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4-
Geologic Problems and Consequences in Construction
In the most general and simplified sense, the major problem during con-
struction is "ground" (i.e. rock or soil) behaving differently than
anticipated. The nature of the ground has significantly different im-
plications for hand mining, drill-and-blast, and shield and/or tunnel
boring machine operations. Equally important to the basic identifica-
tion of "hard" or "soft" ground is the determination of the zone of
transition from "hard" to "soft" or vice versa, as well as the potential
for both extremes to exist in the same place, i.e. "mixed face." In
identifying characteristics important for construction, consideration
must also be given to factors that can affect ground behavior, such as
the presence of water or the construction process itself.
Classifications and predictions based on inaccurate or insufficient
information can easily result in a partial to total difference between
expected and encountered conditions. Especially from the construction
point of view, the geotechnical site investigation should provide the
basis for anticipating in a reliable and specific way the behavior of
the ground.
Using the abbreviated list of "problems encountered" appearing in
the project abstracts (Volume 2), Table 4.1 was prepared to identify
some of the construction consequences. The conditions noted became
construction problems, or escalated to greater and more troublesome
importance, mostly because the contractor was not prepared for them.
This circumstance raises the question of whether these problems
could have been avoided, or their impacts minimized, with a more thor-
ough or different preconstruction geotechnical site investigation. It
is possible that some problems could have been eliminated by making soil
or rock borings at closer spacings, and/or by utilizing geophysical
seismic surveys to gain information, or by applying other investigative
techniques. Still, it is essential to note that not all of the problems
could have been anticipated by additional investigation.
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TABLE 4. 1 Ef feet of Geologic Conditions on Construction
Major Problem Areas
Ground behavior
blocky or slabby
running
flowing
squeezing
swelling
spelling (bursts)
stand-up time
rock loads
in-situ stress
Groundwater
operating nuisance
large quantity
high pressure
corrosive or insoluble salts
Existing conditions
noxious wastes
utilities and structures
obstructions (boulders, piles,
concrete, etc.)
gas
mixed face
Mechanical problems in rock
hard, abrasive
mucking
soft bottom
face fall-out
gripper instability (TBMs)
roof stabbing
drilling and blasting necessary
for line drilling
pressure binding
Soft-ground problems in machine mining
surface subsidence
face instability
water inflow (significant)
material hardness
steering
high rock or intrusions
Compressed air
blowouts
fire
other safety restrictions
contingency dewatering and grouting
Consequences/Requirements
excavation method, special equipment,
immediate support
time loss, special method of control
time loss, special method of control
immediate support
intermediate support
progress shut-down, safety
immediate support
extra steel, long-term support
cave-ins, time loss, special pro-
cedures
inefficiencies, slow-downs, extra
pumping
progress shut-down, handling pro-
cedures
progress shut-down, handling pro-
cedures
damage to excavation equipment,
temporary supports, concrete
safety, inefficiencies, time loss
progress shut-down
progress shut-down, equipment damage
safety, progress shut-down
special procedures, techniques,
equipment
progress rate, tool life
downtime
progress rate, grade and alignment,
special design
progress shut-down
progress rate, alignment
cave-ins, progress shut-down, addi-
tional support
progress rate
progress shut-down, immediate support
damage at surface
progress slow- or shut-down, immedi-
ate support or compaction grouting
progress slow- or shut-down
machine binding, progress shut-down
time loss, grade and alignment cor-
rection
machine damage, blasting, excavation
method
safety, progress shut-down
safety, progress shut-down
productivity
progress rate, safety
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MAJOR PROBLEMS FOR CONSTRUCTION
The project case histories developed during this study probed the nature
of the geotechnical site investigations and subsequent conditions en-
countered during construction. The discussion that follows centers on
the conditions and resulting problems which, based on the case histories
and on the experience of the subcommittee, have been shown to be impor-
tant either because of frequency of occurrence or magnitude of impact.
Stand-Up Time
A stand-up time problem occurs when the ground (rock or soft earth) will
not support itself for a time sufficient to accommodate the construction.
Stand-up time affects five major areas of concern: type of ground sup-
port; equipment selection (e.g., shield); manpower requirements; produc-
tion and schedule; and cost.
Construction Impact
Stand-up time, particularly in blocky ground, dictates in large part
whether ground support systems such as rock bolts and mesh are required,
or other systems such as steel supports or shotcrete. In extreme cases,
a combined system of rock bolts, steel sets, and shotcrete may be re-
quired.
Stand-up time, or lack thereof, dictates when ground support must be
installed, which in turn may have a pronounced effect on rate of prog-
ress and costs. The type and timing of ground support installation will
affect progress either directly or indirectly. In the case of the re-
quirements to install ground support immediately after blasting or me-
chanical excavation, the work involved will be "in the cycle," and thus
will directly extend the round time and the schedule. The work involved
is "unit" production and no short-cuts can be taken.
The method and timing of ground support installation will dictate
the type of equipment required to accomplish the work effectively. The
equipment selected must permit rapid and proper installation of the sup-
port elements and yet not be so cumbersome as to delay other operations
that must be accomplished in the tunnel. Highly specialized equipment
that is capable of installing only a specific ground support system will
be totally costed against the project in question, as opposed to partial
amortization.
As stand-up time strongly influences the type of ground support
system to be used--and consequently the equipment necessary to install
it--manpower will also be affected. Some types of ground support sys-
tems are more labor intensive than others. For example, a simple system
of widely spaced rock bolts will no doubt require much less labor than a
very elaborate system of rock bolts, steel sets, and shotcrete.
Because significant elements of the construction plan are dictated
by the assessment of stand-up time, an unanticipated adverse condition
is disruptive--and sometimes dramatically so. Delayed performance, im-
plementation of a different ground support plan, and increased cost are
the results.
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Data (and Interpretations) Available Prior to Bid
There is no known specific laboratory or field test that can be con-
ducted prior to construction that will accurately predict stand-up time.
However, the use of RQD and core recovery, close inspection of the joint
conditions, consideration of depth of overburden, and observed behavior
of the ground in road cuts, outcrops, or any existing projects adjacent
to the one contemplated, may provide valuable insight into the stand-up
time to be expected. To be useful, collections of observed behavior
need to be accurately and completely described. For example, the amount
of induced ground vibration caused by blasting should be addressed,
along with factors which relate to slaking or squeezing, such as circu-
lating air, water saturation, and water flow.
In cohesive soil, stand-up time is fairly well indicated by the re-
lation of overburden load to undrained shear strength (sometimes called
the "overburden factory. If the overburden factor is 5 or 6, the soil
is only marginally stable. However, if the factor is 3 or 4, stand-up
time will be good.
In cohesionless soil, the stand-up time is less easy to quantify.
The acting water pressure, gradation, and relative density are important.
For example, marginal stability and short stand-up time with a tendency
to running conditions would be indicated by a cohesionless soil having a
uniformity coefficient of less than 3, with less than 5 percent fines
(passing a No. 200 sieve) and a relative density of less than 40 to 50
percent.
To be constructive in terms of potential stand-up time problems, the
geotechnical investigation should include as relevant information sur-
veys of adjacent projects or projects in similar ground conditions, in
addition to site specific data. Complete and accurate logging of rock
core and description of joint spacing, orientation, and roughness would
be useful.
In-Situ Stresses
In-situ stresses in rock are induced by geologic loading, such as may
have occurred dur ing glaciation, or by tectonic activity. The excava-
tion of a tunnel opening creates a change in stress cordite on that can
result in excess movement and, possibly, local failures of the rock sur-
face created by the tunnel.
Construction Impact
The ratio of the in-situ stress to the strength of the rock determines
the scope and degree of deformation or failure that can occur. If the
stress is much higher than the strength, local to large failures can
occur; in the extreme, the rock may behave in a plastic manner, or in a
way similar to soil.
The excavation cycle can be adversely affected by local failure,
such as fall-out at the heading, popping or squeezing rock, and other
movements of rock that directly interfere with the excavation process
and necessitate the installation of special supports. Support through
rock reinforcement (e.g., rock bolts and spires or sets) increases the
excavation cycle time. The selected equipment may then be inappropriate.
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Data (and Interpretations) Available Prior to Bid
The geotechnical site investigation must be structured toward identifi-
cat~on of zones of high stress, especially in known areas of high pre-
load or tectonic activity. Overcoring and/or hydraulic fracturing at
greater depths are methods useful for investigating the in-situ stresses.
Swelling and Squeezing Ground
Swelling occurs when the ground expands in volume by absorbing or ad-
sorbing water and then tends to move into an available opening or to
exert pressure. Squeezing occurs when weak material (generally clayey)
behaves plastically under the weight of overlying ground and slowly ad-
vances inward without perceptible volume increase. Either condition can
be serious, mainly affecting support requirements and excavation equip-
ment, particularly shields.
Construction Impact
Steel sets and lagging can become distorted and out-of-shape with appre-
ciable swelling or squeezing pressures. Distortion may be great enough
to preclude the placement of permanent support, such as concrete lining.
Excavation can be affected if swelling or squeezing ground impedes
equipment. In (admittedly) the most dramatic case, the skin of a shield
may seize due to friction that cannot be overcome by the hydraulic
thrust system. Also, a heaving bottom can ruin tramming equipment and
mucking procedures by inducing severe shock loads.
Data (and Interpretations) Available Prior to Bid
The geotechnical investigation should include analyses both for clay
minerals indicative of swelling (i.e., montmorillonite) and for wet con-
ditions that could induce physical swelling. The boring program should
be structured to provide appropriate samples for laboratory tests and
analyses.
Groundwater
A groundwater problem is the presence in higher volumes than predicted--
or worse, the unanticipated presence--of water during tunneling. The
presence and movement of water strongly influences ground behavior, cre-
ates a requirement for handling, and affects labor and equipment produc-
tivity, and thus cost.
Construction Impact
In the case of rock, as water moves throughout the medium, particularly
in areas having fault and gouge zones,~the water may carry loosened par-
ticles of material into the excavated opening. As this material mi-
grates out of the host medium, voids are created and the matrix becomes
loosened, in turn causing instability in the rock mass.
Water can change physical properties of the ground such as cohesion,
plasticity, and tendency to swell. In the case of soft ground, water
under pressure {even low pressure) may bring smaller particles with it
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as it moves into the opening, thus creating voids. As these voids are
created, the ground will move inward, often resulting in subsidence
above or adjacent to the excavation. Subsidence in soft-ground excava-
tions may be particularly critical in urban areas because of the poten-
tial for damage to adjacent structures. -
Water also creates requirements for handling. The primary handling
necessitates the use of collecting pumps, conveyance lines, and in many
cases a centralized pumping system. The installation and maintenance of
this primary handling system may adversely affect production and sched-
ule. Moreover, a secondary handling system may be required to satisfy
environmental concerns. Features of a secondary system may include
elaborate settling basins and chemical treatment equipment. In addi-
tion, effluent testing is a likely requirement.
The presence of groundwater raises problems with respect to labor
and equipment productivity. In the case of labor-intensive operations
that are to be accomplished in a wet environment, human effectiveness
will be reduced by these less-than-optimal conditions. This reduction
of effectiveness translates directly into production losses. However,
less labor-intensive operations are also subject to the effects of
water. For instance, when excavated material must be trammed over long
distances, abrasion of rotating parts occurs as the equipment travels
through rock slurry covering the floor of the tunnel. In addition, sa-
line water can be detrimental to mechanical equipment because of its
corrosive effects on electrical components. TBMs are particularly sus-
ceptible to corrosion; however, the problem usually occurs only when a
project is located under a saltwater body.
Problems and resulting costs are magnified when the groundwater vol-
umes and pressure are not anticipated. The result is that the primary
and secondary handling systems, ground support systems, excavation
equipment, and general method of attack are usually inadequate. This
impedes production and increases costs.
Data (and Interpretations) Available Prior to Bid
Pump tests to determine groundwater levels and behavior should be per-
formed, because groundwater will often be the key element in the con-
tractor's work plans. Multiple piezometers to measure perched water
tables should be installed and monitored over a considerable period of
time. These measurements will enable the contractor to evaluate methods
of handling excess groundwater.
In a fractured rock medium, pump tests are often advantageous. In
the case of soft ground, chemical analyses and testing for effects of
exposure to air should be performed, in addition to standard pumping
tests taken to equilibrium and drawdown studies.
For rock, there are no highly reliable groundwater prediction mecha-
nisms which can be used and still maintain cost-effective construction.
Nevertheless, a statement concerning estimated volumes and pressures,
based on engineering judgments, should be presented. Particular atten-
tion should be given to the jointing system and its degree of openness.
Finally, data from any adjacent projects should be canvassed carefully
for pertinent information. (Note: A variety of techniques exist that
can be combined for reliability, but the cost is usually far beyond the
resources of general underground construction.)
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Existing Structures
Existing surface and underground structures can be sensitive to the con-
struction operations of dewatering, excavation, and support as well as
to blasting and equipment vibration. If construction operations are
undertaken without adequate forewarning or preparation, these structures
can be damaged. In addition, performance of the contract may be dis-
rupted while emergency protection measures are instituted.
Construction Impact
In earth tunnels, if the soils are compressible, dewatering can cause
subsidence and damage to adjacent surface structures. To correct these
problems requires underpinning of structures, recharging, continuous
sheeting or slurry wall, or combinations of these construction proce-
dures. Depending on the permeability and gradation of the soil, compac-
tion grouting may also be used.
Settlements caused by "loss of ground" at the tunnel face and sup-
ported perimeter, as well as elastic yielding into the excavated space,
may be the determining factors in selecting the methods of excavation
and support, as well as the support plans for structures within the area
of inf luence .
Data (and Interpretations) Available Pr for to Bid
Data relevant to the presence and physical condition of existing struc-
tures should be obtained and provided. Site investigation methods and
laboratory tests should be planned to provide clear information and con-
clusions regarding (a) the behavior of the water table and (b) the com-
pressibility, permeability and gradation (important in recharge and
grouting considerations), density, and strength of the soil. Precon-
struction monitoring of area elevations and detailed inspection of pre-
existing structure distress is desirable. In a rock profile where drill-
and-blast procedures are applicable and sensitive structures exist at
the site or nearby, preconstruction blast/vibration/noise/sensitivity
measurements should be made for comparison with later construction
effects and for use in establishing a public relations program.
Gases
Gases encountered underground can be noxious, toxic, and hazardous, pos-
ing significant problems in construction. If unanticipated, such gases
can obviously be dangerous; if anticipated and planned for, the danger
may be reduced or eliminated.
Construction Impact
When gas is encountered, its properties as well as its chemical and
physical reactions to moisture, air (chemical constituents), high tem-
perature, pressure, etc., must be evaluated. The gas and/or its prod-
ucts must be studied for effect on personnel, corrosive action on mate-
rial and equipment, and potential for explosion. In addition, the
potential for release of gas f rom groundwater should be investigated.
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The tunneling equipment may have to be ~spark-proofed" if explosive
gases are contemplated. A special ventilation and absorption system may
have to be installed for noxious and corrosive gases. Personnel may
need training in the detection and handling of unexpected gases, and
safety equipment may have to be specif fed or issued. Moreover, the
entire excavation procedure may have to be controlled by strict mining
standards.
Data (and Interpretations) Avail able Prior to Bid
Adjacent soil and rock should be evaluated as potential sources of ex-
plosive gases and other noxious or corrosive gases. Chemical testing
should be conducted according to strict standards, especially in sus-
pected problem areas. Groundwater samples must be checked as a source
of gas as well as for reaction with gas. All exploratory boreholes
should be checked for the presence of gas, and it may be advisable to
install special probes within some boreholes to permit recurring checks
of gas type, concentration, and pressure.
Rock Hardness and Strength*
Problems can occur when the rock to be excavated is (a) harder and more
difficult to penetrate than anticipated, or (b) less competent than an-
ticipated. Information about rock hardness and strength is important,
and may be critical, to the success of the tunneling operation.
Construction Impact
Rock hardness and strength affect drill or cutter penetration rates and
equipment wear. At least to some degree, strength affects stand-up time.
The contractor's excavation method, equipment selection, labor esti-
mates, and round-cycle times are based on the preconstruction assessment
of rock hardness and strength. Inadequate information on these factors
introduces risks which may lead to undesirable contingency pricing or to
later disputes. Construction will be significantly affected when rock
hardness and strength turn out to be materially different from those an-
t~cipated. A revised excavation approach and changes in equipment are
required, leading to delays and attendant cost increases.
The adequacy of available information on these parameters affects
the bidder's ability to determine the competency of the rock to be self-
supporting. For TBM operations it is important in determining advance
rates, cutter costs, and types of cutters to use; for drill-and-blast
operations it affects round cycle time and costs of labor and equipment.
Data (and Interpretations) Available Prior to Bid
In any one type of rock formation the strengths may vary by several
thousand psi, or even tens of thousands of psi. Hence, although uncon-
fined compressive strength provides excellent data overall with respect
In this report the term "rock strength" generally refers to the uncon-
fined compressive strength of intact rock cores.
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to a single sample of rock, it has been found that a minimum of 50 to 60
samples per litholog ic unit at tunnel depth are needed to obtain a good
statistical average or range of strength for making j udgments as to
equipment and methods of excavation.
For example, compressive strength has been used for at least 28
years by a major machine manufacturer to predict TBM performance. With-
in a given rock type, compressive strength is useful (although perhaps
not definitive) for predicting TBM progress. When the rock cutter's
normal force load exceeds the compressive strength of the intact rock,
TBM performance is good.
There is no agreed test for abrasiveness, but the percentage of min-
erals with Mobs hardness of 5.5 or greater is commonly reviewed to esti-
mate cutter wear. Silica (quartz) is among the most abrasive minerals.
Percentage by volume of silica in rocks is best determined by thin sec-
tions, but in practice an approximation based on standard petrology
textbooks is often used. Indications are that, at present, an insuffi-
cient number of silicate content determinations are being made of the
material to be excavated (e.g., granite and sandstone). Knowledge of
silica content permits more realistic estimates of the abrasion to which
excavation equipment will be subjected.
The subsurface exploration program, therefore, should be set up to
obtain a level of information that enables the bidder to make reasonable
assumptions in regard to strength and silica content, thereby reducing
uncertainties and lowering project costs.
Deviations in Rock or Soil Elevation
When bed rock is unexpectedly found to protrude to a point within the
excavation limits of an underground opening, or when soft ground in-
trudes into a rock tunnel, it is usually a ser ious problem. The adverse
effects of high rock in a soft-ground tunnel will vary in degree depend-
ing on the extent of the rock, on the elevation of the top of rock, and
also on whether or not what results is a mixed hard-rock/soft-rock face.
For a mixed-face tunnel from inception, the height or top of rock above
the invert will affect both the top heading and bench operations.
Construction Impact
In the case where the rock is higher than expected, the top heading or
the steel support will not have the proper configuration to fit the tun-
nel and will have to be modified. Also, the excavation and mucking
equipment may well prove to be too large for efficient use. Conversely,
in the bench operation the drilling equipment may have booms which are
too high to accommodate vertical drilling. Thus, a change to horizontal
drilling may be required. Because the volume of material to be moved
will be larger than anticipated, the original muckers may be too small.
In a situation where the rock is lower than expected, the top
heading steel will be "too short" and will require expensive splice
welding. If the top-of-rock variance is large enough, multiple top
drifting may be required. In a case of lower top of rock, however, the
bench operation may escape without major adverse impact.
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In the case of a full-face soft ground tunnel from inception, a rock
intrusion, particularly if continuous over any distance, may be disas-
trous. The shield will have been designed to excavate soft ground, not
rock. Blasting will be required and the shield skin, doors, and hydrau-
lics may be subject to critical damage because blast-proofing may not
have been incorporated in the design of the shield. Additionally, the
excavating tool, or "digger," may be too light to excavate even loosened
rock on a sustained basis. In the extreme case of high rock, the shield
may have to be abandoned, necessitating a top heading and bench oper-
ation.
The scheduling impact of a change in construction method may be
great because it will have been unexpected; support steel and other top
heading excavation equipment will not be readily available. A similar
problem will exist with regard to equipment needed for the benching
operation.
Data (and Interpretations) Available Prior to Bid
Core boring is the most reliable indicator of top of rock and of the
weathered transition zone between top of rock and overlying soil. How-
ever, uncertainty will persist for the area between borings because that
area is unexplored; only inferences can be drawn from borings. Seismic
surveys may yield more continuous data, although the quality of those
data is inferior to that gained by coring.
Both core borings and seismic data should be obtained for use in lo-
cating the top of rock. In the case where rock is thought to be close
to an otherwise full-face soft-ground tunnel, or where soft-ground is
thought to be close to an otherwise full-face rock tunnel, extra data
should be collected in order to rule out the (potentially disastrous
possibility of a significant intrusion.
Compressed Air
Compressed-air techniques are confined mainly to earth tunnels driven
through pervious, water-bearing soils or driven under or adjacent to
bodies of water. In shallow tunnels, such as those frequently found in
or near urban centers, excessive water inflow is generally the primary
concern, with face instability being an exacerbating factor. The use of
compressed air to combat face instability alone is more often encoun-
tered in tunnels at great depth or in very weak soils. Compressed air
may be the most reliable or only feasible construction technique for
completing a length of tunnel in certain soils below the water table,
especially when water inflow or face instability, or both, are expected
to be severe.
Construction Imoact
Compressed-air techniques are expensive and can impose risks to con-
struction and to personnel that are not encountered otherwise. There-
fore, serious effort should be devoted to finding an alternative align-
ment. If that process fails, the second most desirable approach is for
the owner to undertake a sufficiently detailed investigation program to
permit dictating procedures. If compressed air is not specified, then
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the low bidder may be the one who is gambling on success with a less
conservative {hence, less expensive) technique. That type of risk-
taking can have serious consequences; failure of the construction method
will shut down and delay not only the particular tunnel section involved
but also (usually) the entire system. The third approach is to select
the alignment according to a comprehensive subsurface exploration,
within the siting constraints for the project.
The decision to use compressed air is not one to be made lightly;
although the technique overcomes some potentially serious construction
problems, it introduces unique problems of its own. Any compressed-air
operation requires the mobilization of expensive equipment in the form
of compressors and air locks. Personnel costs rise because workers must
be medically certified for fitness to work in the environment, and medi-
cal personnel must be available to handle emergencies. In addition,
operational complications and slow-downs are introduced, the most obvi-
ous being that personnel, equipment, and muck must all use air locks to
enter or exit the compressed-air working environment.
At moderate levels of pressure (generally 12 psig or less), the pen-
alty for compressed-air tunneling is not significant in comparison with
more conventional methods. For example, compressor size and capacity
are relatively modest and working shifts are shortened only slightly.
With increasing pressure, compressor expenses mount, but the largest
cost increase is caused by shortening shifts to protect worker health.
When working shifts are limited to one-half or one-third the length of
the usual eight hours, several crews may be required to perform work
normally accomplished by one. Moreover, increasing pressures require
additional time for gradual compression and decompression to protect
against the bends. Thus, the contractor is paying full salary to per-
sonnel who must spend hours of the shift in relatively unproductive work
while confined to the air locks.
In addition to inconvenience and expense, compressed-air tunneling
is subject to unique and self-induced hazards, especially at higher
pressures. For example, the sudden loss of air (i.e., a blow-out)
through an anomaly such as unexpectedly thin overburden can result in
crippling decompression or extensive flooding. Fire is also a major
concern, because a blaze that would be minor in free air can be a con-
flagration in a compressed-air environment. This consideration requires
cautious construction procedures, eliminating the use of many common ma-
terials such as wood lagging for initial support. Certainly it should
severely limit the use of compressed air in gassy ground where explo-
sions are already a potential hazard.
Data (and Interpretations) Available Prior to Bid
In evaluating the feasibility of using compressed air, it is essential
to determine the density and shear strength of the soil, the grain size
distribution, the cohesion, and the hydrostatic and overburden pressures.
Applied air pressure must be given careful analysis because it affects
the safety of crews, adjacent structures, and equipment.
Groundwater levels, flow rates, soil classifications, and soil prop-
erties (particularly density and permeability) are data that must be de-
termined to arrive at a decision about the need for compressed air to
control hydraulic pressure and water inflow. Hydrostatic pressure,
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porosity, and shear strength must be well defined in order to permit a
reasonable analysis of required air pressure, and thus of production and
cost.
When compressed air is being contemplated for a project, considera-
tion should be given to offsetting the exploratory borings from the tun-
nel alignment to minimize piercing of the tunnel opening. Special care
should always be exercised in sealing exploratory borings in soil and
rock.
Geologic and hydrologic investigations may reveal that the proposed
tunnel will encounter a poorly graded gravel, rock, or old timbers as
well as high head and low shear strength, etc. If so, the use of
compressed-air methods could be severely restricted or inappropriate.
As an alternative in such cases, pipe jacking or preconstructed tubes
sunk in place in pre-excavated underwater trenches may be solutions.
COST CONSEQUENCES
An important consideration in devising an exploration program is the
need to correlate the actual investigations to the problem areas of con-
struction, as well as to provide the basis for design assumptions and
engineering cost estimates. The investigation data and interpretations
should be available to the designers during the design stage, as well as
to the bidders who need answers for the problem areas, or an opinion, or
a statement that "we do not know." The design assumptions and criteria
for temporary structures should be clearly stated, with explanations and
qualifications included. Possible or alternative solutions to problems
identified as a result of the investigations should be detailed in the
contract documents, with provisions for approval of procedures and pay-
ment (which should be accomplished in a prompt and ongoing manner during
prosecution of the contract). In addition, if the data are presented
with "unknowns," an equitable solution should be offered in the contract
documents for the possible costs due to lack of information. The equi-
table solution should be a clear statement of the assumed expected
behavior of the soil and/or rock with regard to the anticipated con-
struction. Thus, bids can be appropriately prepared as a function of
production estimates based on a common anticipation of support criteria,
soil/rock behavior, and acceptance of risk by owner and contractor.
Common to all of the problem areas identified in Table 1 at the be-
g inning of this chapter is the fact that had they been identif fed pr for
to construction, the disruption of construction caused by the unexpected
would have been eliminated. If disruptions do not occur, the non-
productive cost of delays and inefficient ies can be avoided.
Performance of effective geotechnical investigations can minimize
these costs by g iving both designers and contractors a better under-
standing of the conditions to be encountered. If problem areas have
been detected and the possible consequences recognized at an early stage
in the design/construct process, all necessary design and construction
activities can be geared to overcome them. In a sense, then, they are
no longer "problem" areas. The cost of overcoming these conditions will
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have been included in the proj ect budget, rather than appearing later as
cost overruns, magnified in amount by disruption costs and, too often,
litigation costs as well.
However, subsurface investigation is something less than an exact
science, and not all problems can be predicted. Therefore, provision
should be made for clear definition and allocation of risk and asso-
ciated cost. Establishing a baseline of information and assumptions
concerning subsurf ace conditions will benefit both owner and contractor.
The owner will receive a reliable cost estimate from the designer, and
the contractor can be accorded appropriate adjustments in the contract
and price for conditions varying materially from those assumed.
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Representative terms from entire chapter:
compressed air
