if the length 

exceeds 500 ft. 

Natural Ventilation n Naturally ventilated 

tunnels rely primarily on atmospheric conditions 

to maintain airflow and a satisfactory environment 

in the tunnel. The piston effect of traffic provides 

additional airflow when the traffic is moving. 

Naturally ventilated tunnels over 1,000 feet (305 

meters) long require emergency mechanical ventilation 

to extract smoke and hot gases generated 

during a fire as defined by NFPA 502 “Standard for 

Road Tunnels, Bridges, and Other Limited Access 

Highways”. Tunnels with lengths between 800 and 

1,000 feet (240 and 305 meters) will require the 

performance of an engineering analysis to determine 

the need for emergency ventilation. Because 

of the uncertainties of natural ventilation, 

especially the effect of adverse meteorological and 

operating conditions, reliance on natural ventilation, 

to maintain carbon monoxide (CO) levels, 

for tunnels over 800 ft (240 m) long should be 

thoroughly evaluated. If the natural ventilation is 

demonstrated to be inadequate, the installation of a 

mechanical system with fans should be considered 

for normal operations. 





Smoke from a fire in a tunnel with only natural 

ventilation moves up the grade driven primarily by 

the buoyant effect of the hot smoke and gases. The 

steeper the grade the faster the smoke will move 

thus restricting the ability of motorists trapped 

between the incident and the portal at the higher 

elevation to evacuate the tunnel safely. 

Mechanical Ventilation n A tunnel that is 

sufficiently long, has heavy traffic flow, or 

experiences adverse atmospheric conditions 

requires mechanical ventilation with fans. Mechanical 

ventilation layouts in road tunnels are either 

of the longitudinal or transverse type. 

Longitudinal Ventilation n This type of 

ventilation introduces or removes air from the 

tunnel at a limited number of points, thus creating 

a longitudinal flow of air along the roadway. 

Ventilation is either by injection, or by jet fans. 

Injection Longitudinal Ventilation is frequently 

used in rail tunnels and is also found in 

road tunnels. Air injected at one end of the tunnel 

mixes with air brought in by the piston effect of the 

incoming traffic. This type of ventilation is most 

effective where traffic is unidirectional. The air 

speed remains uniform throughout the tunnel, and 

the concentration of contaminants increases from 

zero at the entrance to a maximum at the exit. 

Injection longitudinal ventilation with the supply 

at a limited number of locations in the tunnel 

is economical because it requires the least number 

of fans, places the least operating burden 

on these fans, and requires no distribution air 

ducts. 

Jet Fan Longitudinal Ventilation has been 

installed in a significant number of tunnels worldwide. 

Longitudinal ventilation is achieved with 

specially designed axial fans (jet fans) mounted at 

the tunnel ceiling. Such a system eliminates the 

spaces needed to house ventilation fans in a 

separate structure or ventilation building; however, 

it may require a tunnel of greater height or width to 

accommodate jet fans so that they are out of the 

tunnel’s dynamic clearance envelope. This envelope, 

formed by the vertical and horizontal planes 

surrounding the roadway pavement in a tunnel, 

define the maximum limits of predicted vertical 

and lateral movement of vehicles traveling on the 

road at design speed. As the length of the tunnel 

increases, however, the disadvantages of longi- 


tudinal systems, such as excessive air speed in the 

roadway and smoke being drawn the entire length 

of the roadway during an emergency, become 

apparent. 

The longitudinal form of ventilation is the most 

effective method of smoke control in a road tunnel 

with unidirectional traffic as was determined in the 

Memorial Tunnel Fire Ventilation Test Program. A 

longitudinal ventilation system must generate 

sufficient longitudinal air velocity to prevent the 

backlayering of smoke. Backlayering is the movement 

of smoke and hot gases contrary to the 

direction of the ventilation airflow in the tunnel 

roadway. The air velocity necessary to prevent 

backlayering of smoke over the stalled motor 

vehicles is the minimum velocity needed for smoke 

control in a longitudinal ventilation system and is 

known as the critical velocity. 

Transverse Ventilation n Transverse ventilation 

includes systems that distribute supply air 

and collect exhaust air uniformly along the length 

of the tunnel. There are several such systems 

including the full transverse system which 

includes both supply and exhaust air uniformly 

distributed and collected. The semi- or partial 

transverse systems incorporate only one, either 

supply or exhaust air. 

Semi transverse ventilation can be configured 

as either a supply system or an exhaust system. 

Semi transverse ventilation is normally used in 

tunnels up to about 7,000 feet (2,000 meters); 

beyond that length the tunnel air velocity speed 

near the portals may become excessive. 

Supply semi transverse ventilation applied to 

a tunnel with bi-directional traffic produces a 

uniform level of contaminants throughout the 

tunnel because the air and the vehicle exhaust 

gases enter the roadway area at the same uniform 

rate. In a tunnel with unidirectional traffic, 

additional airflow is generated in the roadway by 

the movement of the vehicles, thus reducing the 

contaminant level in portions of the tunnel. 

Because the tunnel airflow is fan-generated, this 

type of ventilation is not adversely affected by 

atmospheric conditions. The supply air travels the 

length of the tunnel in a tunnel duct fitted with 

supply outlets spaced at predetermined distances. 

If a fire occurs in the tunnel, the supply air initially 

dilutes the smoke, which was shown in the 

Memorial Tunnel Fire Ventilation Test Program to 

be an ineffective method for controlling smoke 




from larger fires. Supply semi transverse ventilation 

should be operated in a reversed mode for 

the emergency so that fresh air enters the tunnel 

through the portals to create a longitudinal flow 

of air equivalent to the critical velocity. It also 

provides a tenable environment for fire-fighting 

efforts and emergency egress. 

Exhaust semi transverse ventilation installed in 

a unidirectional tunnel produces a maximum 

contaminant concentration at the exit portal. In a 

bi-directional tunnel, the maximum level of contaminants 

is located near the center of the tunnel. 

In a fire emergency both the exhaust semi 

transverse ventilation system and the reversed 

semi transverse supply system create a longitudinal 

air velocity in the tunnel roadway thus 

extracting smoke and hot gases uniformly along 

the tunnel length. 

Full Transverse Ventilation n Full transverse 

ventilation has been used in extremely long 

tunnels and in tunnels with heavy traffic volume. 

Full transverse ventilation includes both a supply 

duct and an exhaust duct to achieve uniform 

distribution of supply air and uniform collection of 

vitiated air throughout the tunnel length. During a 

fire emergency the exhaust system in the incident 

zone should be operated at the highest available 

capacity while the supply system in the adjacent 

incident zone is operated. This mode of operation 

creates a longitudinal airflow (achieving the critical 

velocity) towards the incident zone and allows the 

smoke and heated gases to be extracted as close as 

possible to the fire and keep the upstream stopped 

traffic clear of smoke. 

Other Ventilation Systems n There are 

many variations and combinations of the systems 

described previously. Most of the hybrid systems 

are configured to solve a particular problem faced 

in the development and planning of the specific 

tunnel, such as excessive air contaminants exiting 

at the portal(s). 

Ventilation System Enhancements n A 

few enhancements are available for the systems 

described previously. The two major enhancements 

are single point extraction and oversized exhaust 

ports. 

Single point extraction is an enhancement to a 

transverse system that adds large openings to the 



exhaust duct. These openings include devices that 

can be operated during a fire emergency to extract 

a large volume of smoke as close to the fire source 

as possible. Tests conducted as a part of the 

Memorial Tunnel Fire Ventilation Test Program 

concluded that this concept is extremely effective 

in reducing the temperature and smoke in the 

tunnel. The size of openings tested ranged from 100 

to 300 ft 2 (9.3 to 28 m 2 ). 

Oversized exhaust ports are simply an expansion 

of the standard exhaust port installed in the 

exhaust duct of a transverse or semi-transverse 

ventilation system. Two methods are used to create 

such a configuration. One is to install on each port 

expansion a damper with a fusible link; the other 

uses a material that when heated to a specific 

temperature melts and opens the airway. Several 

tests of such meltable material were conducted as 

part of the Memorial Tunnel Fire Ventilation Test 

Program but with limited success. 

20.6.5 Elements of Road Tunnel 

Ventilation Systems 

Major components of ventilation systems commonly 

used for road tunnels are described in the 

following. 

Ventilation Buildings (Figs. 20.5 and 

20.6) n Fans, electrical transformers and switchgear, 

control board, and auxiliary equipment are 

housed in ventilation buildings. In short- and 

medium-length tunnels, one building at either 

portal is sufficient. Longer tunnels should have a 

building at each portal. A few of the longest have 

three or four buildings. For underwater tunnels, 

ventilation buildings may be at the water’s edge, 

each building controlling a land and a river section 

of the tunnel. 

Fresh air is taken in through large louver areas 

in the walls of the building. The louvers should be 

protected by bird screens. Louvers are usually 

aluminum and arranged for shedding water. 

Adequate drains should be provided in the fan 

room to remove rainwater, which may blow in 

through the louvers. Vitiated air is discharged 

through vertical stacks, which also should be 

covered by screens. 

Tunnel Ducts are usually of constant area 

throughout their length. Concrete surfaces should 

be smooth for minimum friction. Obstructions, 





Fig. 20.5 Sections through Hampton Roads Tunnel Ventilation Building. (a) Fresh-air supply 

system. 

such as ceiling hangers, should be streamlined or at 

least rounded. Turns in ducts and shafts leading to 

the tunnel should be equipped with noncorrosive 

turning vanes for smooth air flow. 

Flues spaced about 15 ft apart, extended from 

the ducts, supply fresh air slightly above roadway 

level. Ceiling ports are slanted at 458 in the 

direction of air flow in the ducts. All air openings 

should provide the means to adjust size to balance 

the air flow over the length of the tunnel. 

Fans n Two types of fans are available: 

centrifugal fans, used in all tunnels up to about 


1938, and vane axial fans, a later development. 

Centrifugal fans have backward-curved blades and 

are nonoverloading. The efficiencies of welldesigned 

fans of either type are about the same. 

For underwater tunnels, with vertical air shafts in 

the ventilation buildings, the vane axial fans 

require considerably less space and avoid the 

efficiency loss through the fan chambers usually 

associated with centrifugal fans. Blades for vane 

axial fans may have a fixed pitch or may be 

adjustable during operation. When reversed, the 

former type provides 80% of maximum capacity. 

The latter type may be adjusted from 0 to 100% of 

capacity for supply and exhaust, thus permitting 




adjustments to meet variable demands for ventilation 

with fewer fans. The noise level of vane 

axial fans at maximum speed is somewhat 

higher than that for centrifugal fans because of 

greater tip speed. In sensitive surroundings, the 

noise from supply and exhaust fans can be 

dampened by sound baffles. Vane axial fans may 

have external drives or motors built into the hub of 

the impellers. 

Centrifugal fans are operated by squirrel-cage 

motors through chain or multiple V-belt drives. 

The latter eliminate lubrication problems and wear 

on a multiplicity of parts (inherent in chain drives), 

give excellent service, and can be easily replaced. 


Fig. 20.5 (Continued) (b) Exhaust-air system. 


Chains are enclosed in solid housings; belts are 

protected by wire guards. 

For flexibility, the load is divided between 

several fans—at least two, sometimes as many as 

six—for each system. Four is a good number for 

demands exceeding about 600,000 ft 3 /min. 

To further adjust supply to variable demand, fan 

motors are equipped with two-speed windings. 

Three speeds with two motors have been used in 

earlier installations but are not necessary with an 

adequate number of fans. Spare fans may be 

provided as protection against breakdown, or total 

fan capacity may be increased by 10 or 15%. With 

good maintenance, fans are seldom out of 





commission, and the extra capacity of the system is 

sufficient to maintain acceptable conditions for 

limited periods with one unit out of service. 

To protect the exhaust fans in case of a serious 

fire in the tunnel, automatic deluge sprinkler 

systems should be installed to cool the exhaust air. 

Dampers n All fans should be equipped with 

shutoff dampers to prevent short circuiting of air. 

Their operating motors should be interlocked with 

the control of the fan motors for automatic opening 

and closing. Trapdoor-type or multiblade dampers 


Fig. 20.6 Section through ventilation building of the Holland Tunnel. 

are in use; the latter take less space and time to 

operate. 

Fan Control n In short, unattended tunnels, 

fans can be controlled automatically with carbon 

monoxide analyzers. Larger tunnels with heavy 

traffic may have operators stationed in the control 

room. They operate the fans to control conditions in 

the tunnel. At least two independent sources of 

electric power must be available, usually through 

feeders from different parts of the utility system. If 

these are not available, a diesel-engine emergency 




generator sufficient for minimum requirements 

should be installed. 

Carbon Monoxide Analyzers n These take 

continuous air samples from the tunnel and 

analyze them for CO content. The results are 

visually indicated and also recorded on paper tape, 

with time gradations. The recorders are mounted 

on the face of the control board, to guide the 

operator in selection of number of fans and speed 

necessary. 

In a longitudinal or semitransverse supply 

system, air samples are taken from the tunnel 

proper at points of maximum concentration. 

In transverse systems, the samples may be taken 

from the exhaust ducts. 

Haze Control n To measure visibility in 

tunnels affected by haze from exhaust gases, 

instruments have been developed that give a 

reliable indication without excessive maintenance. 

Equipment manufactured for the Port Authority of 

New York and New Jersey uses the scattering of 

ultraviolet light by dust particles. Instruments 

protect the optics by recessing them in tubes 

through which filtered air is exhausted. Another 

type of instrument compares the intensities of two 

branches of a split light beam passing through the 

same optics, one going through a tube filled with 

clean air, the other through tunnel air. 

Ventilation Power Requirements n The 

power requirements and pressure losses are best 

evaluated using the prodedures contained in the 

Tunnel Engineering Handbook (J.O. Bickel and 

T.R. Kuesel, ‘‘Tunnel Engineering Handbook,’’ 

Kluwer Academic Publishers, New York). 

20.7 Tunnel Surveillance 

and Control 

Emergency exhaust ventilations systems in short 

tunnels or tunnels with very light traffic may be 

activated by such instruments in the tunnel as 

carbon monoxide analyzers or fire-alarm or 

telephone boxes connected to the nearest fire and 

police departments. Emergency operation for other 

types of tunnels should be supervised by personnel 

in control centers. 



Control of many newer tunnels is programmed 

for computer operation. The computers, however, 

may be bypassed for manual operation in an 

emergency. 

To permit surveillance of tunnel traffic by 

personnel in the control room, monitors may be 

installed in that room to display views of the entire 

length of the roadways as transmitted by television 

cameras mounted in the tunnel. In a short tunnel, 

each camera covers a specific stretch of roadway 

and transmits to a specific monitor. For a long 

tunnel, to limit the number of monitors required to 

a convenient number, groups of cameras may be 

operated in sequence to transmit to their monitors. 

In an emergency, the sequence can be interrupted 

to permit a specific camera to focus on the region of 

concern. 

Traffic Control n Signal lights generally are 

mounted at the portals of a tunnel and at intervals 

in the interior such that at least one traffic light is 

plainly visible within a safe stopping distance. In a 

tunnel with two-way traffic, the signals facing 

traffic may incorporate red, amber, and green 

lights. Lights on the reverse side of those signals 

may be red and amber, to permit lane alternation 

in an emergency. In a tunnel with two-lane, oneway 

traffic, the signals facing traffic carry red, 

amber, and green lights, whereas lights on the 

reverse side of signals for the left lane may be 

amber and red, to permit two-way traffic in an 

emergency. 

Traffic flow may be monitored by pairs of 

electric induction coils that are embedded in the 

pavement of each lane and that report the flow on 

indicators in the control room. If traffic velocity is 

too slow, for instance, less than 5 to 10 mi/h, the 

traffic lights are changed to amber, for caution. If 

traffic stops, the lights are changed to red. If 

necessary, for slow traffic, the traffic lights may be 

alternated between stop and go to space traffic flow 

into the tunnel. 

Fire Control n Automatic fire detectors may be 

installed in the ceiling throughout a tunnel. When a 

fire occurs, they indicate the location and send an 

alarm to an operator who alerts an emergency 

crew. If the operator verifies the alarm (which 

might have been activated by the heavy exhaust of 

a diesel engine rather than by a fire), an emergency 

program can be started: The emergency crew and 





vehicles are mobilized. For traffic moving toward a 

fire, signal lights are turned to red, while for traffic 

moving away from the fire, signals remain green, to 

permit evacuation. And the ventilation system for 

the affected part of the tunnel is converted to 

exhaust. 

Hydrants generally are installed about 300 ft 

apart, in niches in the tunnel walls, to provide 

water for fire fighting. Water may be obtained from 

municipal water supplies, if available. Otherwise, 

the water mains may be connected to tanks 

providing about 10,000 gal of storage. The tanks 

may be located near each portal and supplied by 

pumps from local sources or from groundwater. 

Booster pumps may be installed to provide at 

least 125-psi pressure for application of water 

on fires. Fire alarms and fire extinguishers for 

control of minor fires may be installed next to the 

hydrants. 

Communications n Emergency telephones 

may be placed along the tunnel side walls for 

communication with an operator in the control 

room. An aerial in the tunnel will permit the 

operator to transmit messages to motorists through 

their car radios and allow them to receive other 

broadcasts while in the tunnel. 

Power Supply n Power should be supplied 

from two independent sources, for example, from 

two different utilities or independent substations 

of one utility. An alternative is a standby diesel 

generating plant capable of supplying power 

at least for ventilation and emergency lighting 

to keep the tunnel in operation. This equipment 

should be supplemented by storage batteries 

to supply instant power for the emergency 

lighting. 

20.8 Tunnel Lighting 

The Occupational Safety and Health Administration 

sets minimum requirements for illumination 

on construction sites: 

5 ft-c—general construction-area lighting, warehouses, 

corridors, exitways, tunnels, and shafts 

3 ft-c—concrete placement, excavation and waste 

areas, accessways, active storage areas, loading 

platforms, refueling, and field maintenance areas 


10 ft-c—batch and screening plants, mechanical 

and electrical rooms, indoor work rooms, rigging 

lofts, indoor toilets, and tunnel and shaft headings 

during drilling, mucking, and scaling. 

For other areas, follow illumination recommendations 

in “Practice for Industrial Lighting,” IES 

RP7, the Illuminating Engineering Society of North 

America. 

For emergency use, every employee underground 

should be equipped with a portable hand 

or cap lamp unless sufficient natural light or an 

emergency lighting system provides sufficient 

illumination along escape paths. Only portable 

lighting meeting OSHA requirements may be used 

within 50 ft of any heading during explosive 

handling. (See also Art. 20.16.) 

[“Construction Industry: OSHA Safety and 

Health Standards 29 CFR 1926/1910,” Superintendent 

of Documents, Government Printing Office, 

Washington, DC 20402.] 

Lighting for Tunnels in Service n Since 

locomotives are equipped with strong headlights, 

railway tunnels are generally not lighted except for 

emergency evacuation. Subaqueous tunnels and 

other tunnels on electrified lines, particularly in 

cities, are equipped with a nominal amount of 

lights, especially in refuge niches. 

Rapid-transit tunnels are lighted sufficiently to 

make obstructions on tracks visible and to facilitate 

maintenance work. The lights are installed and/ 

or shielded to prevent glare in the motorman’s 

eyes. Luminaires are installed in tunnels for 

emergency use. 

For highway tunnels, the most troublesome 

lighting condition is the transition from bright light 

in the approach to the tunnel, the entrance 

(threshold zone) luminance, to the luminance in 

the interior. Guidelines for alleviating this condition 

have been issued by the American Association 

of State Highway and Transportation 

Officials (AASHTO) and the Illuminating Engineering 

Society of North America. Threshold zone 

illumination varies greatly with topography, orientation, 

sun exposure, and season and should be 

evaluated for the most critical condition. Daylight 

penetration through the portal into the threshold 

zone may assist the transition. In addition to the 

threshold zone, two or three transition zones 

gradually reduce the luminance to that of the 




interior. The length of each of these zones should 

be approximately one safe-stopping sight-distance 

(SSSD) at design speed. Reduction between zones 

should not exceed 3:1. 

At night, a pavement luminance of 2–5 cd/m 2 

minimum is recommended for the entire length of 

the tunnel. The approach and exit roadways should 

have a luminance level of no less than one third the 

tunnel interior level for a distance of a SSSD. 

There are four viable types of light sources used 

in tunnels, fluorescent, low-pressure sodium (LPS), 

high-pressure sodium (HPS), and metal halide 

(MH). The advantages and disadvantages of each 

are discussed in greater detail in ANSI/IESNA RP- 

22-96 8.1. These include restrike time in the event of 

momentary power interruption, linearity of source 

to reduce flicker, cost, color rendering, lamp size, 

lamp efficacy, control of light distribution, effects of 

air temperature, lumen depreciation with time, 

glare, the risk of lamp rupture, and keeping 

enclosures dust-tight and water tight. Florescent 

lamps frequently provide the lower illumination 

levels, combined with LPS at threshold and 

transition zones. Lower wattage LPS sources are 

also used in interior zones. HPS and MH lamps 

come in a wide selection of sizes, better lamp life, 

compact size and are easily optically controlled. 

20.9 Tunnel Drainage 

Most tunnels through hills and mountains have 

water problems. Surface water penetrates through 

fissures and percolates through permeable soils. 

Attempts to seal off the rock by grouting, with 

either cement or chemicals, usually are not 

completely successful since very high pressures 

may build up even if flows are low. Cast-in-place 

concrete linings may not be completely watertight. 

Water may find its way through shrinkage cracks in 

the linings into the interior of tunnels. There, it may 

freeze in cold weather and produce an unsightly 

appearance, objectionable in highway tunnels. 

Consequently, provision must be made to drain 

water from tunnels. 

Fire fighting, washing of tunnel interiors, and 

flushing of pavements also introduce water that 

must be drained. 

Although cut-and-cover tunnels can be waterproofed, 

this is difficult with bored tunnels. If the 

water problem is not serious, the most economical 

solution is to seal cracks in the lining that leak. With 



good concrete control, the number of these should 

be small. It is good practice to design tunnels 

assuming that they will leak and therefore provide 

appropriate drainage paths. 

If water appears in considerable quantity during 

rock tunneling operations, tight steel lagging over 

the tunnel supports and grouting may prevent 

leakage. In serious cases, it may be necessary to 

dry-pack between the rock and the tunnel lagging 

to drain water. This is a slow, costly method 

requiring much manual labor. Dry pack behind the 

side walls can easily be placed and is effective in 

preventing the buildup of a hydrostatic head 

behind the lining. Longitudinal drain pipes should 

be installed behind the base of the side walls, with 

the laterals at regular intervals leading to the main 

drain (this is a large drain installed under the 

roadway, for roadway drainage). Water will flow 

through the dry packing and into the base drains. 

In a rock tunnel, heavy flow of water coming 

through a drill hole indicates a water-bearing fault 

or seam. The flow may be stopped by drilling 

additional holes and injecting cement grout. Some 

holes should be slanted to reach beyond the 

periphery. If dense sand or rock flour in the fault 

prevents proper penetration of cement grout, 

chemical grouting may give satisfactory results. 

In special cases, it may be necessary to drill a pilot 

hole well ahead of the face to detect severe water 

conditions, especially substantial quantities under 

heavy pressure. This must be done for rock 

tunneling under deep bodies of water. 

In highway tunnels, drainage inlets should be 

installed at regular intervals along the curbs, with 

cross connections to the main drain. The latter 

should be of generous size, in longer tunnels 

preferably large enough to provide crawl space 

to remove silt accumulations, particularly when 

grades are near horizontal. Traps at drainage inlets 

are undesirable, because of the danger in the event 

of a fuel spill. 

Leakage in well-constructed underwater tunnels, 

either shield-driven or immersed, is usually 

minor. It can be controlled by calking joints in 

segmental liners or by injecting cracks where leaks 

appear. Main sources of water are washing of 

tunnel interior, fire fighting, drippings from 

vehicles, and rain collected in open approaches. 

Pumps are usually sized to handle the full flow 

from one fire hydrant. 

Continuous open gutters recessed into the curbs 

have been used in many subaqueous tunnels. The 





gutters lead water to a low point, where it is 

collected in a sump. Drainage inlets, spaced about 

50 ft apart along each curb and connected to 

longitudinal drain lines embedded in the concrete 

below the curbs, are desirable because they prevent 

propagation of fire by burning fuel in case of a 

serious accident. Drain lines should be at least 8 in 

in diameter. They should be equipped with 

cleanouts every 500 ft. 

In straight, open approaches, transverse interceptors 

about 300 ft apart are most effective in 

preventing water from entering a tunnel. They are 

18 in wide, extend from curb to curb, and are 

covered with gratings, with slots parallel to the 

center line of the roadway. An interceptor is placed 

in front of the tunnel portal and another about 10 ft 

inside. 

In curved, superelevated approaches, drainage 

inlets should be installed at regular intervals along 

the low curb. 

All drainage from open approaches should be 

collected inside the portals in sumps below the 

roadway. Each sump should be divided into a 

settling basin and a suction chamber. Easy access 

must be provided for cleaning out sediments. A 

minimum of three electrically driven, largeclearance 

drainage pumps should be installed, one 

as a standby. Alternating automatic controls rotate 

the pumps in service. High-water-level alarm 

circuits should be extended to the control room. 

Sump and pump capacity, with two pumps 

operating, should be designed for maximum, 

short-duration rainfall for the locality. An intensity 

of 4 in/h, based on a 15-min downpour at a rate of 

8in/h, is ample for most areas. 

A smaller sump should be located at the low 

point of the tunnel. This sump also should be 

divided into a settling and a suction chamber. Two 

automatically controlled drainage pumps, with a 

capacity of 250 gal/min each, may be adequate. 

Their discharge should be carried to one of the 

portal sumps. 

20.10 Water Tunnels 

These may be diversion or intake tunnels for 

hydropower plants, or aqueducts bringing water to 

city and municipal distribution systems. 

Diversion tunnels carry river water around dam 

sites during construction. They are designed to 

carry the maximum expected runoff during this 


period. They may also discharge excess water after 

the reservoir has been filled, or be converted to 

intake tunnels to a powerhouse located in the side 

of the valley below the dam. If they are not needed 

after completion of the project, the diversion 

tunnels are closed with concrete plugs. Extensive 

diversion tunnels have also been built to collect 

water from several watersheds for a central power 

plant. 

Intake tunnels bring water from reservoirs to 

turbines or the heads of penstocks. The tunnels are 

mostly in rock and operate under a positive 

hydrostatic head. In pervious and fissured ground, 

they are lined with reinforced concrete or steel 

plate; in sound rock, a sprayed-concrete lining may 

be adequate to provide a smooth surface as long as 

there is sufficient overburden to exceed the internal 

pressure. 

Many miles of aqueduct tunnels have been built 

for municipal or area water-distribution systems. 

These tunnels are, for the most part, in rock but 

may also contain stretches of soft-ground tunneling. 

They may be under large hydrostatic pressure, 

such as the New York City aqueduct, which crosses 

the Hudson River 600 ft below sea level. 

Tunnels with small or no interior pressure 

generally have a horseshoe section; pressure 

tunnels are circular. Lining is concrete, 6 to 36 in 

thick, depending on size, pressure, and nature of 

rock. Welded steel tubes may be needed when 

pressures are particularly high. Grade tunnels may 

be lined with plain concrete, pressure tunnels with 

reinforced concrete. Diameters range from 7 ft for 

small aqueducts to 50 ft for Hoover Dam diversion 

tunnels. In very sound rock, sprayed-concrete 

lining has been used. Parts of the Colorado River 

aqueduct are lined with continuous steel shells 

against concrete backing, and the inside is 

protected by 2 in of reinforced sprayed concrete. 

To expedite construction, long tunnels are 

subdivided into several headings by shafts or 

adits, about 2 to 5 mi apart. 

20.11 Sewer and Drainage 

Tunnels 

Large cities require miles of tunnels to carry off 

storm runoff and to conduct wastewater to 

treatment plants. These tunnels are built in a 

variety of soils. Some are constructed as box 

culverts by the cut-and-cover method, but most are 




tunneled with tunnel boring machines (TBMs). Size 

varies from about 7 to 15 ft. Drainage tunnels for 

storm water are usually less extensive since they 

can discharge into nearest open waters. 

The cross section of sewer and drainage tunnels 

is usually horseshoe or circular, with concrete 

lining. Quality of concrete is of special importance 

to resist the detrimental effect of wastewater. 

Generally, they are grade tunnels, except for 

siphons under rivers, which are under pressure. 

A circular or egg-shaped section maintains velocity 

at low flow to prevent excessive settling of solids. 

Alignment is dictated by location of treatment 

plants, soil conditions, and the street plan of the 

city. Continuous grades should be maintained 

except for siphons. A minimum grade should be 

maintained for gravity flow. 

20.12 Cut-and-Cover Tunnels 

Shallow-depth tunnels, such as rapid-transit lines 

under city streets, underpasses, land sections of 

underwater tunnels, and end sections of tunnels 

through hills, are built by cut-and-cover methods. 

A trench is excavated from the surface, within 

which a concrete tunnel is constructed. With 

bottom-up construction, the completed tunnel is 

covered up, and the surface reinstated. With topdown 

construction, the walls are constructed first, 

perhaps using bentonite slurry in narrow trenches. 

The roof is constructed next, backfilled and the 

surface reinstated. Excavation and construction of 

the floors below roof level then follow, using access 

from the ends or from glory holes. Both bottom-up 

and top-down construction almost always use castin 

place concrete. Depth of invert on subways and 

underpasses usually does not exceed 35 to 40 ft. For 

connections to subaqueous tunnels, cuts up to 

100 ft have been used under special circumstances, 

and depths to 60 ft are not uncommon. 

The Occupational Safety and Health Administration 

(OSHA) sets standards, regulations, and 

procedures for protection of personnel during 

excavation. OSHA requires that all surface encumbrances 

and underground utility installations, such 

as sewers, electrical and telephone conduits, and 

water pipes, be protected, supported, or removed 

as necessary to safeguard the workers. Also, 

structural ramps used for access or egress should 

be designed by a structural engineer and constructed 

in accordance with the design. For trench 



Fig. 20.7 Taipei Metro Cut-and-Cover. 

excavations that are 4 ft or more deep, a stairway, 

ladder, or ramp should be provided for egress so as 

to require no more than 25 ft of lateral travel for 

workers. 

Among the measures that OSHA specifies for 

safeguarding personnel in excavations are the 

following: Precautions should be taken to prevent 

exposure of personnel to harmful levels of 

atmospheric contaminants (Art. 20.6). If natural 

lighting is inadequate for safe working conditions, 

illumination to meet OSHA requirements for 

excavation should be provided (Art. 20.8). Personnel 

should not be allowed to work in excavations in 

which water accumulates unless the workers are 

protected by safety harnesses and lifelines, water is 

being removed to control the water level within 

safe limits, and special supports or shields are used 

to protect against cave-ins. 

To avoid exposure to falling objects, personnel 

should not be permitted below loads carried by 





Fig. 20.8 63rd Street Tunnel Strutting and Tie-backs. 

lifting or digging equipment. Unless excavations 

are entirely in stable rock or are less than 5 ft deep 

in stable soil, protection should be provided 

against cave-ins. Retaining devices may be 

required to prevent excavated or other materials 

or equipment from falling or rolling into the 

excavation. 

Where space and depth of excavation permit 

and the ground is sufficiently firm, open slopes 

may be used along the sides of the excavation. For 

excavations up to 20 ft deep, OSHA limits the slope 

to a maximum of 1:1 1 ⁄ 2 (348 with the horizontal), 

unless soil tests and analyses indicate steeper 

slopes will be stable. A registered professional 

engineer must design excavations deeper than 

20 ft, and excavations must be monitored by a 

competent person as defined by OSHA. Personnel 

should be protected from loose rock or soil falling 

or rolling from the excavation face. For the 

purpose, loose material may be removed by scaling 

and protective barriers may be installed at intervals 


along the face. If the lower portion of the 

excavation has vertical sides, that region should 

be shielded or supported to a height at least 18 in 

above the vertical sides. 

Groundwater may be lowered, as needed, by 

tiers of wellpoints. This may lower the groundwater 

outside the excavation considerably and 

cause settlements. The lowering of the external 

groundwater can be reduced by the use of slurry 

walls, contiguous or overlapping bored piles, or 

steel sheet piling. Adjacent structures with a risk of 

settlement may require underpinning. Furthermore, 

where lowering of groundwater exposes 

wooden piles to air, deterioration may follow. 

Where space permits, the sides of the trench 

may be sloped back to reduce the need to provide 

support to them. In confined or deep areas, support 

of excavation may be required. The excavation 

support may be temporary walls that are not part 

of the final structure, or they may form part of the 

final structure, especially when excavations are 




Fig. 20.9 Tangent Piles. 

deep. The ease and simplicity of constructing the 

final structure within temporary walls must be 

balanced against cost savings when they are 

incorporated into the final structure. Temporary 

excavation support may be steel sheet piles, soldier 

piles and lagging, and tangent or secant piles. For 

deeper excavations, concrete slurry walls may be 

constructed, usually 2 ft, 3 ft, or 4 ft thick, sometimes 

incorporating soldier piles or beams (SPTC 

walls), and may form part of the final structure. 

Steel sheetpile walls, for depths to about 30 to 

40 ft, supported by wales and cross bracing. The 

walls keep loss of ground to a minimum. 

Soldier piles and lagging, made of steel H 

beams with wood or concrete lagging. These are 

used for greater depth. Lagging must be blocked 

tight against the earth to control loss of ground. 

Soldier piles may be combined with sheetpiles, 

instead of wood lagging, if tight bulkheads are 

required. Wales and cross bracing support the 

walls. 

Concrete slurry walls built in bentonite-slurry 

trenches have been used to prevent loss of ground 



and eliminate or reduce groundwater lowering. 

Sections of trenches about 20 ft long are excavated. 

The trenches are kept filled with bentonite slurry. 

Then, reinforcing cages are lowered into them, and 

concrete is placed to fill the trenches, displacing the 

slurry. Key sections are formed at the ends of the 

trenches. The walls serve as part of the final 

structure or as impervious bulkheads. 

SPTC or California Wall, a combination of 

soldier piles and slurry wall. This was used on 

some stations of BART, and as part of the final 

structure for much of the Central Artery Project in 

Boston. Large wide-flange steel beams are inserted 

in slurry-filled bored holes, the space between the 

beams is excavated under slurry, and excavation 

and pipe holes are filled with concrete. Care must 

be used in excavation to have the concrete solidly 

keyed into the space between the flanges. The steel 

piles in the composite wall act as reinforcing and 

permit easy attachment of interior bracing. 

The fundamental basis for the design of 

excavation support systems is consideration of 

how the soil being supported behaves, and perhaps 

also how the floor of the trench behaves, since 

substantial heave can occur under adverse conditions. 

Any movement of the support system can 

cause soil movement and hence settlement of 

adjacent structures. It is the amount of movement 

that can be tolerated at the adjacent structures that 

often dictates the type and stiffness of the 

excavation support system, with control of 

ground-water levels often being crucial. Structures 

may have to be designed for both short-term and 

long-term effects. One of the primary tools for 

this purpose is soil-structure interaction analysis. 

Frame structures supported by beam-on-anelastic-foundation 

analysis are also commonly 

used. In most cases, a two-dimensional analysis is 

sufficient, although complex areas may require 

three-dimensional modeling, perhaps using finite 

element analysis. 

Many subway and highway structures have 

been built using steel columns and beams with jack 

arches, but this is uncommon today. New structures 

are generally reinforced concrete box structures 

but when the excavation support system also 

forms part of the final structure, it may in practice 

be difficult to obtain full fixity between the walls 

and the slabs. Partial fixity may then be specified, 

and perhaps shear connections also provided. For 

high load on the roof or large spans, the composite 

action of a thin concrete slab on top of steel beams 





has been used. Haunches may help to reduce 

effective spans. Arched tunnel roofs are rare today. 

Design loads include weight of overburden, self 

weight, live load surcharge, potential future 

construction, horizontal earth pressure, hydrostatic 

loads if below the water table, and seismic loads. 

Weight of submerged structures must be adequate 

to prevent floatation excluding all removable items 

from within the tunnel and above it while ignoring 

friction on the sides. 

Tunnel Waterproofing n Tunnels in dry soil 

need no waterproofing on base and walls, but roof 

slabs should have at least minimum waterproofing. 

Tunnels below groundwater level should be 

waterproofed all around. 

Tunnels that are not waterproofed should be 

designed assuming that they will leak, and paths 

provided to remove any leakage water. Many 

tunnels have drainage channels adjacent to exterior 

walls and a second “false” wall a few inches from 

the first onto which the final tunnel finishes are 

attached. Joints are more likely to leak than other 

locations, so that special attention should be paid to 

waterproofing joints even if surface waterproofing 

is not applied. 

Methods of surface waterproofing include 

membranes supplied in roll form with overlapping 

or welded joints, spray-applied membranes, blindside 

waterproofing designed for the outside face of 

walls cast against existing ground or the support 

system, clay-based panels that swell on contact with 

water, and chemical additives to the concrete. Some 

methods of waterproofing require safety precautions 

during application, such as ventilation. Waterproofing 

membranes that adhere to the surface to 

which they are applied help to prevent the spread 

beneath the membrane of any water that could leak 

through a puncture. The repair of leaks appearing 

on the inside of the tunnel may then be as simple as 

injecting offending cracks locally, otherwise new 

leaks may appear as previous leaks are repaired. 

For joints, a large number of extrusions are 

available that are designed to be buried in the 

concrete, half each side of the joint, some of which 

can be injected later if leaks still occur. Joints may 

also be waterproofed using hydrophilic materials, 

gaskets in compression within the joint or bolted 

to the surface each side of the joint, and surface 

applied injectable and reinjectable tubes. 

Most waterproofing must be protected against 

mechanical damage (for example, during back- 


filling or the placement of reinforcement), and 

against noxious gases (vehicle exhaust, for example) 

and fire within the tunnel. Protection methods have 

included a layer of concrete, plywood boards, 

and brick (against vertical faces). Heat resisting 

materials and cover plates with gaskets have been 

used to protect joints within tunnels. 

To save on excavation width, waterproofing for 

walls may be applied to trench bulkheads and 

concrete placed against it. 

20.13 Rock Tunneling 

Standards, regulations, and procedures of the 


should be adhered to in rock excavations, as in all 

construction operations. (See Arts. 20.6, 20.8, and 

20.12.) 

Tunneling in rock today is primarily by drilland-blast 

or by using a TBM (tunnel boring 

machine). Drill-and-blast tunnels can be any shape, 

whereas most TBMs are only capable of drilling 

circular holes. A rule of thumb is that a tunnel 

requires at least one diameter of cover, although 

less may be possible. Where rock quality is 

particularly good, the tunnel may be unlined or 

may only need mesh, sprayed concrete and rock 

bolts or dowels. More fractured rock may require 

significant temporary ground support such as steel 

sets and lattice girders until a final lining is 

completed. Fracture zones may be particularly 

difficult to cross due to high flows of water under 

high pressure, and due to the quantities of loose 

material. For some purposes, lining may be 

required to promote flow or to prevent ingress of 

water. 

(Kuesel, T. R., Tunnel Stabilization and Lining, 

in ‘‘Tunnel Engineering Handbook,’’ Bickel, J. O., 

Kuesel, T. R., and King E. H., Editors, Chapman & 

Hall, 1996. U.S. Army Corps of Engineers Manual, 

1997, Design of Tunnels and Shafts in Rock, EM 

1110-2-2901.) 

For rock excavation, the most important 

geological conditions to be anticipated are the 

presence of faults, usually involving areas of badly 

fractured rock; direction and degree of stratification; 

fissures and seams; presence of water, which 

may be cold or hot or contain corrosive or irritating 

ingredients; pockets of explosive or toxic gas; and 

rock strain. The petrography is of lesser importance 




unless the rock is highly abrasive, causing 

excessive wear of drills. 

Too much information can never be provided 

for the engineer, to produce a realistic design, and 

for the contractors, to prepare sound bids. Even at 

best, unforeseen difficulties must be expected. 

In addition to geological surveys and borings 

(Art. 20.5), engineers may use electric-resistivity 

measurements and gamma-ray absorption for 

information on depth and characteristics of rock 

formations. Information also may be obtained from 

the U.S. Geological Survey, which has extended its 

scope and geophysical studies beyond the mining 

field. Where geological conditions are particularly 

hard to evaluate or are especially severe, exploratory 

pilot tunnels, about 10 10 ft, may be driven 

part way from each end or for the entire length of a 

tunnel, prior to final design and advertising of 

construction. TBMs may be used for pilot tunnels 

Table 20.2 Load Hp in Feet on Rock on Support in Tunnel* 

either used as a seperate service tunnel, or 

abandoned, or enlarged to form the final tunnel. 

In these pilot tunnels, internal rock stresses can be 

measured by pressure cells and strain gages 

inserted in transverse drill holes, and the nature 

of the rock, foliation, blockiness, and pressure of 

faults and water can be inspected. 

Tunnel Boring Machines (TBM) n The high 

initial cost of hard-rock TBMs tends to restrict their 

use to longer tunnels. Although ideally suited to 

circular tunnels due to the rotary motion of the 

cutting heads, variations of the excavated shape 

may be technically feasible, as have been done with 

a few soft-ground TBMs. Large grippers are jacked 

outwards, either to each side or top and bottom, 

and hold the main part of the machine and transfer 

the applied thrust to the adjacent firm rock. Disk 

Rock condition Hp, ft Remarks 

1. Hard and intact Zero Light lining on rock bolts only if spalling or 

popping occurs 

2. Hard, stratified, or schistose 0 to 0.5B Light supports. Load may change erratically 

from point to point 

3. Massive, moderately jointed 0 to 0.25B 

4. Moderately blocky and seamy † 

0.35(B þ Ht)to 1.10(B þ Ht) 

No side pressure 

5. Very blocky and seamy 0.35(B þ H t) 

to 1.10(B þ Ht) 

6. Completely crushed, chemically 

intact † 


7. Squeezing rock, moderate depth 1.10(B þ Ht) to 

2.10(B þ Ht) 

8. Squeezing rock, great depth 2.10(B þ Ht) to 

4.50(B þ Ht) 

9. Swelling rock Up to 250ft, 

regardless of 

value (B þ Ht) 

Little or no side pressure 

1.10(B þ H t) Considerable side pressure. Requires 

continuous support of lower ends of ribs 

or circular ribs 

Heavy side pressure; invert struts required. 

Circular ribs recommended. 

Same as for Type 7 


Circular ribs. In extreme cases, use yielding 

supports 

* If depth of rock over tunnel is more than 1.5(B þ Ht), where B is width and Ht is height of tunnel. From R. V. Proctor and T. L. White, 

“Rock Tunnels and Steel Supports,” Commercial Shearing & Stamping Co., Youngstown, Ohio. 

† If roof of tunnel is permanently above the water table, values for Types 4 and 6 can be reduced by 50%. 





cutters mounted in the cutter-head face roll at high 

pressure on the exposed rock face and crush the 

rock, the tailings from which are mechanically 

removed. After an advance of up to six feet or so, 

the grippers must be retracted, moved forwards 

and jacked out again. Advance rates are very 

dependent on the hardness of the rock and its 

integrity, and the wear and tear that the rock may 

cause. In swelling rock, extra precautions must be 

taken to ensure that the machine does not get stuck. 

As the rock quality diminishes, the machine will 

need to resemble a soft ground TBM more and 

more. Rock TBMs tend to be launched from a 

mined chamber in which the whole machine can be 

assembled. 

Headings n In the past, when mucking was 

done by hand loading into mine cars, and drill 

equipment was cumbersome, excavation was 

advanced in drifts or headings. In weak rock or 

for very wide tunnels, this method is still used. A 

top heading may be advanced first. This permits 

installation of crown supports if needed. The rest is 

excavated by benching down from the top heading. 

These different levels make transportation of 

excavated material inconvenient. In wide tunnels, 

side headings may be advanced. In that case, legs 

of steel sets (supports for side walls and roof) are 

placed, where necessary. The side headings are 

followed by a top heading and erection of the arch 

supports. The remaining block can be attacked 

from the face or from the side drifts. 

A bottom heading or pilot tunnel may be used 

instead. Enlargement proceeds at several places 

along the heading simultaneously. The pilot tunnel 

has to be large enough to allow in and out traffic 

and should be timbered to protect it. 

In very long tunnels, a parallel heading, 40 ft or 

more from the tunnel axis, expedites excavation by 

providing access to several working faces through 

cross drifts. From this pilot tunnel, transverse 

headings are driven at several points to the main 

tunnel axis, from which tunnel excavation can 

proceed in both directions. The parallel heading 

carries all traffic to the different faces and serves as 

a drainage and ventilation tunnel. This method was 

used in the 12-mi Simplon tunnel, where the 

parallel drift was later enlarged to a full-sized 

single-track railroad clearance, and in the Moffat 

and New Cascade tunnels. 


A center heading may also be used in large rock 

tunnels. From it, the section is enlarged to full size 

by radial drilling. 

Full-Face Tunneling n To save time and 

labor, full-face rock excavation is used wherever 

feasible, for efficient mechanization of the operation. 

Large track or rubber-wheel-mounted jumbo 

frames carry high-speed drills. As an alternative to 

drill and blast, roadheaders are sometimes used on 

weaker rock. They are smaller excavators equipped 

with ripper or point-attack teeth mounted on 

rotating balls attached to slewing and elevating 

arms. Being more maneuverable, roadheaders can 

excavate openings of almost any shape. Mucking 

(removal of excavation) is done by large, mechanized 

loaders. Muck is carried in diesel trucks, 

where permissible, or in trains of large mine cars 

pulled by battery-powered locomotives if laws 

prohibit use of internal combustion engines. 

Excavation Limits n Contract plans prescribe 

excavation profiles. An inner A line is the 

minimum theoretical section to be excavated; to 

this is added a tolerance, usually 6 in to the B line or 

payment line. Any overbreak beyond this is at the 

contractor’s risk and has to be filled at the 

contractor’s expense. 

Blasting n Drilling pattern and blasting 

charges are governed by the rock characteristics, 

fragmentation desired for mucking, and external 

conditions, such as proximity of sensitive structures. 

The procedure should be worked out by an 

experienced blasting expert and may have to be 

modified during construction. The center group of 

holes, fired first, are drilled convergent, so that a 

conical shape is blasted. Blasting proceeds toward 

the periphery with short-time delays. A 6- or 8-indiameter 

center, or “burn” hole, without charge, 

acts as a relief opening, improving blasting effect. 

Rounds are usually about 10 ft deep but may be 

more or less, depending on the rock. Line drilling, 

a ring of straight holes, fairly closely spaced 

around the periphery, is used if as smooth a section 

as possible is desired. 

Temporary Supports n Practically all rock 

tunnels need some temporary supports. Timber 

may be used in pilot tunnels and small headings. 

For larger tunnel cross sections, steel sets are 




more economical because of their strength and 

ease of installation. These are made of I beams 

cold-rolled into shape. For small tunnels with 

circular arches, the sets may be continuous 

frames. In larger tunnels or for flat arches, the 

sets consist of separate posts and arches (Fig. 20.11). 

Where roof supports only are necessary, the 

arches may be supported on plates resting on 

rock ledges. Steel sections are usually uniform for 

the entire tunnel, and spacing of sets is varied 

according to rock loads. Normal spacing is 4 ft. 

but spacing may be reduced to 2 ft or increased to 

as much as 6 ft. 

The sets should be erected as soon as scaling of 

loose rock has been completed. Blocking should 

immediately be wedged between the steel and the 

rock surface at 3- to 5-ft intervals to prevent rock 

movement from starting. The steel frames should 

allow space at the crown, between the lower flange 

and the concrete surface, for a pipe for placing 

concrete. 

Timber or steel lagging should be placed 

between the sets. The amount of lagging depends 

on rock conditions. Lagging may be practically 

solid, or there may be gaps of various widths 

between the sheets, as required by circumstances. 

Badly fragmented rock may require metal panning 

between sets if water is present. The pans are made 


Fig. 20.10 Drill and Blast (TARP). 


of interlocking channels. The space between pans 

and rock should be dry-packed to allow water to 

run off into the drainage system. 

The concentrated loads on the sets at blocking 

points produce bending moments in the frames. 

Table 20.2 presents formulas for loads on supports 

in rock tunnels (R. V. Proctor and T. L. White, “Rock 

Tunnels and Steel Supports,” Commercial Shearing 

and Stamping Co., Youngstown, Ohio). 

Through badly faulted rock or pressure areas, 

circular tunnel sections and ring supports are 

preferable, particularly in seismic areas (Fig. 20.12). 

Rock Bolts n In good rock, but also for some 

rock that may be classified as poor, rock bolts may 

be used to secure the excavation. They are usually 

1 in in diameter and 8 ft long. They may be 

coupled, however. The bolts provide anchorage in 

sound rock, where they are held by wedges driven 

into split ends when the bolts are inserted or by 

expansion sleeves gripping the sides of the hole 

when the bolts are threaded in. The bolts are tested 

for pull-out and prestressed by nuts bearing 

against face plates on the rock surface. Untensioned 

deformed bars, bolts, or steel or glass-fiber 

tubes are used as rock reinforcement and fully 

grouted with cement or high-strength resin grout. 





Fig. 20.11 Typical cross section through the Lehigh Tunnel on the Pennsylvania Turnpike Extension. 

Half section B shows the bracing, or sets. 

They are stressed by deformation of the rock, 

which is monitored by extensometers and convergence 

measurement until equilibrium is reached. If 

necessary, additional untensioned or tensioned 

bolts are inserted. All rock bolts in permanent 

installations should be grouted as protection 

against corrosion. 

Another type of bolt has a perforated sleeve, 

which is placed in a hole in the rock and filled with 

grout. As the bolt is pushed into the hole, the grout 

is squeezed through the perforations and against 

the rock. Bond between bolt, grout, and rock 

provides the holding force. 

Shotcrete n Use of sprayed concrete (gunite or 

shotcrete) as preliminary tunnel support for rock 

tunnels was developed in Europe and has also been 

successful in North America. As soon as possible 

after blasting, while mucking is going on, a layer of 


concrete is sprayed on the roof. The concrete is 

made with a well-graded aggregate, up to 3 ⁄4-in size, 

which is frequently dry-mixed with cement and an 

accelerating agent. The mixture is ejected through a 

nozzle under pressure by special pumps. Mixing 

water is added at the nozzle. Initial set takes place 

in about 30 to 120 s, final set in 12 min. 

Also often used is a wet mix, for which 

aggregate, cement, and water are placed in the 

mixer and additive is injected as a liquid at the 

nozzle. Addition of about 5% by volume of 

microsilicate greatly improves adherence of shotcrete 

to the rock and reduces reinforcing steel 

1 1 

requirements. Addition of 

= 

2- to1 

= 

2-in steel fiber 

to a mix in the amount of 1 ⁄ 2 to 1% by weight 

considerably increases the ultimate strength and 

toughness of the shotcrete. 

Thickness of the initial layer may vary from 2 to 

4 in, depending on rock conditions. Additional 




layers may be sprayed on as needed. Total 

thickness may be as much as 8 in. 

The nozzle may be held directly by an operator 

or attached to a boom manipulated by a worker 

stationed under the protective roof of the jumbo. 

Automatic application has been successful in a 

machine-bored tunnel (Heitersberg Tunnel in 

Switzerland). Robots, controlled by an operator 

on the jumbo, can be used to apply either dry or 

wet mix shotcrete. 

Shotcrete is sprayed on the sidewalls after 

completion of mucking. Heavy water inflow must 

be intercepted and drained through inserts in the 

shotcrete. Well-trained operators and careful 

supervision and control are essential for good 

results. Properly executed, the method can be used 

successfully for fractured rock. 



Fig. 20.12 Typical section through Berkeley Hill Rock Tunnel (heavily faulted rock) for the 

San Francisco Bay Area Rapid Transit. 

Strength of concrete in place reaches 200 to 

250 psi in 2 h, 1400 to 1500 psi in 12 h. The ultimate 

compressive strength of 4000 to 5500 psi is about 

15% less than that of the same concrete without 

accelerator. 

Waterproofing n Above the groundwater 

table, waterproofing is usually applied to ceilings 

in transportation tunnels to prevent dripping. 

Drainage paths may be provided along the base 

of the walls to handle any water that does appear. 

False walls with finishes are often used to hide 

walls where leakage is expected. For most tunnels 

below the groundwater table, a waterproofing 

membrane enveloping the tunnel is used between 

the initial ground support and the final lining. If the 





tunnel lining is undrained, the final lining will 

carry the full groundwater pressure and should be 

designed accordingly. Where drainage is provided 

outside the waterproofing membrane, the final 

lining may be designed for a reduced groundwater 

pressure. Waterproofing membranes may also 

need to resist deleterious gases or liquids expected 

to be present. 

Leakage n See Art. 20.13. 

(J. O. Bickel and T. R. Kuesel, “Tunnel 

Engineering Handbook,” Van Nostrand Reinhold 

Company, New York.) 

20.14 Tunnels in Firm 

Materials 


requirements should be satisfied in underground 

excavations. (See Arts. 20.6, 20.8, and 20.12.) 

Materials, other than rock, that may be 

encountered in tunneling are sands of various 

densities and grain sizes; sands mixed with silt or 

clay; clays, either pure or containing silt or sand, 

and varying from relatively plastic with high water 

content to firm and dry; and alluvial mixtures of 

sand and gravel or glacial till. To improve the 

properties of poorer ground, or to reduce water 

infiltration, ground improvement prior to mining 

may be undertaken. This may take the form of the 

injection of cementitious or chemical grout, or it 

might physically mix soil with these materials. 

Bentonite has also been used; environmental issues 

associated with the use of the selected material 

should be considered. If not subject to hydrostatic 

pressure of free water, materials may be excavated 

by mining. Temporary support is given by timber 

or steel framing in headings whose size and 

number depend on local conditions. 

Mining of headings in all these materials 

requires the driving of poling boards, supported 

by cross timbers and posts to hold the roof. As 

excavation is advanced on a face as steep as the 

material will stand, these boards are driven further, 

with the rear supported by the frame, the front by 

the soil. A new support is set under the forward 

end of the poling boards and the process repeated. 

The sides of the heading are held by boards 

supported by the posts, as required. Figure 20.13 

illustrates the basic procedure for this type of 

excavation. 


Fig. 20.13 Timber bents support poling boards 

in basic earth mining. 

Steel supports are often used instead of timber, 

particularly for large headings. Steel lances, made 

of small wide-flange beams with wedge-shaped 

points, may be used instead of wood poling boards. 

The lances are long enough to be supported on two 

frames and driven by jacks or air hammers into the 

soft face for a distance equal to the support spacing. 

In loose soil or running sand, the face is 

supported by breast boards. A shallow slot about 

2 ft deep and one or two poling boards wide is 

excavated in the top of the face, and a short vertical 

breast board is placed immediately, to hold the face 

and support the forward end of the poling. After 

this slot has been excavated across the heading and 

all vertical breast boards set, a cap is installed, 

supported by short posts. The rest of the face may 

then be excavated downward and held by 

horizontal breast boards (see Fig. 20.14). 

The size of the heading should be as large as soil 

characteristics allow, but not less than 5 ft wide by 

7 ft high. Steel bents shaped to the tunnel arch are 

preferable to timber framing, if economical, 

considering both price and speed of operation. 

Poling may be timber or steel. 

Fig. 20.14 Mining in running ground requires 

breast boards. 




Steel liner plates are available in various shapes 

and sizes. They may be used to support the ground 

if a limited excavated area of the roof or arch will 

stand long enough for insertion of the liner plates, 

starting at the top of the arch and working down. 

The flange of each plate is bolted to the previously 

erected liner. 

In small tunnels, ribbed or corrugated liner 

plates may give adequate support. In large tunnels 

or under heavier loads, the plates are backed up by 

steel ribs, against which they are blocked. Liner 

plates without flanges may also be used as lagging 

or poling. See also Art. 20.17. 

To prevent settlement or unbalanced load, all 

voids behind the liner plates should be filled by 

injection of pea gravel or cement grout. 

Small tunnels may consist of a single heading. 

For large tunnels, various combinations of headings 

are used. Some of these are known by the 

country of their origin, as American, Austrian, 

Belgian, English, German, or Italian methods, but 

are used in many variations. Originally, the 

methods required wood supports, but now steel 

supports are favored, where economical. 

Sequential Excavation Method (SEM) n 

Also known as the New Austrian Tunneling 

Method (NATM), SEM was developed in Austria 

but is now used worldwide. It is a tunneling 

method adapted to the excavation of variable and 

non-circular cross-section reaches of tunnel, such 

as highway ramps and subway stations. This 

underground method of excavation divides the 

space (cross-section) to be excavated into segments, 

then mines the segments sequentially, one portion 

at a time. Excavation sequencing by the American 

method, Austrian system and Belgian method are 

outlined below. 

The excavation can be carried out with common 

mining methods and equipment (often a backhoe), 

chosen according to the soil conditions; tunnelboring 

machines are not used. Ground conditions 

are assessed at the face of the tunnel or from the 

side of a small tunnel, which helps to decide how to 

proceed in the best way and determines the choice 

of equipment and lining. It should be noted that the 

combination of ground treatment and SEM for the 

excavation of uniform cross-section tunnels would 

generally be more expensive than the use of 

pressurized face TBM construction under American 

underground construction labor and economic 



conditions. Thus the application of SEM would 

be limited economically to variable geometry 

structures. However for shallow tunnels, such 

structures could probably be more economically 

constructed using cut-and-cover techniques. 

SEM requires extremely dry conditions; dewatering 

is often necessary before the excavation 

can proceed. SEM involves careful sequencing of 

the excavation as well as installation of supports. 

Shotcrete (a kind of concrete sprayed from highpowered 

hoses) may be used to line the tunnel or 

support the face, and grouting (the injection of a 

cementing or chemical agent into the soil) may be 

used to increase the soil’s strength and reduce its 

permeability. Because of the requirements of this 

method, the rate of excavation is slow. Use of this 

method in saturated, non-cohesive granular soils 

would require the use of groundwater control and 

ground improvement techniques. One real concern 

with the use of SEM in granular soils is sudden 

uncontrollable ground loss, often resulting in 

surface sinkholes. This can happen when the 

selected ground improvement method is unsuccessful 

because of localized variation in ground 

conditions. 

One method often used to control groundwater 

is compressed air. However, the high air pressures 

often required might make the use of compressed 

air tunneling uneconomical in comparison to other 

possible methods. It is for similar reasons that shield 

tunneling using compressed air has been replaced 

by tunneling with pressurized face tunnel boring 

machines. Another commonly used method of 

controlling groundwater is dewatering. However, 

unrestricted dewatering can have a significant 

effect on adjacent foundations. An approach that 

has been used with variable success overseas has 

been to install groundwater cut-off walls (slurry 

walls, etc.) along both sides of the right-of-way and 

then dewatering inside the cut-off. When dewatering 

sands, running or fast-raveling ground, conditions 

may result so that some form of ground 

improvement, such as closely spaced groutable 

spiles or horizontal jet grouting above the crown of 

the excavation could be required. Two other ground 

improvement methods that could be used are jet 

grouting and chemical grouting. Each method 

would be used to create a block of stabilized ground 

through which the tunnels could be excavated. 

American Method n As shown in Fig. 20.15a, 

excavation starts with (1) a top heading at the 





tunnel crown, which is supported by poling, posts, 

and caps. Next, the excavation is widened between 

two bents and the top arch segments adjoining the 

crown are set, supported by extra posts or struts. 

(2) The excavation then is benched down along the 

sides, and another segment of ribs is set on each 

side. (3) and (4) These are doweled to the upper 

part and supported by temporary sills. This 

process is repeated to the invert sill. The bench 

finally is excavated to full section. (5) Ground 

between ribs is held by lagging, and voids are 

packed. This method is suitable in reasonably firm 

material. 

Austrian System n As shown in Fig. 20.15b,a 

full-height center heading is advanced. It either 

starts with a top heading and is cut down to the 

invert in short lengths or starts as separate bottom 

and top headings. (1) and (2) In the latter case, the 

core between the two is excavated for short 

distances, and the short posts replaced with long 

ones. (3) The arch section is widened in short 

lengths and is held by segmental arch ribs and 

longitudinal poling boards. (4) The arch ribs are 

supported by struts from the center-cut framing 

and sills at the spring line. The rest of the excavation 

is advanced to full face in short increments, and 

posts are set to support the sills. (5) This method is 

suitable for reasonably stable soil. 

Belgian Method n As shown in Fig. 20.15c, in 

firm ground, the upper half of the tunnel is 

excavated, starting with a center heading from the 

crown to the spring line. (1) This is widened to both 

sides, the ground being held by transverse polings. 


Fig. 20.15 Some excavation procedures for large tunnels: (a) American method. (b) Austrian method. 

(c) Belgian method. 

These are supported by longitudinal timbers, in 

turn supported by struts extending fanlike from a 

sill in the center heading. (2) Next, a center cut is 

excavated to the invert (3), leaving benches to 

support the arch of the tunnel lining. Slots are cut 

into the benches at intervals to underpin the arches. 

The rest of the bench then is removed to complete 

the side walls (4), after which the invert is concreted. 

The excavation may be advanced a considerable 

distance before the tunnel lining need be inserted. 




20.15 Shield Tunnel in 

Free Air 

This section describes shield tunneling where the 

face is essentially open and exposed at ambient air 

pressure, and Section 20.16 when exposed under 

compressed air. Both these methods are less 

common today than tunneling by the tunnel boring 

machines (TBM) described in Section 20.19, now 

widely used. 

Shield tunneling is generally used in noncohesive, 

soft ground composed of loose sand, gravel, 

or silt and in all types of clay, or in mixtures of any 

of these. It is indispensable for tunneling in these 

materials below the water table. 

Requirements of the Occupational Safety and 

Health Administration (OSHA) for underground 

construction should be complied with in operations 

with shields. OSHA requires the following, 

in particular: Lateral or other hazardous movement 

of a shield when subjected to a sudden lateral load 




should be restricted. Personnel accessing shielded 

areas should be protected against cave-ins. Personnel 

should not be permitted in shields when they 

are being installed, removed, or moved vertically. 

Excavations may extend up to 2 ft below a shield 

bottom, if the shield is designed to resist the forces 

from the full depth of the trench and if soil will not 

be lost from behind or below the bottom of the 

shield. (See also Arts. 20.6 and 20.8.) 

The shield is a cylinder made of welded steel 

plate (Fig. 20.16). It has a diameter slightly larger 

than the outside of the tunnel lining. The plate is 

stiffened by two interior ring girders, the first one 

installed a short distance behind the cutting edge. 



Depending on the diameter and loads, the 

girders are braced by horizontal and vertical steel 

struts. The cutting edge is beveled and reinforced 

by welded steel plates to a thickness of up to 3 or 

4 in. For loose ground, the upper half of the 

shield is extended forward 12 to 18 in to form a 

protective hood. 

The tail of the shield overlaps slightly the end of 

the finished lining and provides space for at least 

one liner ring, and for underwater tunnels is 

usually long enough to accommodate two rings. 

The inside of the tail clears the lining by about 1 in 

all around. For working in soft clay, the front of the 

shield may be closed by a steel bulkhead with door- 

Fig. 20.16 Longitudinal section through a shield used for tunneling through soft ground in free air. 





equipped openings through which material is 

excavated. Soft clay may be extruded through the 

openings while the shield advances. 

Working platforms that can be advanced and 

retracted by hydraulic jacks are mounted on the 

shield bracing (Fig. 20.16). They give access to all 

parts of the face and, by keeping in contact with it, 

support it if necessary during shoving. Additional 

breasting jacks can be mounted in the bracing to 

hold breast boards against the face if it needs 

extensive support. 

Shield Advancement n Hydraulic jacks (Fig. 

20.16) for advancing the shield are set on the webs 

of the ring girders close to the periphery of the 

shield. The shove jacks are evenly spaced around 

the perimeter and exert pressure against the 

forward ring girder, which is stiffened by brackets 

welded to the skin of the cutting edge. Jack 

plungers are equipped with shoes bearing against 

the tunnel lining. The stroke of the jacks is slightly 

more than the width of a liner ring. 

A rotating erector arm is mounted inside the tail 

to pick up and place liner segments. Hydraulic 

pumps mounted behind the shield supply 5000- to 

6000-psi pressure to the jacks, erector arm motor, 

and other hydraulically operated equipment. 

Control valves for these devices are mounted on a 

panel in the shield. 

The method of operation, excavation, and speed 

of advance vary greatly according to the type of 

soil. In sand and gravel, the face usually has to be 

held by breast boards (Fig. 20.16), which are braced 

by telescoping struts, breasting jacks, or the 

working platforms. The breasting may have to be 

carried down to the invert of the face, which is 

excavated to the cutting edge of the hood. If 

compressed air is used, the breasting may be 

carried part way down to where the air pressure 

balances the hydrostatic head, the lower part of the 

face taking its natural slope. In firm materials, silty 

sand, or stiff clay, the full face may be excavated 

without breasting. Average progress for the 31-ftdiameter 

Queens Midtown Tunnel, New York City, 

in these materials was between 7 and 8 ft in 24 h. 

Shields are not well-suited for rock tunneling, 

but rock or mixed faces, partly rock and partly soil, 

may be encountered in parts of soft-ground 

tunnels. If the rock is high enough, a bottom 

heading may be excavated ahead of the shield and 

a concrete cradle placed, with steel rails embedded, 


to exact line and grade to support the shield as it 

advances. A similar bottom heading may be used 

in a full rock face if the full cross section cannot be 

excavated. Then, the rock may be blasted around 

the periphery of the rest of the cutting edge to 

permit advancing the shield. Progress in mixed 

face in the Queens Midtown Tunnel averaged 

about 3 to 4 ft per 24-h day. 

Best progress is made in plastic material 

through which the shield may be shoved blind, 

that is, without taking any soil into the inside, the 

volume being displaced by compressing or 

heaving of the surrounding material. To counteract 

the tendency of the tunnel to heave behind the 

shield, because of buoyancy, enough soil may be 

admitted through small openings in the face 

bulkhead and left in the invert to balance the 

forces until the interior lining is placed. This 

method is called shoving half-blind. In the first 

tube of the Lincoln Tunnel, New York City, about 

20% of the material was taken in. If displacement 

or heaving of the soil may cause disturbance of 

adjacent structures, such as buildings or another 

tube nearby already in place, the openings should 

be adjusted to admit nearly all the displaced 

material. This was done in the second and third 

tubes of the Lincoln Tunnel, through openings 

aggregating 5 to 20% of the face area. Average 

daily progress was about 30 ft. 

Shields are usually started from shafts sunk to 

the invert grade. These shafts may be specially 

constructed for this or may later be part of a 

ventilation building. An opening is provided in the 

shaft wall to fit the shield and is closed by a timber 

bulkhead during sinking operations. The shield is 

erected on a concrete cradle at the base of the shaft. 

The opposite shaft wall forms the abutment for the 

jacking forces. A few rings are erected behind the 

shield, which is advanced through the opening 

after removal of the bulkhead. 

The shield is steered by varying the pressure of 

the shoving jacks around the periphery. On large 

tunnels, the total jacking force may be 3000 to 

5000 tons. If the shield has a tendency to rise, more 

pressure is applied at the top than at the bottom. 

Similar corrections are made for other directions. 

If the soil is relatively loose, it is excavated at the 

face by hand tools. In hard-packed silty sands or 

very stiff clay, air spades are used. Relatively soft 

clay may be cut by clay knives. The muck may be 

shoveled by hand on a short conveyor in the shield, 

but more commonly, a large hydraulic scraper or 




hoe is used to loosen the soil and scoop it onto the 

conveyor. From there, it is discharged on a loading 

conveyor mounted on a movable carriage behind 

the shield. The loading conveyor dumps it into 

mine cars, usually of about 4-yd 3 capacity in large 

tunnels. The muck trains are rolled back through 

the tunnels to an access shaft. The individual cars 

are hoisted up the access shaft and dumped into 

hoppers for discharging into trucks. 

Tunnel Linings n Except in very stiff or 

compact soils, segmental ring liners are used in 

shield tunnels. These used to be of cast iron but 

today steel or precast concrete is used. The 

segments are brought in by mine cars, unloaded 

by hoists mounted on the conveyor carriage, and 

deposited within reach of the erector arm. This is a 

telescoping, counterweighted arm pivoted on the 

center line of the tunnel for full rotation by a 

hydraulic motor (Fig. 20.17). A gripper at its 

outer end engages lugs or bars in the segments 

and places these, starting at the bottom. A 

short, tapered segment forms the key. See also 

Art. 20.17. 



Packing n Since the shield has a larger 

diameter than the lining, a void exists around the 

liner rings. This may permit a cave-in and cause 

settlement. The usual practice when segmental 

liners are used is to inject pea gravel into this void 

through grout holes in the liners immediately after 

the shield has been advanced (Fig. 20.16). Cement 

grout is later injected into the gravel to solidify it. In 

a section of the Victoria line of the London subway 

in deep, very stiff clay, an articulated cast-iron 

lining was installed and expanded against the clay 

behind the shield. The adjacent rings were pressed 

into contact by the jacking forces but were 

not bolted. Expansion of steel ribs with wood 

lagging has also been used to achieve tight fit 

against the soil. 

Semicircular or semielliptical shields have 

been used as temporary supports for the roof or 

arch of excavations, mostly in dry or dewatered 

soils, for example, for tunnels at shallow depth 

where open-cut operations are prohibited by 

circumstances. They are advanced in a manner 

similar to that for circular shields. 




Fig. 20.17 Section through a conventional shield (used in 1930 for the Detroit, Mich.—Windsor, Ont., 

Tunnel) for tunneling with compressed air. 





20.16 Compressed-Air 

Tunneling 

Although tunnel shields in free air are effective in 

naturally dry soil or ground that can be dewatered 

(Art. 20.15), compressed air is needed while 

tunneling below the water table, particularly in 

subaqueous tunnels. The air pressure counteracts 

the hydrostatic head. Also, the pressure reduces the 

water content of the soil at the face, making it more 

stable and safer to excavate. 

Legal issues surrounding safety and health 

issues of compressed air tunneling have reduced 

use of this method. It still has to be used from time 

to time to remove obstructions in front of tunnel 

boring machines that they cannot handle, and to 

carry out maintenance and repairs on some of the 

machines. 

Air Pressure n Theoretically, the air pressure 

required to balance the hydrostatic head is 0.43 psi 

for fresh water and 0.44 psi for seawater per foot of 

depth. Actually, the pressure depends on the 

properties of the soil as well as on the method of 

excavation. In open material, such as pervious 

coarse sand and gravel, the full air pressure would 

be required, whereas in impervious soils, such as 

stiff clay, no pressure at all may be needed. A 

careful analysis of the soil at regular intervals along 

the alignment is needed to estimate the maximum 

air pressure and air quantities required. Closed 

shields for tunnels in the Hudson River silt 

operated with as little as 16-psi air pressure in 

depths up to 100 ft. In the sand and gravel under 

the East River in New York, the hydrostatic head 

was balanced for about one-quarter or one-third 

the diameter above the invert. To reduce loss of air 

at the top of the face, breast boards were plastered 

with clay. 

Blowout Prevention n With the air pressure 

balancing the head at the bottom of the face, there is 

an excess of pressure at the top. If the weight of 

cover over the tunnel is insufficient to hold the 

excess air pressure safely, a heavy clay blanket may 

be placed on the river bottom over the tunnel 

heading to prevent a blowout at the top of the face. 

If the air pressure equals the water pressure at the 

invert, the excess pressure at the top of a 30-ftdiameter 

face would be 13 psi in seawater, or 

1870 lb/ft 2 . For a 10-ft natural cover of 50-lb/ft 3 


material, the blanket would have to make up the 

deficiency of 1370 lb/ft 2 . At 60 lb/ft 3 submerged 

weight, 23 ft of clay would be required. Navigation 

requirements may make it necessary to remove the 

blanket after completion of the tunnel. Clay for this 

blanket should be relatively soft so that it will 

readily coalesce into an impervious layer. 

Bulkheads n When the shield is started from a 

shaft, an airtight deck is built above the tunnel, to 

hold the pressure until the shield is advanced some 

distance. An airtight bulkhead is then built into the 

tunnel a sufficient distance behind the shield to 

provide space for the loading conveyor and a few 

mine cars. To keep the volume filled with 

compressed air within reasonable limits and to 

comply with safety laws, new bulkheads are built 

as the tunnel advances and the old bulkheads are 

removed. Usually, regulations permit a maximum 

distance of 1000 ft between the face and the 

bulkhead, which may be constructed of steel or 

concrete. 

Air Locks n Worker and material locks are 

built into the airtight bulkhead. The locks are 

airtight cylinders at least 5 ft in diameter and have 

gasketed doors (Fig. 20.17). Worker locks should 

provide at least 30 ft 3 of air space per occupant. 

Compressed air is admitted to the lock from the 

high-pressure side or from the compressed-air line 

and is exhausted through a connection to the freeair 

side. Valves of these connections are controlled 

from the inside in worker locks and generally from 

the outside in material locks. The door at the highpressure 

side opens from the lock into the tunnel; 

the door at the free-air end opens into the lock 

chamber. Doors are held tight by the air pressure 

and cannot be opened until pressures on both sides 

are equalized. Pressure gages are provided in the 

locks as well as in the tunnel. 

The material lock is at the level of the mine-car 

track. The lock should be large enough to 

accommodate several mine cars. 

The worker lock is at a higher level and must 

have not less than 5 ft clear head space. This lock is 

equipped with benches for workers to sit down on. 

In large tunnels, two sets of locks may be used to 

speed up operations. If there is danger of rapid 

flooding, an extra worker lock may be placed as 

high as possible, and a hanging safety walk 

extended at this level from the lock to the shield. 




A safety screen placed in the upper part of the 

tunnel near the heading will trap air above this 

safety walk in case of flooding and permit workers 

to escape. Some safety laws require installation of 

two worker locks. 

A special decompression chamber capable of 

accommodating an entire shift of workers should 

be available when decompression time required is 

more than 75 min. A passageway should be 

provided to give workers in a man lock access to 

the special chamber. 

Safety and Health n For all compressed-air 

work, a well-equipped first-aid station and 

decompression chamber are required, staffed by a 

trained attendant at all times. A physician must be 

available at all times for emergency calls while 

work is in progress. 

Most states and countries have laws regulating 

the working hours and locking rates for compressed-air 

work. Regulations of the U.S. Occupational 

Safety and Health Administration for 

work in compressed air as well as for construction 

in general and all underground operations should 

be observed. (See also Arts. 20.6, 20.8, and 20.15.) 

OSHA requires that a record be kept outside 

worker locks of the time in each shift that workers 

spend in compression and decompression. A copy 

should be given to the supervising physician. 

During the first minute of compression in a lock, 

pressure may be increased up to 3 psig and should 

be held at that level, and again at 7 psig, long 

enough to determine if anyone in the lock is being 

adversely affected. After the first minute, pressure 

may be raised gradually at a rate up to 10 psi/min. 

If personnel experience discomfort, pressure 

should be reduced to atmospheric and distressed 

personnel should be evacuated from the lock. 

Except in emergencies, pressure in a lock should 

not exceed 50 psi. Temperature in a lock should be 

at least 70 8F but not more than 90 8F., whereas 

temperature in compressed-air working areas 

should not exceed 85 8F. 

Unless air pressure in the working chamber is 

less than 12 psig, decompression in a worker lock 

should be automatic. Manual controls, however, 

should be provided inside and outside the lock to 

override the automatic mechanism in emergencies. 

The lock should have a window at least 4 in in 

diameter to permit observation of the occupants 



from the working chamber and free-air side of 

the lock. 

OSHA requires also that at least 30 ft 3 /min of 

ventilation air be supplied per worker in the 

working chamber. In addition, OSHA specifies that 

at least 10 ft-c be provided by electric lights on 

walkways, ladders, stairways, or working levels. 

Two independent supply sources should be used, 

including an emergency source that becomes 

operative if the regular source should fail. External 

parts of electrical equipment, including lighting 

fixtures, when installed within 8 ft of the floor, 

should be made of grounded metal or noncombustible, 

nonabsorptive, insulating material. 

OSHA also requires that sanitary, comfortable 

dressing and drying rooms be provided for 

workers employed in compressed air. Facilities 

should include at least one shower for every 10 

workers and at least one toilet for every 15 workers. 

Fire-fighting equipment should be available at all 

times in working chambers and worker locks. 

OSHA requirements for total decompression 

time, which depends on the air pressure in the 

working chamber and the time of worker exposure 

to that pressure, are listed in Table 20.3. Decompression 

should take place in two or more stages, 

but not more than four. (Four stages are required 

for pressures of 40 psig or more.) In Stage 1, 

pressure may be reduced at a rate up to 5 psi/min 

from 10 to 16 psi, but not to less than 4 psig. In later 

stages, the rate of pressure reduction may not 

exceed 1 psi/min. Local rules, however, also 

should be checked. Limits of union contracts, 

though, are sometimes more stringent than legal 

requirements. 

The amount of air required for compressed-air 

tunneling depends on so many variables that exact 

rules cannot be given. To determine the size of the 

compressor plant for a given job requires a great 

deal of judgment by the engineer, based on past 

experience. Low-pressure machines are installed 

for the tunnel air and high-pressure units for air 

tools. Adequate standby capacity must be provided 

by using a number of compressors. High-pressure 

air may be used as an emergency tunnel supply by 

interconnecting the compressors through reducing 

valves. 

Shieldless Tunneling n Some tunnels have 

been built in water-bearing ground by using 

compressed air in conjunction with liner plates, 





Table 20.3 Total Decompression Time, Min, after Construction Work in Compressed Air* 

Working- 

Working period, h 

chamber, 

psig 

1 

⁄2 1 1 1 ⁄ 2 2 3 4 5 6 7 8 Over 8 

9 to 12 3 3 3 3 3 3 3 3 3 3 3 

14 6 6 6 6 6 6 6 6 16 16 33 

16 7 7 7 7 7 7 17 33 48 48 62 

18 7 7 7 8 11 17 48 63 63 73 87 

20 7 7 8 15 15 43 63 73 83 103 113 

22 9 9 16 24 38 68 93 103 113 128 133 

24 11 12 23 27 52 92 117 122 127 137 151 

26 13 14 29 34 69 104 126 141 142 142 163 

28 15 23 31 41 98 127 143 153 153 165 183 

30 17 28 38 62 105 143 165 168 178 188 204 

32 19 35 43 85 126 163 178 193 203 213 226 

34 21 39 58 98 151 178 195 218 223 233 248 

36 24 44 63 113 170 198 223 233 243 253 273 

38 28 49 73 128 178 203 223 238 253 263 278 

40 31 49 84 143 183 213 233 248 258 278 288 

42 37 56 102 144 189 215 245 260 263 268 293 

44 43 64 118 154 199 234 254 264 269 269 293 

46 44 74 139 171 214 244 269 274 289 299 318 

48 51 89 144 189 229 269 299 309 319 319 

50 58 94 164 209 249 279 309 329 

without a shield. Considerable lengths of 7- to 12ft-diameter 

interceptor sewers in New York were 

constructed this way. A few miles of Chicago 

subway were built in soft clay with steel ribs and 

liner plates under compressed air. 

[J. O. Bickel and T. R. Kuesel, “Tunnel 


Company, New York; “Construction Industry: 

OSHA Safety and Health Standards (29 CFR 

1926/1910,” Superintendent of Documents, 

Government Printing Office, Washington, DC 

20402 (www.gpo.gov).] 

20.17 Tunnel Linings 

Unlined Tunnels n Tunnels in very sound 

rock, not affected by exposure to air, humidity, or 

freezing, and where appearance is immaterial, are 

left unlined. This is the case with many railroad 

tunnels. 


* For normal conditions, as specified by the Occupational Safety and Health Administration in “Construction Industry: OSHA 

Safety and Health Standards (29CFR 1926/1910),” revised 1991. 

Unlined water tunnels in rock are susceptible to 

leakage either into or out of the tunnel, depending 

upon the relative pressures. There is therefore a 

risk that material could be washed out of weak 

zones and fissures, potentially leading to instability 

unless lined. However, Norwegian hydropower 

tunnels in good crystalline rock are often unlined 

for most of their length. 

Shotcrete Lining n Where rock is structurally 

sound but may deteriorate through contact with 

water or atmospheric conditions, it can be 

protected by coating with sprayed concrete, 

reinforced with wire fabric or fibers, or unreinforced 

(Art. 20.13). Such a lining may also be used 

in water tunnels in good rock to provide a smooth 

surface, reducing the friction factor and turbulence. 

Cast-in-Place Concrete n Most tunnels in 

rock, and all tunnels in softer ground, require a 

solid lining. Highway tunnels of any importance 

are always lined for appearance and better lighting 




conditions. Stone or brick masonry has been used to 

a great extent in the past, but currently concrete is 

preferred. The thickness of the permanent concrete 

lining is determined by the size of the tunnel, 

loading conditions, and the minimum required to 

embed the steel ribs of any primary lining. 

The lining is placed in sections 20 to 30 ft long. 

Segmental steel forms are universally used and 

must be properly braced to support the weight of 

the fresh concrete. The walls are usually concreted 

first, up to the spring line. Next come the arch 

pours. It is important that the space between the 

forms and the rock or soil surface be completely 

filled. Grout pipes should be inserted in the arch 

concrete to permit filling any voids with sand-andcement 

grout. 

Concrete is placed through ports in the steel 

lining or pumped through a pipe introduced in the 

crown, a so-called slick line. Placement starts at the 

back of the pour, and the pipe is withdrawn slowly. 

A combination of both methods may be used. 

Concrete is either pumped or injected by slugs of 

compressed air. Admixtures are added to get an 

easily placed mix with low water content and to 

reduce concrete shrinkage. If there is leakage 

of water, it usually occurs at shrinkage cracks, 

which may be sealed with a plastic compound. Or 

the water may be carried off by copper drainage 

channels installed in chases cut in the concrete 

(Art. 20.9). 

Footings for side walls in rock tunnels are cut 

into the rock below grade. They give adequate 

stability unless squeezing ground is encountered, 

in which case a concrete invert lining is placed. In 

soft ground, a concrete slab is placed, to serve as 

pavement in highway tunnels. If heavy side 

pressure exists, this slab may have to be made 

heavier to prevent buckling. 

Unreinforced Concrete Lining n A concrete 

lining is placed to protect the rock and 

provide a smooth interior surface. Where the 

concrete lining is exposed to compression stresses 

only, it may be unreinforced. Most shafts not 

subject to internal pressure are lined with 

unreinforced concrete. Shrinkage and temperature 

cracks are probable and may cause leakage. Where 

there is a risk of non-uniform loading, unreinforced 

liners are not used, such as in squeezing ground 

and through soil overburden. 



(Recommendations in Respect of the Use of 

Plain Concrete in Tunnels, AFTES c/o SNCF,17 

Rue d’Amsterdam, F75008 Paris, France.) 

Reinforced Concrete Lining n In most 

cases, reinforcing steel will be required to withstand 

tension and bending stresses. Reinforcement 

is usually required at least on the inside face to 

resist temperature stresses and shrinkage, although 

reinforcement elsewhere may be needed to resist 

moments. 

Linings for Shield Tunnels n Linings for 

shield tunnels may be one-pass or two-pass. A onepass 

lining system is when the final lining is also 

the initial lining, usually for tunnels in soil. With a 

two-pass lining system, an initial lining is installed 

behind the shield just sufficient to allow the shield 

to advance while a waterproofing membrane is 

installed and the final cast-in-place reinforced 

concrete lining is prepared. The advance rate is 

thus usually faster and costs fall. The initial lining 

may be segmental rings with minimal bolting for 

ease of erection (Fig. 20.18), or steel ribs with 

lagging. Precast concrete segments are now widely 

used and the use of cast iron and fabricated steel 

are rare due to their high cost. Although the initial 

lining may be designed as part of the final lining, 

any leakage through the seals would result in the 

full hydrostatic pressure acting on the inside final 

lining for which it should be designed. 

Pipe in Tunnel n Water and sewer tunnels up 

to 14 ft diameter are often provided with an 

internal pipe that forms the inner lining. After the 

pipe is secured against movement, the space 

between the initial ground support and the pipe 

is filled with cellular or mass concrete. Sewer pipes 

may require a further interior lining to protect 

against corrosive liquids and gases. Water tunnels 

with a high internal pressure exceeding the 

expected external pressures are usually provided 

with a steel lining if a reinforced concrete lining is 

insufficiently strong. Since the pipe may be 

dewatered, it must also be designed for the external 

pressure, which, if the pipe has leaked, may equal 

the internal pressure. 

(U.S. Army Corps of Engineers Manual, 1997, 

Design of Tunnels and Shafts in Rock, EM 1110- 

2-2901.) 





Fig. 20.18 Cross section shows typical segmental cast-iron lining for a tunnel (Lincoln Tunnel under 

the Hudson River). 

In stiff soils, steel ribs, usually 4-in H beams, and 

wood lagging may be used as primary lining. The 

ribs are usually spaced 4 ft c to c and are erected in 

the tail of the shield. Precut and dressed wood 

lagging is placed solidly around the circumference 

between the flanges of the ribs. This lagging also 

transfers the jacking forces to the tunnel lining. 

Precast-concrete lagging has also been used 

successfully. 

Segments are made as long as convenient 

handling permits, usually 6 to 7 ft. The width of 

the rings depends on the distance the face can be 

safely excavated ahead of the shield and weight to 

be handled. The wider the rings, the longer the tail 

of the shield and hence the more difficult the 

steering of the shield. Early tunnels had 18-in-wide 

rings. Recent tunnels have gone to 30 or 32 in. 


Segments are made to close tolerances on all 

sides. They are connected by high-strength bolts. 

Longitudinal joints are offset in successive rings. 

The flanges have recesses along their matching 

edges for calking. These grooves used to be filled 

with lead or impregnated asbestos calking strips, 

pounded in manually. Synthetic sealers, such as 

silicone rubber and polysulfides, can be injected 

into the grooves by calking guns. These compounds 

adhere to the metal sufficiently to form an effective 

seal under pressures usually encountered in 

underwater tunnels. 

Each cast-iron segment is provided with a 2-in 

grout plug for injection of pea gravel and grout into 

the space between the lining and the soil. Bolt holes 

are sealed with grommets of impregnated fabric or 

plastic grommets, the latter being particularly 




effective. Bolts are tightened with hydraulic or 

pneumatic wrenches where possible otherwise 

with hand wrenches. 

Welded steel segments, similar in shape to castiron 

segments, have been used for economic 

reasons in some subaqueous tunnels. They were 

welded in jigs to tolerances as close as practicable, 

but flanges were not machined and no calking 

grooves were provided. Difficulties were experienced 

in making them watertight with gaskets. 

An improved design includes calking grooves 

and fabrication tolerances similar to those for 

cast iron. 

Precast Concrete n Precast segments are 

essential to increasing the speed of machine 

tunneling. A compromise must be reached between 

the segment size and the number of segments to be 

installed, directly affecting the weight of the 

segments, the size of the equipment needed to 

handle the segments, and the number of operations 

to be carried out. The width of the segments is 

governed by the stroke of the jacks pushing the 

head of the shield, usually in the range of three to 

five feet. Tapered rings, narrower on one side, are 

used on bends. At least three segments per ring are 

required, with five to eight being more common. 

The closing segment in a ring is usually smaller and 

wedge shaped to facilitate insertion. Joints in 

adjacent rings are usually staggered so that all 

joints are discontinuous, helping to stiffen the 

rings. 

Connection details to adjacent segments vary 

widely and can be flanged (Fig. 20.19). Straight 

bolts with nuts, washers and grommets are the 

most common, but the use of curved recessed bolts 

result in smaller pockets. Gaining popularity are 

straight bolts placed at an angle to minimize 

recesses; the bolts couple into sockets cast into the 

adjacent section. Dowels may also be used between 

adjacent rings. The bolts ensure that the rubber or 

neoprene seals between segments are compressed. 

The addition of a hydrophilic seal near the outside 

face may reduce leakage even further. Due to the 

very close tolerances needed to ensure seals remain 

watertight and that the diameter remains constant, 

a high degree of mechanization with steel forms is 

used. The segments must be installed within the 

shield tail and the space behind them (the tail void) 

grouted at a pressure at least equal to the external 

pressure, making lateral alignment modifications 



very difficult. It is not uncommon for most bolts to 

be retrieved once the grout is set. Secondary linings 

are not essential. 

Heavy, interlocking concrete blocks have been 

used successfully in relatively dry or impervious 

soil. They present difficulties when exposed to 

water pressure due to leakage. 

Except where steel rings and lagging or concrete 

blocks are used as primary lining, no secondary 

concrete lining is used, unless required for 

appearance and interior finish of highway tunnels. 

In this case, a concrete lining of the minimum 

thickness practicable is placed. When the tunnel is 

to be faced with tile, provision should be made for 

attaching it. (To facilitate maintenance and improve 

lighting, walls and ceilings of highway tunnels are 

usually finished with ceramic tiles.) To provide 

good adherence of the scratch coat, scoring wires 

may be welded longitudinally on the steel forms 

for the lining to provide a rough concrete surface. 

Coating of smooth concrete surfaces with epoxy 

compound may result in satisfactory finishes at 

less cost. 

See also Art. 20.18. 

20.18 Design of Tunnel 

Linings 

Article 20.17 discusses the types of linings usually 

used for tunnels. The following paragraphs 

describe design of a liner ring. 

A liner ring is statically indeterminate. A onepass 

lining is designed for transport and erection 

loads, loads during grouting, and ground loads 

including seismic. In lieu of computer analyses, 

which might be as simple as a two-dimensional 

analysis of a grid framework supported on springs, 

or as complex as finite element or finite difference 

three-dimensional analyses using soil-structure 

interaction for each step of the construction 

process, stresses in the liner ring may be computed 

after the ring is made statically determinate by a 

cut at the top and one end is fixed (Fig. 20.20). 

For a circular ring of constant cross section 

symmetrically loaded the thrust at the crown C is 

Tc ¼ 2 

pR 

ð p 

0 

M cos f df (20:2) 






Fig. 20.19 Typical liner segments used for rapid-transit tunnels. 




Fig. 20.20 Stresses in liner ring may be 

computed by assuming it cut at crown C. 

The vertical shear at the crown is zero, and the 

moment is 

ðp 1 

Mc ¼ RTc Mdf (20:3) 

p 0 

where R ¼ radius of ring 

M ¼ bending moment at any point U due to 

loads on CU 

f ¼ angle between U and crown C 

With the thrust and moment at the crown known, 

the stresses at any point on the ring can be 

computed, as for an arch (Art. 6.71). 

(A set of equations is presented in Chapter 15B 

‘‘Tunnel Structures, Structural Engineering Handbook,’’ 

2000 Update for ENGnetBASE, Edited by 

Wai-Fah Chen and Lian Duan, CRC Press, 2000 

(www.crcpress.com).) 

Loads on a lining include its own weight and 

internal loads, weight of soil above the tunnel 

(submerged soil for tunnels below water level), 

reaction due to vertical loads, uniform horizontal 

pressure due to soil and water above the crown, 

and triangular horizontal pressure due to soil and 

water below the crown. 

Magnitude of loads on tunnel liners depends on 

types of soil, depth below surface, loads from 

adjacent foundations, and surface loads. These will 

require careful analysis, in which observations 

made on previous tunnels in similar materials will 

be most helpful. 

In rock, the quality of the rock will affect the 

loads that are carried by the tunnel, and loads 

carried by any initial rock support may effect the 



loads carried by the secondary lining. Compression 

of competent rock due to outward displacement of 

the tunnel lining in a pressure tunnel may also 

need to be considered. Often those linings must be 

designed to take the full internal pressure. Beyond 

100 psi internal pressure, reinforced concrete liners 

may no longer be sufficient and steel liners may be 

needed. If a tunnel is watertight, the interior lining 

is usually designed to carry at least the full external 

water pressure, since leaks in any outer linings will 

eventually lead to full transfer of the hydraulic 

head. If the tunnel is drained, at least some of the 

hydrostatic head should be considered. Blasting 

may also disturb the rock locally, leading to loads 

different to those of a bored tunnel. 

Following the derivation of moments, axial 

thrust and shears, the concrete cross section can be 

designed accordingly, and steel or fiber reinforcement 

placed accordingly as needed. Tension cracks 

in themselves do not necessarily result in failure, 

whereas through-cracks (often caused by shrinkage) 

can cause leakage and corrode exposed steel. It 

is usually undesirable for cracks to extend more 

than halfway through the section. Typical steel 

reinforcement for crack control may reach 0.28% or 

more of the section area. Restraints at the exterior 

face due to keying into an irregular rock surface 

may change the calculated behavior. Linings with 

irregular width are more likely to crack at the 

thinnest sections or at initial ground support 

embedments. Waterstops are used at construction 

joints to reduce leakage. 

Because of flexibility, tunnel liner rings can offer 

only limited resistance to bending produced by 

unbalanced vertical and horizontal forces. The 

lining and soil will distort together until a state of 

equilibrium is obtained. If the deflection, in, 

exceeds more than 1.5D/10, where D is the tunnel 

diameter, ft, the lining may have to be temporarily 

braced with tie rods when it leaves the shield until 

the final loading conditions and passive pressures 

have been developed. In certain soft materials, 

when shields were shoved blind (without material 

being excavated), initial horizontal pressures 

exceeded the vertical loads, so that the vertical 

diameter lengthened temporarily. Ultimately, the 

section reverted to approximately its initial circular 

configuration. 

When a lining is in rock, determination of the 

loads imposed on the lining need to be done with 

care. Stable rock may distribute the stresses around 

the tunnel, and if impervious, may leave any 





tunnel lining virtually unloaded. Following excavation, 

rock that has not yet reached stability can 

still be moving, extreme examples of which are 

squeezing and swelling rock. Further displacements 

may be little affected by the presence of the 

lining in such cases, or may depend upon the 

relative stiffnesses of the two, so that the lining 

must be designed accordingly. Concrete cast 

against irregular rock may also be keyed into the 

rock and result in composite action. If the rock is 

anisotropic, material properties and movements 

may depend upon direction. Some external loads, 

such as groundwater pressure and some clays, are 

independent of displacements. Depending upon 

the porosity of the surrounding material, water 

pressure can eventually build up to the full 

hydrostatic load even in a rock tunnel. 

Potable water supply tunnels may need to be 

made watertight when passing though areas where 

the inflow of groundwater is not acceptable. Where 

groundwater contains fine silt or chemicals that 

could clog drainage facilities on which the tunnel 

design is based, regular maintenance is required to 

keep drainage paths clear, or else new drainage 

paths must be provided or the tunnel designed as 

watertight. Sewage tunnels frequently generate 

hydrogen sulfide and so require extra protection 

against corrosion, such as using an internal PVC or 

HDPE membrane cast into the internal tunnel 

lining. 

Precast Lining Segments n Analysis and 

design must cover all aspects of fabrication, 

storage, transportation, installation, jacking loads, 

and expected loads in service. Allowance for creep 

and shrinkage during all stages is required. It is not 

uncommon for segments to be installed slightly 

askew, resulting in extra bolting forces and nonuniform 

loads. Curved bolts although easier to 

install, require extra reinforcement. Attention to 

reinforcement details at corners can help to reduce 

damage. Compressed gaskets tend to create tensile 

stresses that may require reinforcement. Durability 

of the tunnel lining is highly desirable, and may be 

enhanced by using low permeability crack-free 

concrete, the use of pozzolans to resist sulfate 

attack and microsilica to improve strength, protecting 

the bolts and reinforcement against corrosion 

such as by applying epoxy coating to the fabricated 

reinforcement, and by the use of external waterproofing, 

including quality grout and a bitumen 

coating to the exterior face. 


[Kuesel, T. R., Tunnel Stabilization and Lining, 

in ‘‘Tunnel Engineering Handbook,’’ Bickel, J. O., 

Kuesel, T. R., and King E. H., Editors, Chapman & 

Hall, 1996 (www.wkap.nl)] [U.S. Army Corps of 

Engineers Manual, 1997, Design of Tunnels and 

Shafts in Rock, EM 1110-2-2901 (www.USACE. 

ARMY.MIL/inet/USACE-docs/eng-manuals).] (The 

design, sizing and construction of precast concrete 

segments installed at the rear of a tunnel boring 

machine, 1997, translated into English 1999, French 

Tunneling Association AFTES c/o SNCF, 17 Rue 

d’Amsterdam, F75008 Paris, France.) 

20.19 Machine Tunneling 

To reduce costs and increase the speed of the everincreasing 

amount of tunnel construction, a 

number of tunnel-boring machines (TBM) for rock 

and soft ground have been developed. Universal 

machines for mixed ground of rock and soft 

material (mixed face) are designed for each specific 

location and have opened up new possibilities for 

machine tunneling. 

Hard Rock TBMs n Rock-boring machines 

consist of a rotating head, either solid or with 

spokes, on which are mounted cutting tools 

suitable for the type of rock. The machines are 

mounted on large frames, which carry the driving 

machinery and auxiliaries, including a series of 

hydraulic jacks to exert heavy pressure against the 

face. Chisel cutters serve for soft rock, disk cutters 

break harder rock by wedge action, and toothed 

roller cutters with tungsten carbide inserts cut the 

hardest rocks. A critical factor in evaluating 

production is the amount of down time for 

maintenance and replacement of cutters and 

their cost. Most long tunnels in rock use hard-rock 

TBMs. 

Soft Ground TBM n Two main types of tunnel 

boring machine (TBM) are used in soft ground, a 

slurry TBM and an earth pressure balance (EPB) 

TBM. Both types operate with a sealed front 

compartment that is kept under sufficient pressure 

to stabilize the face and minimize ground movement. 

EPB TBMs have been limited to diameters 

under 33 ft due to the high torque needed to drive 

the rotating cutter head, although other forms of 

drive may overcome this limitation. Slurry TBMs 

have been built up to 50 ft diameter, and larger 

sizes are planned. Settlements at the surface in soft 




ground are directly related to the percentage loss of 

material outside the tunnel. Typical loss of material 

lies between 0.5% and 2.5%. Factors affecting the 

loss include the properties of the material 

traversed, the face pressure used, the design of the 

shield, and the rate of advance. Tunnels exist where 

the loss has been zero. Soft ground TBMs are 

generally launched from a relatively small shaft, 

with subsequent parts of the machine being added 

as progress is made. 

The sealed front compartment of a slurry TBM is 

usually filled with a bentonite slurry held in 


Fig. 20.21 Preparing the Cairo Metro Shield for Assembly. 


equilibrium with the soil and groundwater 

pressures acting at the face. The equilibrium is 

often balanced by a compressed air reservoir and 

flow controls. The slurry also acts as a lubricant and 

holds loosened soil in suspension. The main 

disadvantage of a slurry TBM is that the slurry 

must be continuously circulated through a separator, 

often located on the surface, to remove the 

excavated material before returning the reconditioned 

slurry to the face. One main advantage is 

that the underground operators never come into 

contact with the excavated material. The slurry 





TBM also has better face control, especially in 

mixed-face and bouldery ground. Many slurry 

TBMs also incorporate a boulder-crushing unit. 

Soil excavated by the rotating cutter head of an 

EPM TBM falls behind the head and is removed by 

screw conveyor that discharges either onto a 

conveyor belt or directly into muck cars. The 

excavated material may be conditioned by the 

addition of water, clay or by biodegradable 

additives to assist in lubrication and to provide 

better pressure drop through the screw conveyor, 

thus preventing the direct exit of groundwater. The 

rate of advance must be closely coupled to the rate 

of advance to avoid excessive ground movement. 

Flowing Ground n Tunneling machines have 

been developed for use in flowing ground, to 

confine the pressure to a small space between the 

face and a bulkhead behind the cutting wheel. They 

use a pressurized slurry composed of bentonite or 

of excavated material, to balance the pressure. The 

solids are settled out of the recirculated slurry, and 

their volume is accurately measured to determine 

the advance of the machine. 

In very soft materials, the volume of material 

excavated must match the advance of the machine, 

or else sink holes may appear or mounds may be 

pushed up, in both cases causing unacceptably 

large ground movements. 

20.20 Immersed Tunnels 

Immersed tunnels should be considered for all 

water crossings. They are the shallowest form of 

tunnel, requiring only minimal protection against 

sinking ships and dropping anchors, with 5 ft of 

granular material and scour protection often being 

adequate. Because they are so shallow, they result 

in the shortest combined length of tunnel and 

approaches, and because they may serve their 

function better than other choices, the overall cost 

can be less. Approach gradients can also be flatter. 

Immersed tunnels can be constructed in ground 

conditions that would make bored tunneling 

difficult or expensive, such as the soft alluvial 

deposits characteristic of large river estuaries, but 

when rock has to be excavated under water, 

immersed tunneling may be less cost effective. 

Consequently, the ideal alignment for an immersed 

tunnel may not coincide with the ideal alignment 

for a bridge. Alignments do not need to be straight, 


so immersed tunnels can be designed to suit design 

speeds, existing land uses, topography, and 

connections to existing road or rail systems. 

(L.C.F. Ingerslev, “Water Crossings—the 

Options,” Tunneling and Underground Space Technology, 

Vol. 13, No. 4, pp. 357–363, Elsevier Science 

Ltd., 1998.) 

Immersed tunnels consist of very large precast 

concrete or concrete-filled steel tunnel elements. 

They are fabricated in convenient lengths on 

shipways, in drydocks, or in improvised floodable 

basins, sealed with bulkheads at each end, and then 

floated out. They may require outfitting at a pier 

close to their final destination before being towed 

to their final location, immersed, lowered into a 

prepared trench, and joined to previously placed 

tunnel elements. After any further foundation 

works have been completed as discussed below, 

immersed tunnels are backfilled and the bed 

reinstated (Fig. 20.22). Where intrusion into the 

water column is permitted, the final bed level may 

be higher than the original. The side slopes of the 

excavated trench depend upon the soil characteristics; 

often a slope of 1:1.5 is feasible under 

temporary conditions, although flatter grades and 

an allowance for possible sloughing may be 

required in softer materials. 

Floating Tunnels n For particularly deep 

after-crossings at a number of locations, designs 

have been proposed for tunnels that remain totally 

exposed within the water column. These “floating” 

tunnels may be supported below the water surface 

on piers rising from the bed, unsupported if the 

distance between ends is short, supported from 

floating pontoons, or even held down by cables 

if positively buoyant. The text in Section 20.20 

applies to floating tunnels as well as immersed 

tunnels, except for foundations and backfilling. 

Whereas immersed tunnels need only consider 

dynamic loads up to time that they are finally 

placed, floating tunnels have to be designed for 

dynamic loads throughout their life. They appear 

to be particularly attractive for deep narrow 

waterways, since the overall length of tunnel may 

be significantly shortened compared to other forms 

of tunnel. While most immersed tunnels are built 

for water depths of between 5 m and 20 m, 

concepts for 100 m depth have been prepared. 

Floating tunnels could avoid the need to be so 




deep, as long as risks from ship and submarine 

collision can be handled. 

(“International Tunneling Association 

Immersed and Floating Tunnels Working Group 

State-of-the-Art Report,” Second Edition, Pergamon, 

April 1997.) 

Internal Dimensions n Highway design or 

client requirements should determine the required 

number of traffic lanes, tracks, or internal spaces. It 

is usual to avoid climbing lanes within immersed 

tunnels elements themselves, and nominal widths 

of emergency lanes or shoulders have almost 

always been used to minimize costs. If tunnels are 

particularly long, extra width may have to be 

provided at intervals to permit emergency stopping. 

Curbs, or more usually barriers, are provided 

to protect the walls from traffic impact. Barriers 

over 2 ft high may make the lane width seem 

narrower and slow motorists. Emergency access 

into an adjacent tunnel should be available, say at 

300 ft intervals, which would require an emergency 

“walkway” at least 2 ft wide on top of the adjacent 

barrier. Such emergency “cross-passages” may 

need to be provided at intervals of say 100 meters. 

Extra space may be needed for tunnel and other 

utilities, construction and misalignment tolerances, 

lighting, lane signs, and highway signs, while 

keeping the clearance height as low as possible. 

Escape ducts, when provided, should be slightly 

over-pressurized relative to adjacent ducts to 

prevent entry of noxious fumes. With the minimum 

spaces determined, space allowances for any 

necessary ventilation system (such as for jet fans 

or additional ducts) can then be evaluated. The 

critical design case may be for moving or stalled 



Fig. 20.22 Immersed tunnels are set in a trench, which is then backfilled. 

traffic, but a fuel fire usually governs. Permitted 

classes of vehicles may be restricted by legislation 

or owner requirements to limit it the potential size 

of the fire. Finally, additional air space may be 

needed for the tunnel elements to be able to just 

float when completed with bulkheads in place, and 

perhaps space for additional ballast, either inside 

or out, to stay submerged when completed and 

bulkheads removed. Floating tunnels that rely on 

buoyancy must have sufficient compartmentalized 

buoyancy to stay afloat in case of accidental 

damage to two adjacent compartments. 

[L.C.F. Ingerslev et al, Chapter 15B Tunnel 

Structures, ‘‘Structural Engineering Handbook,’’ 

2000 Update for ENGnetBASE, Edited by Wai-Fah 

Chen and Lian Duan, CRC Press, 2000 (www. 

crcpress.com).] 

Construction n The technique of immersed 

tunneling is often less risky than bored tunneling, 

since tunnel element manufacture can be better 

controlled due to the construction of the elements 

in a controlled environment in the dry. As a result, 

immersed tunnels are nearly always much more 

watertight and therefore drier than bored tunnels. 

Two main types of tunnel have emerged, known 

as steel and concrete. Steel tunnels use structural 

steel, usually in the form of stiffened plate, working 

compositely with the interior concrete, whereas 

concrete tunnels do not, relying on steel reinforcing 

bars or prestressing cables. The number of concrete 

tunnels is a almost twice that of steel tunnels. Steel 

tunnels can have a draft of as little as about 8 ft, 

whereas concrete tunnels have a draft of almost the 

full depth. Tunnel cross-sections may have flat 

sides or curved sides. Historically, concrete tunnels 





have been circular, or curved with a flat bottom, but 

the predominant shape has been rectangular (Fig. 

20.23), which is particularly attractive for wide 

highways and combined road/rail tunnels. Steel 

tunnels have been circular, curved with a flat 

bottom, and rectangular (particularly in Japan), but 

the predominant shape in the past in the US has 

been a circular shell within an octagonal shape, 

with ventilation ducts above and below the roadway, 

either in single tube or binocular versions. This 

arrangement of ventilation ducts may change, since 

current techniques permit the use of longitudinal 

ventilation in much longer tunnels, often obviating 

the need for separate ventilation ducts. Steel 

tunnels can be categorized into three sub-types: 

† Single shell, where the structural shell plate 

works compositely with the interior reinforced 

concrete and the shell plate requires corrosion 

protection (Fig. 20.25). 

† Double shell, where the structural shell plate 

works compositely with interior reinforced 

concrete and is protected by external concrete 

placed within a non-structural form plate (Fig. 

20.26). This shape has also been used in pairs for 

several tunnels, the most recent being Ted 

Williams in Boston, Massachusetts. Double shell 

tunnels are only found in the US. 

† Sandwich, where structural steel plates, both 

internal and external, are connected by diaphragms 

and the internal space is filled with 

unreinforced self-compacting concrete. 


Concrete Tunnels n All but four of the 

concrete transportation tunnels built have been 

rectangular (Fig. 20.23). They are for road, rail, or 

for both road and rail. A number of other tunnels 

carrying pedestrians, utilities, sewage or water 

have also been built. Road and rail traffic are 

carried in separate ducts. Of all the road tunnels, 

only one carries four lanes in a single duct. All the 

others have three or less lanes per duct. In order to 

keep profiles as shallow as possible, any air ducts 

are usually located at the sides, rather than above 

or below the traffic duct. Because concrete tunnels 

are much heavier than steel tunnels when they are 

launched, they are usually constructed within dry 

docks or purpose-built casting basins (graving 

docks) capable of being flooded for removal of the 

elements. For many narrower tunnels, some form 

of catamaran lay barge has been used to support 

them during their immersion and placing, whereas 

some wider tunnels have had a pontoon placed on 

top near each end from which the tunnel was 

lowered. In most cases, watertightness has been 

assured by some form of exterior membrane, which 

itself may require protection. Ideally, the membrane 

should adhere to the concrete to limit the 

spread of any leakage through it. An outer steel 

membrane can yield and still remain watertight 

despite significant deformations. 

An element is a length of tunnel that is floated 

and immersed as a single rigid unit. For a few 

Dutch tunnels and the Øresund tunnel (between 

Denmark and Sweden), the rigidity has been 

temporary and was later released, elements 

consisting of a number of discrete segments 

Fig. 20.23 Box-section concrete immersed tunnel (Deas Island Tunnel). 




stressed together longitudinally for ease of transportation 

and placing. After placing and release of 

the segments, each may act as a mini-element free 

to move at the segment joints. The ability to use 

discrete segments can depend upon subsurface 

conditions, acceptable displacements, and sufficient 

capacity to resist seismic effects. 

Most tunnel elements are cast in bays, similar to 

segments, but continuous across the joints. Typically 

the floor slab is cast first. The walls and roof 

may be cast in either one or two operations. Special 

efforts must be made to reduce or preferably 

eliminate cracking in the concrete during fabrication. 

Testing and repair of leaks should be 

completed before submerging elements. 

(L.C.F. Ingerslev, “Concrete Immersed Tunnels: 

The Design Process,” Immersed Tunnel Techniques, 

The Institution of Civil Engineers, Telford, UK, 1989.) 

Steel Single-Shell Tunnels n Of the eight 

single-shell tunnels with some external curvature, 


Fig. 20.24 Western Harbour Crossing, Hong kong. 


three are for rail in Tokyo, Japan, and three are for 

rail in the US including the unique two over two 

configuration of the 63 rd Street Tunnel in Manhattan. 

The Baytown tunnel in Texas was circular with 

two lanes for highway, and the Cross Harbour 

Tunnel in Hong Kong is similar but binocular to 

give two lanes in each direction. The Detroit River 

tunnel (1910) and the Harlem River tunnel (1914) 

may not quite fit into this category, being the first 

two immersed transportation tunnels ever built, 

but do have similarities to single-shell tunnels and 

carry rail. 

Figure 20.25 shows the cross-section of the San 

Francisco Bay Area Rapid Transit (BART) trans-bay 

tube with one track in each direction, separated by 

an exhaust air duct and a service passage. The 57 

elements have a total length of about 19,000 ft. The 

steel shell is welded to make it continuous across 

the joints, as is the reinforced concrete lining, to 

provide security against major earthquake loading. 

At the landfall junctions with the ventilation 





buildings, special earthquake joints permit relative 

movement in all directions. The shell plate is 

protected against corrosion by a coating and a 

cathodic protection system. 

All eight rectangular single-shell tunnels are 

located in Japan. They are very similar in layout to 

concrete tunnels, the difference being that the outer 

steel shell works compositely with the concrete, 

whereas even if an outer steel plate is present as a 

waterproofing membrane in a similar concrete 

tunnel, it is not considered in the strength design. 

Steel Double-Shell Tunnels n Fifteen of 

these tunnels have been built, of which five are 

binocular, the most recent being the Ted Williams 

Tunnel is Boston. Figure 20.26 shows a two-lane 

tunnel with a circular interior section. The steel shell 

plate was 31 ft diameter and about 300 ft long. 

Exterior diaphragms, approximately octagonal, are 

spaced about 15 ft apart, and longitudinal ribs of the 

bars and T sections stiffen the shell. Outside form 

plates were attached to the diaphragms and supported 

by angle struts extended from the shell stiffeners. 

The tubes were erected on shipways. All welds 

were tested for watertightness with a compressedair 

stream and soap solution. Before the ends were 

closed with welded watertight bulkheads, the 

reinforcing steel for the interior concrete lining 

was placed. The keel concrete in the space between 

the outside form plates and the bottom of the shell 

was cast before launching. The concrete lining and 

roadway slab were cast while the element was 


Fig. 20.25 Transbay Tube of the San Francisco Rapid Transit System (steel single shell). 

floating at a fitting-out pier, by pumping concrete 

through hatches in the top of the shell into 

segmental steel forms. Pouring sequences were 

regulated to control increments of water pressure 

on the shell plate and longitudinal bending 

moments. The hatches were closed with welded 

plates, and the exterior concrete cap was cast and 

enough tremie concrete placed in the side pockets 

to reduce freeboard to about 1 ft. Most, if not all, of 

these tunnels have been immersed using a 

catamaran lay barge consisting of a barge each 

side of the tunnel element connected by two 

transverse beams from which the element is 

suspended (Fig. 20.28). Additional tremie concrete 

can then be added to give the required negative 

buoyancy for immersing, placing, and joining. 

More tremie concrete may be needed to achieve the 

final factor of safety against floating. 

Figure 20.27 shows a combination of two such 

cylindrical sections for one of the two four-lane 

tubes of the Ft. McHenry Tunnel under Baltimore 

Harbor. 

Steel Sandwich Tunnels n Although postulated 

elsewhere many years earlier, steel sandwich 

tunnels have become a reality in Japan. 

Rectangular in shape, the principle behind this 

form of construction is that there is a steel skin 

inside and outside the tunnel, both acting 

compositely with the concrete between them. The 

plate is stiffened with flat and L-shaped ribs, and 

the interior is divided up into cells by diaphragms 




in two directions. Self-compacting non-shrink 

concrete is pumped into each cell though one hole 

while air is released through others. Osaka South 

Port and Kobe Port tunnels are being constructed 

by this method, the latter being the only steel 

tunnel so far to carry three lanes per duct. 

Foundations n There are two basic systems in 

use for supporting immersed tunnels on line and 

grade, a screeded foundation, and a pumped sand 

foundation. In addition, a few tunnels are founded 

on piles where soils are particularly soft or special 

conditions prevail. Such conditions can include 

earthquake where stone piles may help to dissipate 

excess pore water pressure and prevent soil 

liquefaction. 

(LC.F. Ingerslev, “Immersed Tunnel Foundations,” 

Comitato Organizzatore del Congresso, 



Fig. 20.26 Cylindrical steel double-shell immersed tunnel (Hampton Roads Tunnel). 

“AITES-ITA 2001, World Tunnel Congress: Progress 

in Tunnelling after 2000,” Proceedings pp 

209–216, Milan, June 2001.) 

With a screeded foundation (Fig. 20.22), the 

tunnel is founded on a leveled bed of sand or stone 

2 ft to 3 ft thick, placed prior to the immersed 

tunnel. The leveling has been done by dragging 

either a heavy grid of steel beams or a steel box 

filled with the foundation material along the 

alignment, suspending them from a carriage on 

rails set parallel to the required grade. The material 

has also been placed in narrow passes transverse to 

the alignment using a pipe, the elevation of which 

was computer controlled. 

For a pumped sand foundation, the tunnel is 

founded on a sand or mortar foundation of similar 

thickness, placed after the tunnel element is 

temporarily supported in place. The element can 

be set on two light pile bents that have been 








Any use is subject to the Terms of Use as given at the website. 

Fig. 20.27 Half of steel immersed tunnel combines two cylindrical sections (Ft. McHenry Tunnel).

constructed to the correct grade. The sand can be 

placed through movable pipes inserted and withdrawn 

from the sides beneath the structure (sand 

jetting), or through fixed pipes embedded with the 

structure (sand-flow) with connections either external 

on the roof or sides, or internal through valves. 

One method of sand jetting, invented by the 

Danish firm of Christiani & Nielson, is particularly 

effective: A sand slurry is injected through a 

movable nozzle, and the surplus water is pumped 

off by another nozzle, the rolling motion depositing 

the sand in a compact layer. With sand flow, the 

rolling current deposits the sand around the 

discharge point in a firm circular layer (pancakes), 

often allowed to grow 20–26 ft radius. Discharge 

points are built into the underside of the element 

and are connected to pipes leading to convenient 

points at which the pumped sand supply may be 

connected. 

As well as pile bents, elements have been 

temporarily supported by jacks, penetrating 

through the base of the section. The jacks bear on 

previously placed concrete blocks. By adjusting the 

jacks, the section is brought to exact grade. Then, 

the sand foundation course is flushed in. 

Immersion, Placing and Joining n Loosely 

termed the “sinking” operation, these three 

operations are performed with a high degree of 

control and therefore of accuracy. Immersion and 

lowering of each element is regulated by winches 

on special barges or pontoons, or by cranes, from 

which they are suspended. Alignment is controlled 

by instruments set on fixed points and sighting on 

targets mounted on temporary towers attached to 

the ends of the sections or by sonar to pre-installed 

targets to avoid using temporary towers. Steel-shell 

elements have historically been connected with 

short lengths of shell, which project beyond the end 

bulkheads. The gap between the ends was covered 

by hood plates extended from the lower and upper 

half of the shell extensions. Form plates were 

inserted into guides on the vertical edges of the 

bulkheads. The space around the joint was filled 

with tremie concrete as a preliminary seal. The 

inside of the joint was drained, and closure plates 

were welded to interior ribs of the shell extensions. 

Finally, the concrete lining was completed. 

Making rigid immersion joints today using 

tremie concrete is unusual, with rubber-gasket 

joints being almost universally used. Flexible 

joints are generally sealed with a temporary 



immersion gasket or soft nosed gasket (Ginatype) 

in compression, attached to the end of one 

of the elements and mating with a flat steel face 

on the other. The use of a secondary independent 

flexible seal, capable of being replaced from 

within the tunnel, is common practice (often an 

omega-shaped seal). Each seal should be capable 

of resisting the external hydrostatic pressure and 

should allow for expected future movements. 

Jacks pull the tubes into contact to provide an 

initial seal. The joint is then drained, activating 

the full hydrostatic pressure on the opposite 

end of the tube. The pressure compresses the 

gaskets completely, providing a secure seal. Then, 

the bulkheads between the connected tubes can 

be opened and the joint completed from the 

inside. 

Depending upon the construction sequence, 

the last element may need to be inserted in the 

remaining space, rather than appended to the end 

of the previous element. In order to achieve this, a 

small final gap will remain. This closure or final 

joint corresponds to a short length of tunnel that 

will need to be constructed in a special way. 

Methods used have include tremie concrete to seal 

a rigid joint, and for flexible joints: 

† dewatering to complete the joint in the dry from 

the inside; 

† terminal block where a short closure section is 

slid out from within one side until it meets the 

other and any remaining gap is closed with a 

rubber gasket in compression; 

† wedge-shaped block dropped into the remaining 

gap until it is sealed against both sides. 

Backfill n Up to about half the height of the 

element, the trench is backfilled with well-graded 

self-compacting material to lock the elements 

securely into place. Ordinary backfill is placed to 

a depth of at least 5 ft over the top of the tunnel. If 

any part of the tunnel projects above the natural 

bottom, dikes should be built at least 50 ft away on 

both sides to a height of 5 ft above the tunnel. The 

space between the dikes should be filled with 

backfill, covered with a stone blanket to prevent 

scour where necessary. 

Design n Tunnel elements are designed as rigid 

structures to resist dead loads, live loads, exceptional 

loads and extreme loads. Dead load includes 





mean water level. Live load includes seabed 

erosion and siltation, and variations in water level, 

current, storm loads and earthquakes, each with a 

return period of 5 years or less. Exceptional loads 

include loss of support (subsidence) below the 

tunnel or to one side, and storms and extreme 

water levels with a probability of being exceeded 

once during the design life. Extreme loads include 

sunken ships, ship collision, water-filled tunnel, 

explosion (e.g. vehicular), fire, the design seismic 

event predicted for the location, and the resulting 

movement of soils. Conditions to be investigated 

should include normal, abnormal, extreme, and 

construction. 

20.21 Shafts 

In tunnel work, shafts are starting points for 

excavation in rock or firm material or shields. For 

long tunnels, such as aqueducts, several shafts 

are used to divide construction into shorter 

sections that can be worked simultaneously. For 

vehicular tunnels, especially subaqueous shield 

tunnels, the shafts are used as bases for 

ventilation buildings. In construction of shafts, 

regulations of the Occupational Safety and Health 

Administration should be observed (Arts. 20.6, 

20.8, and 20.12 to 20.16). 

Timber shafts are mined and braced in the same 

manner as tunnels in similar material. Usually, 

poling boards 5 to 6 ft long are driven into the 

ground and braced at regular intervals by 

rectangular timber frames. Then, the soil is 

excavated to the ends of the polings and a new 

frame installed at this level. 

A relatively shallow shaft may be started 

oversize with sheeting 10 to 20 ft long driven 

vertically on the outside of the frame bracing. 

Intermediate frames are installed as the excavation 

proceeds. At the bottom of the tier of sheeting, the 

sides are stepped in to make room for the next tier 

of vertical sheeting. 

In rock shafts, timbering is used to prevent loose 

rocks from falling off the walls. Its placement usually 

lags an appreciable distance behind the excavation. 

Steel liner plates alone, or in combination with 

horizontal ribs, may be used in soft ground where 

excavation can be made in increments equal to the 

width of the liner plates. H beams driven vertically 

as soldier piles, with wood or steel lagging and 

horizontal bracing, may be used for rectangular 


shafts. Enclosures of vertical steel sheetpiling, for 

round or rectangular shafts, are suitable for waterbearing 

ground. 

Where ground conditions are poor and waterbearing, 

shafts may be constructed with a caisson 

(hollow box), with compressed air as needed to 

exclude water. Gravity pulls the caisson down as 

excavation proceeds. Since its weight is relatively 

small, the caisson may have to be temporarily 

ballasted or jetted for sinking. The depth to which a 

caisson may be sunk is limited by the high cost of 

compressed-air work, which results from the short 

working hours permitted under high pressure. 

Open-bottom shafts with heavy walls, often 

circular or subdivided into compartments, may be 

built on the ground and sunk by excavating the 

ground underneath. In dry soil, the excavation may 

be done directly; if water is present, clamshell 

buckets and high-pressure jets may be used to 

loosen the soil and remove it. On reaching the 

proper depth, the bottom of the shaft is closed by 

tremie concrete. 

As an alternative method for shaft construction, 

water-bearing ground may be frozen in a circular 

ring around the shaft location and the excavation 

made in the dry. Closed-end pipes are driven 

vertically into the ground around the periphery, 

and open-end smaller pipes inserted into them. A 

refrigerant, usually brine, is circulated at temperatures 

as low as 230 8F from the interconnected 

inner pipes into the larger ones and from them 

returned to the refrigeration plant. Several months 

may be required to freeze a deep ring solidly. The 

ventilation shaft of the Scheldt River Tunnel in 

Antwerp was built in this manner, as were a 

number of mine shafts in Germany and France. 




20.22 Seismic Analysis and 

Design 

Earthquake loads, or more correctly seismic loads, 

are included among the loads on a structure that 

are required to be considered by most current 

design codes. Seismic effects can occur during the 

construction phase and should therefore also be 

considered during that period; an appropriate level 

of risk should be agreed with the owner. The 




seismic hazard at a tunnel site can be quantified by 

a project-specific seismic hazard assessment. Typically, 

a functional evaluation earthquake (FEE), 

likely to occur not more than once during the design 

life, is used first to design the structure for either 

limited or full performance following a seismic 

event, as agreed with the owner. Next as appropriate, 

either the safety evaluation earthquake (SEE) 

or the maximum credible earthquake (MCE), both 

concerned not only with life safety but also with the 

survivability of the structure under the most severe 

seismic event considered at the location, is checked 

to ensure compliance with minimum performance. 

If necessary, the strength of some parts of the 

structure may have to be enhanced to comply. 

Structures buried in soil are generally constrained 

to follow the seismic deformations of the 

ground in which they are located. The stiffness of 

the tunnel is generally small relative to the soil, so 

that in all but soft soils, tunnel deformations will 


Fig. 20.28 Lay Barge. 


approximate to ground deformations, a conservative 

assumption. For a tunnel in rock, tunnel 

deformations will match those of the rock, but in 

softer soils, the tunnel will resist soil pressures. The 

response of the tunnel to the free-field soil 

displacements (as if the tunnel were absent) will 

depend upon both the stiffness of the tunnel and 

that of the soil. While the complex seismic analyses 

may be solved numerically using computers, some 

simplified procedures have been published. Simplified 

beam-on-elastic-foundation analysis can 

also be used to account for the soil-structure 

interaction effects of soil deformations, especially 

in soft soils. Horizontal shear S-waves, depending 

upon the angle of approach, cause transverse 

bending or axial waves and produce the largest 

strains that are usually governing. Compression Pwaves 

should also be considered. At sites where 

there are deep deposits of soil, Rayleigh R-waves 

may govern the induced strains. Racking (ovaling) 





deformations in the plane of the cross-section can 

occur in tunnels, but not usually in vertical 

shafts, and is caused primarily by seismic 

waves propagating perpendicular to the tunnel 

longitudinal axis. Vertically propagating shear 

waves are generally considered the most critical 

type of waves for this mode of deformation. Axial 

and curvature deformations are induced by 

components of seismic waves that propagate along 

the longitudinal axis. 

The effects of a seismic event on a tunnel as a 

whole can be integrated to give an effective 

acceleration at the tunnel location, expressed as a 

seismic coefficient times the acceleration due to 

gravity. Three seismic coefficients are usually 


Fig. 20.29 TARP Calumet Shaft. 

obtained, for longitudinal, lateral and vertical 

effects. Internal elements, not in contact with the 

soil and with a natural frequency approaching that 

of the seismic waves, may need to be designed to 

substantially larger seismic coefficients. The vertical 

seismic coefficient can be reasonably assumed 

to be two-thirds of the design peak horizontal 

acceleration divided by the gravity. 

Special precautions are needed for tunnels in 

soils that might liquefy or slip, especially so if 

crossing active faults. Liquefaction may cause 

tunnels to float up. Since it is virtually impossible 

to design against these conditions, the best policy 

is either to improve or replace the soil in question 

or to avoid it. Faults are best avoided, but if that 




is not an option, then the tunnel must be 

designed to accommodate expected displacements, 

and perhaps the surrounding material 

may also need to permit relative movement of the 

tunnel. Locations of especially critical design in a 

tunnel are at changes of inertia and soil properties 

(where there will be contrasting responses), 

and at joints that might open up and cause 

flooding. Ductility in structures is particularly 

important for structures to survive and for life 

safety. 

(Wang, J, ‘‘Seismic Design of Tunnels—A 

Simple State-of-the-Art Design Approach,’’ 



Parsons Brinckerhoff Monograph No. 7, 1993.) (St. 

John, C. M., and Zahrah, T. F., Aseismic Design of 

Underground Structures, Tunneling and Underground 

Space Technology, Vol. 2, No. 2, 1987.) 

(Chapter 15B Tunnel Structures, ‘‘Structural 

Engineering Handbook,’’ 2000 Update for 

ENGnetBASE, Edited by Wai-Fah Chen and Lian 

Duan, CRC Press, 2000 (www.crcpress.com).) 

(Earthquake Analysis, L. C. F. Ingerslev and 

O. Kiyomiya, International Tunneling Association 

Immersed and Floating Tunnels Working Group, 

‘‘State-of-the Art Report,’’ Second Edition, Pergamon, 

April 1997.)

TUNNEL ENGINEERING

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