TUNNEL ENGINEERING

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.)
TUNNEL ENGINEERING
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.
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