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OCR for page 113
8-
Conclusions and Recommendations
The basic objective of this study is to discover improvements in practice
and procedures that will enable planning and conducting more ef fective
geotechnical s ite investigation programs. This chapter presents the
subcommittee's judgments on matters that bear on achieving the study
objective.
The conclusions drawn generally offer
practice and areas that could be improved.
a view of current industry
~ In a few cases the conclu-
sions are s imply observations of fact and required no particular analy-
sis or deliberation. The reader will find some suggestions for changes
in current practice--suggestions that are implicit in the way the con-
clusion is stated.
The recommendations offer the more specific statements on how
i .
the
tunneling industry can generally upgrade subsurface investigations and
expand their uses. Eight of the recommendations are firm proposals for
policies that can be implemented within a short time. The others con-
cern areas where research and development would benef it predictions and
exploration techniques.
The judgments presented herein are not all strictly verified by the
data contained in the case histories. Some of the judgments were influ-
enced by subcommittee experience and knowledge of projects that could
not be documented in detail, but include many more than 87 projects over
a 20-year time span. Even though the bases for development dif fered,
all of the conclus ions and recommendations are equally valid in the view
of the subcommittee. In addition, although the subcommittee's study was
conf ined pr imar fly to mined tunnels at relatively shallow depth, the
f indings can be applied to most underground construction proj ects
because the principles of subsurface investigation and contracting are
· . .
so same ar.
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,.
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CONCLUSIONS
It is in the owner's best interests to conduct an effective and thor-
ough site investigation and then to make a complete disclosure of it to
bidders.
An owner has no legal duty to conduct a site investigation. However,
if one is conducted, a variety of legal precedents would make the owner
responsible if actual site conditions are found to differ materially
from those indicated by preconstruction subsurface explorations. Pre-
contract uncertainty as to risk promotes increased costs for contingen-
cies; post-contract uncertainty as to risk allocation fosters disputes
and litigation.
Disclaimers in contract documents are generally ineffective as a matter
of law, as well as being inequitable and inexcusable in most circum-
stances.
Unexpected subsurf ace conditions are the primary cause of disputes
and litigation ar is ing from contracts for underground construction. The
geotechnical investigation is a central element in the definition and
allocation of risk. Disclaimers of information supplied are an inade-
quate means of managing risk. It is the policy of most federal agencies
and many owners to bear the risk of subsurface uncertainties and provide
for differing site condition and changed condition clauses.
Contracting documents and procedures can provide for resolution of un-
certain or unknowable geological processes or conditions before and dur-
ing construction, rather than afterwards.
The provision of clauses covering differing or changed conditions
does not necessarily also provide a mechanism for prompt resolution of
the issue. Adopting a baseline of risks (or a basis of geotechnical
data) before construction would permit timely recognition of a contract
change and provision for cost adjustment during construction, if the
conditions encountered vary materially. This should assist in reducing
or eliminating contingencies for possible delays and disputes, and lead
to more realistic cost estimates and more competitive bidding.
On major projects especially, it is important that (a) the owner employ
a multi-disciplined team including engineering geologists, engineers, and
a construction specialist to develop subsurface data and evaluate their
impact on design and construction; (b) designers and geologists possess
a thorough working knowledge of construction methods and equipment so
that the proper geotechnical data are secured and design is consistent
with construction systems; and (c) contractors employ geologists experi-
enced in underground work to evaluate and interpret the data provided at
the time of bidding, thus ensuring that all the information obtained is
fully considered in preparing bids.
The most extensive and effective geologic site investigations are of
limited value if not incorporated fully into the design, estimating, and
bidding processes. Too often, the significance of geologic site condi-
tions is not emphasized appropriately in siting, budgeting, and design.
Important information either may not have been considered due to poor
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communication between various disciplines, or may never have been ob-
tained due to a failure to recognize the need. Current and developing
underground construction methods demand greater attention to the collec-
tion and application of geologic information. The exploration programs,
interpretation techniques, and even the potential of the investigators)
to conduct an effective investigation must be evaluated. It Is essential
that the owner, designer, and contractor know when additional skills or
knowledge may be required. It is the user's responsibility to help
ensure that geotechnical investigations reduce, rather than contribute
to, risk and the incidence of unanticipated adverse conditions.
Site investigations have to proceed through, but should not always end
with, completion of the feasibility/alignment setting/final design pro-
grams.
Owners must recognize that the preconstruction site investigation
should be an iterative process. A project comprises several phases, and
appropriate data must be collected and analyzed to support the require-
ments of all phases. Anomalies should not be left unresolved by the
presumed "final" program, but further explored by another program, and
then another, if necessary. All geotechnical data that an owner can
sustain economically should be developed. This philosophy should extend
to developing additional information when it is important for good bids,
even if the information no longer is directly relevant to the design
itself. For example, an easily performed but generally ignored investi-
gation procedure is the continued reading of groundwater levels in ob-
servation wells as long as there is time to print the information for
use by bidders. There Is always the possibility of a late-developing
change in the groundwater table having major ramifications for construc-
tion operations. Moreover, it is not too late to continue exploration
after a project is already let for bid; bidders may require data that
entails additional exploration. In some instances, post-bid and even
post-award investigations may be justif led.
Procedures for logging, documenting, and preserving samples from bore-
holes require improvement.
Boreholes should be observed and logged by experienced engineering
geologists. Modern drilling techniques and equipment should be used to
allow optimal core recovery. Color photos of all cores should be taken
soon after removal from the borehole in order to document the condition
of the cores at the time of drilling. Cores frequently deteriorate with
time; samples are removed for testing and, through handling, are mixed
up or disturbed. Efforts should also be made to preserve cores until at
least the completion of construction. Permanent retention of the cores
at the project site or an associated facility would be the most desir-
able approach. For cores that deteriorate rapidly, special preservation
techniques such as wrapping in plastic or sealed tubes may be necessary
for adequate preservation.
Soil sampling procedures are also in need of improvement and stan-
dard' zation. For example, soil sampling should be essentially cons, nuous
through the level of the planned tunnel. Use of high torque equipment
such as rotary drilling or hollow-stem augers should be restricted in
overburden, par t icularly below the planned crown of the tunnel.
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Geophys ical methods can be used to advantage, especially in coordination
with boreholes.
Geophysical methods have the potential to greatly expand knowledge
of the subsurface when used to interpolate between boreholes. Many geo-
physical techniques are not widely used or applied to construction proj-
ects, but are worthy of continued investigation and development. Seismic
refraction surveys profiling tile rock surface between boreholes help
eliminate the problems associated with high rock in the invert of a soft-
ground tunnel and soil i ntrusion in the crown of a rock tunnel. Other
techniques such as resistivity' gravity and magnetic survey can be used
to identify anomalies where borings should be made . Ideally, geophys i-
cal surveys should be performed prior to dr illing the final des ign bor-
ings to allow optimum placement of bor ings to check different conditions
indicated by the geophys ical surveys .
Groundwater and its effects on the subsurface materials merit greater
attention in exploration programs.
The presence of water accounts, either directly or indirectly, for
the majority of construction problems. Most major tunnel projects should
have one or more long-term pump tests, executed in accordance with good
standard practice and conducted so as to test the various formations and
conditions to be encountered during construction. These tests should
include observation wells to directly observe pumping effects, as well
as drawdown and recovery. Chemical tests of groundwater should be per-
formed on a routine basis. Recent advances in computer modeling of
groundwater flow may have applications in improving the ability to pre-
dict f low into the excavation, and thus are worthy of investigation.
Laboratory testing of the subsurface materials generally needs to be
increased.
Experience has shown, for example, that in rock tunnels at least 50
to 60 unconfined compression tests for each significant lithologic unit
are necessary to adequately characterize the range and means of strength
values. Silica content is rarely determined in testing programs, yet it
is an important parameter in allowing the contractor to predict advance
rates and abrasive wear on equipment. In the same vein, sufficient and
careful testing of overburden and soft-ground material is important.
Truly adequate testing calls for supplementing standard split spoon sam-
ples with undisturbed samples from each stratum or zone that affects the
tunnel. It must be noted that testing of disturbed rock or soil samples
places severe limitations on the value of the resulting data.
Exploratory edits and shafts are generally justified only when abso-
lutely essential to obtain critical design data or when a substantial
benefit to construction is indicated.
These exploratory techniques are very expens ive and are of ques tion-
able cost-benef it in many cases . In some cases, pilot tunnels have
actually increased problems dur ins construction of the pro ject; misalign-
ment or exceptionally poor work in the edits or shafts may increase the
cost of the final opening. An alternative view is that a significant
portion of the pilot tunnel or shaft nay be charged to subsequent work
if the final opening incorporates the pilot tunnel or shaft. Generally,
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however, the money expended on an exploratory edit or shaft may be used
more effectively for additional boreholes, groundwater investigations,
laboratory testing, or engineering evaluations.
Maintenance of technical data obtained during design and construction
of underground projects often is not pursued by owners or demanded of
their consultants and contractors.
A surprising quantity of exploration, design, and construction data
is poorly recorded, filed without easy access, lost, or discarded by
owners, construction contractors, and others. In conducting this study,
the subcommittee found that records for older projects, as well as for
some more recent projects, were often difficult to locate or impossible
to obtain. This was because they had either been stored in a manner that
discouraged file searching, or simply destroyed. For newer projects,
the difficulty in locating information was generally caused by poor rec-
ordkeeping. Although this was more often the case for agencies involved
in only one construction program, records were not-always reasonably
available for reference from agencies that build and operate tunnel after
tunnel. Experience has shown that relatively few major underground proj-
ects fail to develop problems during their operational lifetimes. In
many cases, data obtained in the exploration, design, and construction
phases of the project are essential to defining the cause of the problem
and the best method of correction. If records are not available, the
data must be obtained again and the procedure can be time consuming and
costly. The difficulty and expense involved in securing suitable data
can sometimes lead to inadequate or even "patchwork" solutions.
RECoMMENDATIONS
Expenditures for geotechnical site exploration should be increased to
an average of 3.0 percent of estimated project cost, for better overall
results.
The low level of expenditure typical of current practice does not
correlate well with estimated and actual costs or with construction
problems and claims. Overall, increas ing exploration can be expected to
decrease the incidence and severity of construction difficulties and
eliminate a significant portion of the extra costs associated with unan-
ticipated geologic conditions, including project delays, claims, and
litigation. Increased exploration should lead to more reliable engi-
neers' estimates and owners' budgets, as well as more accurate bids. It
is possible that increased exploration would result in higher engineer's
estimates and higher owners' budgets, thereby reducing the direct cash
savings resulting from fewer claimse However, savings still would ac-
crue from eliminating attendant delays, lawyers' fees, and hidden costs.
The level of exploratory borings should be increased to an average of
1.5 linear ft of borehole per route ft of tunnel alignment, for better
overall results.
Current boring practice is not consistent with the evidence that
boreholes are the best single exploration technique for providing
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reliable information to designers and contractors. Borings provide
actual physical samples for direct observation and testing, a feature
that makes them less subject to misinterpretation than more indirect
methods. However, some factors (including the great depths and/or dif-
ficult surface access of some sites) prevent this investigation tech-
nique from being given the intensive use it merits. Exploration at 0.6
lin ft of borehole per route ft generally initiates a decrease in the
deviations between the final tunnel cost and both the bid price and en-
gineer 's estimate. However, an increase to 1.5 produces more desirable
results. Beyond this level of effort, the risks of geologic uncertain-
ties, although not eliminated, may be reduced to the point of diminish-
ing returns for borehole footage drilled as a matter of general practice.
The optimum level for borehole footage entails an increase higher in
magnitude than the optimum level for exploration expenditures, but the
recommendations are not incompatible. A substantial portion of the cost
of any drilling program is devoted to initial mobilization, and the more
modest programs incur maximum charges per ft of boreho'e. However , as
the number and/or depth of boreholes increases, the unit prices flatten
out and even decrease. Moreover, the cost of the overall exploration
program includes expenses for report writing and other miscellaneous
items which do not rise in direct proportion to borehole footage.
The owner should make all his geotechnical information available to
bidders, while at the same time eliminating disclaimers regarding the
accuracy of the data or the interpretations.
In the past there has been a tendency among owners to give bidders
as little of their interpretive information as possible in order not to
be held responsible for any mistakes made in extrapolation from hard
data. Owners would make available the logs of boreholes--because they
are presumably factual--but withhold the geologic reports because of
their interpretive nature. The result was that various contractors were
bidding on different bases, depending on their personal experience or
access to knowledge apart from the boring logs. Bidding contingencies
tended to be high to cover the construction unknowns. This situation is
undesirable and can be mitigated significantly if the owner will present
all the geotechnical information, and without disclaimers. The owner
bears some responsibility for errors in the subsurface predictions, but
it creates a fairer bidding atmosphere and can ultimately lower construc-
tion costs.
All geologic reports should be incorporated as part of the contract
documents .
Some owners follow the philosophy of making all of their subsurface
data available to bidders, but not making it a binding contract document.
The material is presented for examination, yet not provided or sold with
the contract drawings and specifications. Geotechnical documents made
available in this manner are often accompanied by a disclaimer stating
the owner will not be held responsible for any interpretations or use
made thereof. One consequence of this procedure is that some bidders
may not rely on the information in spite of its pass ible accuracy and
may not plan their construction operations with all salient facts in
mind. A second consequence is that if litigation over changed condition
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claims is instituted, much time can be spent in arguments over whether
the geologic information (or misinformation) can or cannot be blamed on
the owner. The procedures should be simplified, even though the owner
will then be more surely liable for any errors in interpretations. The
result--more consistent and accurate bids--will be worth the added
responsibility.
Designers of mined tunnels should compile a ~Geotechnical Design Re-
port, ~ which should be bound into the specifications and be available
for use by bidders, the eventual contractor, and the resident engineer.
A geologic site investigation is generally completed by the middle
stages of design and, therefore, the geologic report cannot comment on
many of the late-developing plans worked out by the designer. As a
result, bidders are uninformed on many important design/construction
matters that may have been given serious consideration prior to the let-
ting of bids. The goal of the Geotechnical Design Report should be to
explain the geotechnical rationale for design and the anticipated effect
of geology on construction. Such reports should result in much better
informed bidders, improved construction procedures, and probably lowered
costs associated with a reduction in bidding contingencies and changed
condition claims. The WMATA Geotechnical Design Reports (Appendix D)
illustrate standard items that should be described i n such reports. In
addition, including one of the systems for rock classifications (e.g. ,
RSR, BAR, Q-System, or Terzaghi) may be useful, provided that the system
is appl fed proper ly .
Monitoring of ambient conditions prior to construction should be under-
taken to establish a baseline of information for comparison during and
at ter construction.
Records of specific data can be useful in preventing or settling
disputes related to construction conditions or effects, as well as in
protecting both owner and contractor from frivolous claims. The process
can range from visual inspection of structures within a zone of vibra-
tion, to a detailed survey of existing damage in adjacent structures, to
long-term measurement of groundwater levels. For construction in rock
where drill-and-blast procedures are applicable and sensitive structures
exist at the site or nearby, preconstruction blast/vibration/noise/sen-
sitivity measurements should be made to compare with later effects and
to use in establishing a public relations program. A crack survey, ele-
vation benchmarks, and vibration measurements of non-construction activ-
ities should also be undertaken.
Pre-bid conferences and site tours should be conducted to ensure that
all bidders have access to the maximum amount of project information.
The end result of a subsurface investigation should be to place as
much geotechnical information as possible in the hands of bidders. A
good site tour can help accomplish this by allowing bidders to get the
slay of the land" and see the physical features of the project for them-
selves. However, such tours may lose some of their effectiveness if not
conducted by a knowledgeable owner representative. Those bidders not
familiar with the territory or the project can miss important features by
being left to discover everything for themselves, and bidder ignorance
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is not in anyone's best interest. In cases where a test edit or shaft
has been constructed, the conducted site tour becomes a matter of even
more concern.
In the same way, a pre-bid conference is a good way of assuring that
all potential bidders have an opportunity to clarify any confusing issues
in the contract documents. Some owners choose not to spend time with
such conferences because attendees tend to raise few issues for fear of
revealing to competitors their amount of knowledge or their bidding
strategies. This situation should not deter owners from making the
effort. A conference should always include an oral geotechnical brief-
ing by the project designer. This feature is especially important where
some policy or circumstance has made it difficult for bidders to obtain
the appropriate geotechnical reports or boring information. In addition,
the bidders' responses to that briefing can assist the owner in evaluat-
ing the effectiveness of the site investigation.
Geologic information from preconstruction explorations and as-built
tunnel mapping and construction procedures should be compiled in a report
detailing project completion.
It is rare to f ind wrap-up reports that descr ibe the mapped tunnel
geology and construction procedures, even among owners who build tunnel
after tunnel. Without such a report, there is no formal way for an owner
to confirm geologic predictions and find out where assumptions were right
or wrong. There is also no easy way to resurrect records of operations
and apply the experience to future projects in order to avoid the repe-
tition of errors. Such information can be invaluable in the event of
damage to or malfunction of the tunnel during its operational life.
There are cases where post-construction problems (e.g., drain clogging,
lining distress) were difficult to diagnose and correct because actual
construction (or geologic) conditions were not recorded. As a minimum,
the as-built geotechnical conditions should be reviewed by the original
exploration team. If practicable, the original team should assist in
the post-construction mapping. It is in the owner's interest to create
such a record for improving design, contracting, and construction manage-
ment techniques. Expense would be involved because the "as-built" report
could approach the size of the original design report, but it would be
to the owner's long-term economic benefit to engage in the effort.
Investigation methods and predictions should be improved for three spe-
cif ic conditions: in-situ stress, stand-up time, and groundwater .
In-situ stress is one of the conditions not always adequately pre-
dicted by designers. A.better understanding of the geologic history of
the site is needed, e.g., eroded cover, normal variation of rock
strength, tectonic activity. However, merely paying more attention to
the situation during exploration might not be as effective as hoped,
because the instruments and predictive techniques need further develop-
ment. Research is especially needed for predicting stresses at great
depth (more than 1,000 ft), particularly when coupled with below average
rock strength (less than 6,000 psi compressive strength).
Estimates of stand-up time developed from information available prior
to construction are usually indefinite (or not provided). Reliable esti-
mates are important for design and bidding . Stand-up time is a ma jor
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consideration in selecting appropriate construction methods, equipment,
and support system. The Rock Mass Rating (KMR) system may show promise
here. In addition, RQDs (when properly determined and recorded), coupled
with close inspection of joint and fracture conditions (roughness, fill-
ing materials, degree of continuity, spacing, and amount of opening) are
useful tools of a semi-quantitative nature.
The occurrence, behavior, and effects of groundwater account either
directly or indirectly for the majority of problems encountered in under-
ground construction. This situation is a strong indicator of the need
for research and development. First, there is a lack of good quality
field pump tests--pump down with observation wells, along with recovery
tests. Second, there is inadequate understanding of the effects on
ground stability that can result from even a small amount of water flow.
In rock tunnels, small quantities of water can substantially reduce fric-
tion along joint surfaces; its exit pressure can dramatically affect
otherwise stable rock. Water can also cause swelling and induce squeez-
ing in certain types of rock. Development of a data base would assist
in sorting and evaluating the complexities of the problems presented by
groundwater. One effective and relatively inexpensive way to establish
a good data base would be for owners and contractors to begin keeping
careful records on quantities of groundwater flowing into the various
reaches of tunnels during excavation. Currently, such data are recorded
on an irregular basis, and thus much valuable information is irretriev-
ably lost. Ideally, the records should be supplemented with notations
regarding the nature and extent of any problems and the effects on con-
struction.
Improved horizontal drilling techniques should be developed that can
recover rock core and penetrate long distances without wandering from
line and grade.
The need is especially severe for tunnels beneath mountains where,
except for portal areas, difficult access and/or great depth generally
make the necessary number of vertical boreholes prohibitively expensive.
The ability to core drill accurately from a portal and along the tunnel
alignment would help investigators to determine not only what is there,
but also the true boundaries and thicknesses of geologic features as
they would ultimately be encountered in the advancing excavation.
Research and development should be conducted to expand the capabilities
of geophysical or other remote sensing methods for obtaining geotechnical
data between boreholes and from the surface down to depths too great or
too costly for boreholes.
Although boreholes provide the best kind of geotechnical information
from within their own confines, interpretation or extrapolation is essen-
tial to project that knowledge to some useful distance beyond the bore-
hole. A higher degree of interpretation/extrapolation is required to
glean information from depths too great for economical borehole penetra-
tion. The process can be greatly abetted by reliable techniques of geo-
physics and remote sensing. However, in comparison with some industries
{e.g., petroleum exploration), engineering investigations make minimal
use of these more indirect methods of data collection. A major reason
is their relative lack of preciseness, which can lead an owner to the
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uncomfortable perception that the data are readily subject to more than
one interpretation. Considering the ability of remote sensing techniques
to cover continuous extents of ground, subsurface investigation would be
vastly enhanced if those techniques could be developed to the point that
their results were as reliable as borings and trusted equally by both
designers and contractors. It should be noted that some federally
financed research on deep remote sensing methods is being conducted; the
methods are showing promise but still require extensive testing to prove
dependability.
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Representative terms from entire chapter:
geotechnical information
