Geotechnical risk management for tunneling beneath open water

Underground Space – the 4 th Dimension of Metropolises – Barták, Hrdina, Romancov & Zlámal (eds)
© 2007 Taylor & Francis Group, London, ISBN 978-0-415-40807-3
Geotechnical risk management for tunneling beneath open water
D.P. Richards
Parsons Brinckerhoff, Portland OR, USA.
B. Nilsen
Norwegian University of Science and Technology (NTNU), Trondheim, Norway
ABSTRACT: Tunnel construction beneath open bodies of water presents certain types of risks not encountered
when tunneling beneath “dry land”. If major problems develop with tunnel equipment, ground control, or ground
water control, construction personnel are exposed to risk of catastrophic flooding in the tunnel and recovery
operations from the surface are generally not possible. Under these construction conditions, management of
geotechnical risk during construction is of paramount importance. This paper addresses some of the more critical
risks that must be considered and evaluated for such tunnel construction, starting with the planning process,
and continuing through design and construction. Due to the increased difficulty and expense of geotechnical site
exploration under water, the extent of the site investigation is sometimes decreased rather than increased, increasing
the geotechnical uncertainty, requiring more detailed planning of construction methods and sequences, and
appropriate redundancies and contingencies. The paper discusses the investingation, planning and construction
processes for both soft ground and hard rock tunnels, and both conventional and machine excavation.
1 INTRODUCTION
The planning, design and construction of underground
facilities beneath open water is always a challenge. In
contrast to on-shore tunnel operations, when ground
and/or ground water control problems are encountered,
the physical constraint of no access from above for
recovery operations when problems are encountered,
introduces a higher degree of risk during construction,
requiring an increase in the level of effort in the
site investigation, design detailing and construction
planning.
2 GEOTECHNICAL RISKS
Geotechnical risks are always a challenge in any tunnel
project, and must be accounted for to minimize risk
during construction, but for under water projects, the
risks are often higher due to site investigation limitations
and restrictions in addressing ground and ground
water control problems during construction. Geotechnical
risks that must be identified and accounted for
in the investigation, design, and construction planning
include:
• General profiling of the bottom surface
• Identification of low spots in the bottom profile
• Depth and character of sediments
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• Variations in type and character of bedrock
• Location and extent of geological weaknesses
• Identification of rock structure patterns
• Characterization of the in-situ permeability
Potential risks associated with construction of hard
rock subsea tunnels may be illustrated by the following
examples from Norwegian projects (Blindheim et.al.,
2005):
• The Bjorøy tunnel (1996), where a more than 10 m
wide fault zone filled with clay, sand and coal fragments
quite unexpectedly encountered. This was
a zone of extremely high permeability and very
poor stability, and a very time-consuming procedure
involving stepwise grouting, drainage, spiling and
shotcrete arches was necessary to tunnel through it.
• The North Cape tunnel (1999), where flat lying, broken
sedimentary rocks caused very poor stability,
resulting in slow progress and requiring shotcrete
and concrete lining at the face. The difficult conditions
were not realized from the pre-investigations
due to the relatively high seismic velocity of the flat
lying layers (4,500–5,500 m/sec).
• The Oslofjord tunnel (2000), where a deep cleft
filled with Quaternary soil was encountered, necessitating
ground freezing to tunnel through.A distinct
weakness zone was detected prior to tunnelling.
Despite very comprehensive pre-investigations

100
200
BH 4-1 BH 2 Tarva fault
BH 3-1
BH 3-2
BH 5-1
Hitra loose material
Fröya
0
2.5
1
low seismic velocity zone
weakness zone in tunnel
core drill hole
0 500 1000 1500 2000 3000 4000 5000
Figure 1. Longitudinal section illustrating low velocity/weakness zones in the Frøya tunnel (from Nilsen & Palmstrøm,
2001).
including traditional refraction seismics as well as
directional core drilling and seismic tomography it
was not found that the zone was eroded to a deep
cleft.A by-pass tunnel was prepared to allow continued
tunneling under the fjord. The soil filled section
was frozen (at 120 m water pressure) and excavated
through.
• At one of the most recent Norwegian projects, the
Frøya sub sea tunnel, a pattern of very distinct,
regional faults was identified by pre-construction
investigations. Some of these faults could be followed
over a distance of more than 200 km.
Challenging ground conditions were documented,
including high permeability zones as well as weakness
zones containing very loose, sandy material
and extremely active swelling clay. The locations of
the main weakness zones are shown in Figure 1.
In the planning of the Frøya tunnel, great benefit
was gained from the previous construction of
the nearby Hitra tunnel, where the major fault in
the fjord was found to consist mainly of a mixture
of heavily crushed rock and clay minerals (including
swelling clay). The water seepage through the
zone was minimal, and a combination of short blast
rounds, spiling, steel fiber reinforced shotcrete,
straps and conventional rock bolts, was used for
getting through it.
The risks noted above for conventionally excavated
drill and blast tunnels are similar for hard rock TBM
tunnels, but they are sometimes more difficult to cope
with using a TBM, due to limited access to the face for
grouting or other ground improvement procedures, as
well as problems in getting around aTBM if it becomes
stuck. Aside from ground water inflows being a potential
risk, other geological risks such as unanticipated
faults or intrusions, either associated with an increase
in fracture frequency, increases (or decreases) in rock
strength, and increased abrasion also become more
significant for a TBM excavation.
Significant risks for soft ground tunnels are mostly
related to pressurized face TBM driven tunnels, since
compressed air as a ground and/or ground water control
technology is being replaced by improvements
in soft ground TBM technology. Considering this
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though, there are still significant risks, mainly related
to the presence of unanticipated ground conditions
such as higher permeability, higher/lower cohesion,
presence of boulders, buried obstruction, abandoned
exploratory boreholes, extremely weak (or strong)
materials, increased abrasiveness etc.
3 SITE INVESTIGATION AND PLANNING
3.1 General
A comprehensive site investigation is critical to the
successful planning and execution of a tunnel beneath
open water. Due to the additional expense and technical
difficulties involved in off-shore drilling, vertical
borings penetrating to tunnel level are often limited,
and are often supplemented by directional drilling
beneath the water, and by the use of marine geophysics.
Geophysical techniques often considered for
such off-shore investigations include:
• Hydrographic surveys: River, lake, or ocean floor
bathymetric mapping
• Multibeam sonar: Multichannel hydrographic surveying
that records a wide swath of data
• Sub-bottom sonar profiling: Detection of layering
beneath the floors of rivers, lakes, or oceans
• Marine Seismic Reflection: Multichannel profiling
of sediment and rock layering beneath the floors of
rivers, lakes, or oceans
• Marine Seismic Refraction: For mapping of thickness
and character of sediments, rock mass characterization
and faults/weakness zones based on
seismic velocity (see also Figure 1)
• Seismic tomography: Investigation of rock mass
character between drill holes or between drill hole
and sea floor
• Marine Magnetometry: For mapping of rock type
boundaries and weathering
• Electrical Imaging: Two-dimensional (crosssectional)
imaging of sub-bottom electrical properties
to detect and delineate sand, clay or gravel
lenses, bedrock fractures – sensitive to water
salinity.

Figure 2. Conventional and directional core drilling at critical part of the Oslofjord tunnel (after Palmstrøm et.al., 2003).
In addition, conventional site investigation requirements
are applicable, including sampling for
determination of strength, weathering, mineralogy,
deformation characteristics, porosity, abrasion potential,
as required to make decisions with respect to
applicable construction methods. For hard rock this
would be a decision between tunnel boring machine
and conventional excavation by drill and blast techniques.
For soft ground, this would be a decision
between types of pressurized closed face tunnel boring
machines – slurry shield or EPB technology.
3.2 Norwegian hard rock practice
As a supplement to normal geological surveys, seismic
investigations are crucial in the first stages. Acoustic
profiling is first carried out for a large area to determine
the most suitable corridor. Extensive refraction
seismics is then carried out to select the best alignment
and to provide information about soil deposits above
the bedrock and about weakness (low velocity) zones
in the bedrock. If possible, directional core drilling as
illustrated in Figure 2 is used from shore to the critical
deepest points of the alignment, which typically also
could be the location of major fault zones. Core drilling
from drilling ships has been applied in a few cases in
Norway, when other drilling has not been feasible.
The costs for the site investigations for Norwegian
sub sea tunnels typically amount to 3–7% of
the construction costs. The established practice of site
investigations has proven to be reliable, but exceptions
have occurred as noted above.
Optimizing the minimum rock cover is crucial
for safety as well as economy. Increased rock cover
makes the tunnel unnecessarily long, representing
extra construction costs and increased operating and
traffic costs over the project’s lifetime. Too little rock
cover may cause severe stability problems, unacceptable
work safety, heavy water ingress, and need for
excessive grouting and high pumping costs.
A minimum rock cover of 50 m is basically required.
A rock cover of less than 50 m can be accepted when
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detailed site investigations have demonstrated fair rock
mass conditions (taking into account the typical occurrence
of fault zones at the deepest point). This is left
open to interpretation and rock cover less than 20 m
has been used, but is then typically restricted to shallow
waters and for good rock conditions.
3.3 Hard rock TBM tunnels
Investigation requirements for TBM tunnels in hard
rock are similar to those for conventional excavation,
since the risks are similar. However, as noted above,
ground and/or ground water control problems are often
more difficult to cope with for aTBM excavation, often
making the understanding and reliability of geological
conditions a bit more critical.
3.4 Soft ground tunnels
Soft ground tunnels passing beneath open water are
usually relatively shallow. For example, transportation
tunnels passing beneath rivers or shallow bays or channels
usually have the depth restricted by the gradient
of the approaches to the tunnel.
While site investigation practices for such soft
ground tunnels are very similar to those identified
above for hard rock tunnels, they must also include
adequate material property characterization to allow
informed decisions to be made with respect to soft
ground TBM technology. The usual material property
tests contributing to the ability to make these
decisions include density, permeability, gradation, and
plasticity.
For these applications, conventional sampling by
both Standard Penetration Testing (SPT), and recovery
of “undisturbed” samples are both commonly used, in
addition to CPT (Cone PenetrationTest). For such shallow
soft ground applications in historic urban areas,
it is often required to identify buried obstructions
such as sunken ships and abandoned foundation piles,
both of which can often be detected by geophysical
techniques.

4 CONSTRUCTIONAL ASPECTS
4.1 General
It is during the construction planning phase, that final
decisions must be made for the appropriate selection
of means and methods of construction best suited to
the subsurface geological conditions identified during
the site investigation phase. These selected means
and methods of construction must be an integral component
of the risk management process, and must
be chosen to best satisfy the intent of the mitigation
measures for reducing risk exposure.
Key considerations in selecting construction means
and methods for hard rock tunnels include:
• Rock types, distribution, and conditions
• Rock properties; strength, hardness, abrasiveness
• Discontinuity characteristics; frequency, orientations,
continuity, roughness
• Major structural features; fault number, orientation,
thickness, character
Key considerations in selection of construction
means and methods for soft ground tunnels include:
• Thickness of overburden cover
• Soil type, layering, stiffness/relative density
• Soil properties; plasticity, gradation, permeability
• Hard inclusions; boulders, cemented zones
• Hydrostatic water pressure
• Ground water chemistry
4.2 Hard rock conventional excavation tunnels
The Norwegian hard rock sub sea tunnels have all been
excavated by drilling and blasting. This method provides
great flexibility and adaptability to varying rock
mass conditions and is cost effective.
Systematic percussive probe drilling with the
drilling jumbo is most critical for safety. By applying
criteria related to inflow per probe hole on when
to pre-grout, the remaining inflow can be controlled
and adapted to preset quantities for economical pumping,
which is normally 300 liters/min per km. Grouting
against water pressures of 2∼3 MPa can be efficiently
achieved with modern packers, pumps and grouting
materials. Grouting pressures up to 10MPa are today
quite common. Follow-up at the tunnel face by well
qualified engineering geologists (and rock engineers)
is of great importance.
In the Norwegian projects, all rock support structures
are drained, whether they are made of castin-place
concrete (mostly horseshoe) lining, sprayed
concrete ribs or sprayed concrete. Sprayed concrete
is dominantly applied as wet mix steel fiber reinforced.
Extensive testing demonstrates that, if the
thickness of the sprayed concrete is above a minimum
of 60–70 mm, and the concrete quality is good (C45),
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corrosion of steel fibers is not a problem. Rock bolts
have extensive corrosion protection. Rock support is
always adjusted to the actual rock mass conditions,
with heavy support applied only in very poor stability
conditions. The use of spiling and shotcrete ribs has
mostly replaced a concrete lining.
4.3 Hard rock TBM tunnels
Although numerous rock TBMs are operating around
the world, many below the static ground water table,
not all of these pass beneath open water. As with any
rockTBM, good ground conditions usually allow good
TBM excavation rates. However, as noted by McLearie
et.al. (2001) for bored rock tunnels recently completed
in Hong Kong, tunnel excavation success was largely
dependent upon ground and ground water conditions,
with intensely fractured zones (faults and dikes) often
introducing large amounts of ground water infiltration,
requiring extensive cut-off grouting before the TBM
could progress, delaying the schedule and increasing
the costs.
At the opposite end of the spectrum, was the Channel
Tunnel, where ground conditions were ideal for
TBM excavation in the relatively consistent chalks
and chalk marls (Varley et.al., 1992 and Barthes et.al.,
1994a). As noted by Barthes et.al., (1994b), although
the chalks were relatively consistent on both sides of
the channel, the French side was more faulted, with
higher in-situ permeabilities, and these factors influenced
the selection of TBMs as well as other means
and methods of construction.
4.4 Soft ground tunnels
A fairly recent example of a large diameter soft ground
tunnel excavated by a pressurized face TBM beneath
open water, was the crossing beneath the Nile River
by the Cairo Metro Line 2 bored tunnels in Egypt
(Richards et.al., 1998). This was excavated by a 9.43
meter diameter slurry shield TBM in dense saturated
sands and crossed two separate branches of the river.
A subsurface profile is shown in Figure 3.
Prior to starting the tunnel bore beneath the river,
a risk assessment was performed, addressing both
design, construction, and long term performance
issues. Based upon this risk assessment, a number
of design issues were double checked to ensure
adequate construction term and long term performance
of the tunnel. This risk assessment was also
used for construction planning, in which emergency
response plans, contingency plans and appropriate
redundant construction systems were identified and
implemented.
Key issues identified to minimize risk while tunneling
beneath the river included: maintenance of
face stability, lining ring shape, ring position, reduce
blowout risk, avoid bridge foundation, avoid TBM

Figure 3. Nile crossing subsurface profile (Richards et.al., 1998).
break downs. Construction related contingency measures
put into place to achieve these goals included:
back-up electrical power, back-up compressed air supply,
back-up locomotives, redundant slurry pumping
equipment, extra reserve supply of bentonite, TBM
improvement and/or modifications before passing
beneath the river channel, and redundant communications
between underground and topside personnel.
The tunnel beneath the river was completed in
two separate tunnel drives, with continuous operation
until the TBM had safely passed beneath the open
water portion of the alignment. Both were completed
without mishap, with no problems encountered which
could not be handled by the contingency measures
put into place prior to starting the excavation beneath
the river.
5 RISK EVALUATION AND MANAGEMENT
To better control the complexity of underground
projects, new codes and guidelines for design, modern
quality systems and special methodologies for risk
analyses have been introduced. Such tools provide a
better basis for planning of investigation as well as
construction and time/cost estimation.
The basic principle for determining the extent of
investigation should always be related to the type
and complexity of the project, and to the prevailing
geological conditions.
A risk analysis may be divided into the following
steps (Nilsen et.al., 1999; Pennington, et.al., 2006):
1. Identification of hazards and damage events
2. Assessment of probabilities for hazards and damage
events identified
3. Description and valuation of consequences including
analysis of initiating events
4. Calculation of pre-mitigation risk rating
5. Identification of appropriate mitigation measures
for each risk
6. Calculation of post-mitigation risk rating
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Within a project, risk analysis and quality control/assurance
should be focused upon:
• Identifying and elimination or reducing hazards
• Reducing the probability of initiating events
• Finding barriers to stop damage events
• Reducing the consequence of possible damage
• Implementation of mitigation measures
The types of geotechnical risks to be considered
include:
• Hard rock tunnels
– Geology more complicated than anticipated
– Weathering deeper than anticipated
– Rock stronger or weaker than anticipated
– More/wider fault zones than anticipated
– Rock more fractured than anticipated
– Rock more abrasive than anticipated
– Rock mass (including fractured/faulted zones)
permeability higher than anticipated
– More voids (e.g. karst) than anticipated
• Soft ground tunnels
– Bottom profile more irregular than anticipated
– Sediments softer/harder or stiffer/denser than
anticipated
– Soils more granular /cohesive than anticipated
– In-situ permeability greater than anticipated
– Soil profile more irregular than anticipated
– More boulders or inclusions than anticipated
Hazards and uncertainties are not only related to
the soil or rock, but also with people involved in a
project. A major task within risk analysis is to understand,
describe and handle uncertainties. The Frøya
sub sea tunnel case may be used to illustrate this. Here,
the refraction seismic measurements showed more low
velocity (weakness) zones than for any of the subsea
tunnel previously constructed in Norway. In addition,
the core drillings penetrated long sections of rocks with
weakness zones having a higher degree of alteration
than is normal in Norwegian hard rocks. The results

of the investigations indicated that the Frøya tunnel
required thorough evaluations to assess its feasibility,
and that special routines be implemented during
planning and construction.
As a part of this, two groups were established to
evaluate the feasibility of the tunnel. In their two
independent reports excavation methods and rock support
were analyzed, supported by a cost estimate and
risk assessment. Construction time and cost estimates
were based on a detailed prognosis of the expected
ground conditions, and the estimate proved to correspond
quite well with reality (Palmstrøm et.al, 2000).
6 DISCUSSION AND CONCLUSIONS
Tunneling beneath open water is usually more challenging
them landside operation, often with a higher
degree of uncertainty and risk. The following lessons
of general relevancy for the planning of future projects
are applicable.
• The extent of ground investigation and planning
should always reflect the complexity of the geology,
the type of project, and the degree of risk.
• Risk assessment should start in the planning phase
and carry on in design and construction.
• The results from the investigations should be properly
documented and their use in calculations and
assessments shown.
• The geological setting and understanding of the
tectonic are vital for all large tunnel projects.
• Ground investigations where the extent is based on
bidding, may cause vital information to be lost, and
should never be accepted.
• Sufficient time must be allocated for necessary
investigations and testing.
• The ground investigations should continue through
the entire construction period. Tunnel mapping and
following up should be done by experienced engineering
geologists representing owner as well as
contractor.
• Risk analysis, assessment of uncertainties and
development of mitigation measures are critical.
• The tender documents, including geological/
hydrological/hydrographic reports, should be thoroughly
prepared, with full quality control.
• An independent reference committee should be
established. For the construction period, strict
requirements should be put on the engineer’s and
contractor’s competency and qualifications.
• Planning and investigation should always carry risk
assessment forward to the detailed design and construction
phases, since residual risk remains, even
after significant and relevant site investigations.
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