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Geotechnical Site Investigations
Once, an "adequate" tunnel exploration program consisted of a boring at
each portal and another boring halfway down the tunnel line. In many
cases this was an adequate program because the tunneling design methods
were very conservative and the construction methods were easily adapted
to a variety of ground conditions. However, in some cases serious prob-
lems developed, and ultimately the costs of providing a contingency for
every possible situation became excessive.
The development of rock tunnel boring machines in the late 1950s was
the precursor of numerous faster, more efficient, and less labor-inten-
sive tunnel construction techniques. A similar development has occurred
in tunnel design, bringing, for example, more sophisticated tunnel liner/
ground interactive analysis which permits the use of thinner, stronger,
and safer lining systems. These developments, however, have not been
without their price. As tunnel engineering has become more exact, it
has demanded more exact prediction of ground conditions to make the im-
proved techniques work.
In many cases the parties responsible for the exploration of under-
ground excavations--including shafts, tunnels, chambers, and underground
mines--have risen to meet the need for better predictions. However, the
number of disputes arising from unanticipated adverse effects of ground
behavior on a contractor's operations has also risen. It is also true
that the development of new tunnel concepts, designs, and construction
techniques is continuing and that the demand for accurate data about the
ground to be excavated will increase. For example, the safe operation
of a proposed underground nuclear-waste storage facility must be predi-
cated on a total understanding of geologic and hydrologic regimes at the
site.
Field and laboratory techniques to be used for developing the re-
quired site investigation data are not addressed in this chapter. These
techniques are described in a number of publications, some of which are
listed in the Selected Bibliography that accompanies this report. Those
publications provide guidance to the factors that must be considered in
underground site investigations, such as the types and methods of explo-
ration, the number of explorations and their locations, and the kinds of
tests. Descriptions of specific underground projects and their particu-
lar problems are provided in later chapters of this report.
This chapter evaluates the advantages and liabilities associated
with site investigations, discusses the use of site investigation data
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and the basis for providing data to potential users, considers the con-
tent of the geotechnical report, and notes the effects of geology on the
cost of construction. Understanding the general rationale for geotech-
nical site investigations is essential to the case history analysis that
constitutes the primary thrust of this study. This chapter is intended
to set forth an overview of the concept and functions of the site inves-
tigation process. The overview, in turn, serves as a link between the
data and the conclusions developed from interpretation of the case his-
tories.
ADVANTAGES o RISKS ~ AD LI - ILITIES
A technically sound and thorough geotechnical site investigation program
is an essential ingredient in obtaining the lowest fair cost for under-
ground construction. To accomplish that end, the program must not only
be optimal in design for the particular conditions at the site, but must
also be sensitive to needed refinements in the scope of traditional
data, data reporting, and interpretation in order to take full advantage
of new cost-reducing construction methods, equipment, and concepts in
project design. Along with fairer tunneling cost, other advantages that
will result include greater project suitability, longevity, and safety.
The adequacy of a site investigation program cannot be measured by
cost alone, because the cost, however large or small, is not always a
valid indication of effectiveness. The ultimate goal--wh~ch is to de-
termine with reasonable accuracy the nature of subsurface formations and
how they will react or behave during tunneling--comes at highly variable
costs, depending on the state of prior (or equivalent) tunneling knowl-
edge in the area, as well as on the geologic complexity of the proposed
site.
The knowledge and skills necessary to achieve a sound and thorough
geotechnical investigation are not possessed by all investigators.
Therefore, the user {owner, designer, contractor) of the geotechnical
data should be responsible for evaluating the investigator's capability
to conduct an effective exploration program, and to know when special
skills or additional knowledge may be needed. As a case in point,
knowledge of construction methods and equipment is essential to the
investigator's capability to plan and manage an effective site investi-
gation. Moreover, such knowledge could be critical where specialized
tunneling equipment or methods are being considered.
There will always be significant physical and financial risks asso-
ciated with tunnel construction. The use of new, specialized equipment
and techniques may actually increase those risks. To consistently re-
duce the risk potential, the emphasis of future geotechnical investiga-
tions must be directed toward optimizing the scope of investigation and
data reports for each site. Critical attention must be given to the
prospective use of new tunneling equipment and techniques, and to the
ability of the investigator to provide an exploration program and
evaluation suited to the equipment and techniques.
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However, it must be recognized that geotechnical investigation for
underground structures is not an exact science; not all problem areas
can be predicted. Owners must recognize this circumstance and provide a
contractual method for defining and clearly allocating the risk and as-
sociated costs. Experience has shown that the best way to define and
allocate the risk is by establishing a baseline of geotechnical data,
interpreting the data using the best possible talent, and presenting the
interpretations to all the bidding contractors. If, after exposure in
the tunnel, the geotechnical conditions encountered vary materially,
then an equitable change in the contract should be recognized, and a
cost adjustment should be made.
Establishing these geotechnical and contractual parameters will lead
to more realistic cost estimates by both designers and contractors, al-
low more competitive bidding, and eliminate the need for most contingen-
cies in the event of adverse conditions. The result should be a reduc-
tion in the incidence and degree of cost overruns.
PHASING OF GEOTECHNICAL EXPLORATIONS
Site investigation is an iterative process. Early in the first phase of
project investigations (during the planning stage, for example) maximum
use should be made of existing data, including past local experience,
available literature, and field examination of all of the potential
sites. The aim is to gather as much information as possible at the low-
est cost, since the viability of the project is still unknown. The
emphasis should be on defining regional geotechnical aspects and condi-
tions.
If the project is continued, the second phase of the geotechnical
investigation should build on the knowledge gained in the previous phase
to begin establishing the specific site characteristics. For example,
an air photo analysis of the site should be conducted, geologic field
mapping accomplished, and a boring plan developed identifying the gener-
al characteristics of the soil and/or rock and the geologic structure.
Borings are usually widely spaced, and laboratory tests on recovered
samples emphasize the basic properties of the materials. Data should be
examined as they are produced to evaluate their validity, then plotted
to establish the materials through which the project will be excavated.
A preliminary design is often developed on the basis of second phase ex-
ploration. In this manner, the design engineer can identify those areas
for which there may not be sufficiently detailed geotechnical data to
permit the design to be developed.
Ideally, prior to proceeding with the third phase of the investiga-
tion, the accumulated data should be gathered and thoroughly analyzed by
experienced geologists, design engineers, and construction engineers.
The primary concern of this interdisciplinary team should be to identify
unexplored potential geotechnical problems that could affect the design
and/or construction. If the geology is simple, all of the potential
problems may have been identified and there will be no need for addi-
tional explorations. However, any potential problems should be thor-
oughly evaluated prior to final design.
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Thus, the need for a third phase of explorations depends on whether
questionable areas are identified by analysis of the earlier phases of
exploration. This phase should be planned carefully. It is often dur-
ing the third phase that specific features to be encountered by the
project are explored, such as fault zones, lithologic contacts, hydro-
logic conditions, and in-situ stress . The most important element of
this phase is input of the design eng ineer; this phase should provide
answers to specific questions regarding the alignment.
Although this third phase normally concludes the preconstruction ex-
plorations, in special cases additional explorations may continue if the
data required are slated for use by the bidding contractors. For exam-
ple, it may not be necessary for the design engineer to know specifi-
cally the bounds and volume of an aggregate source or muck disposal area
to be used on a project. However, those data may be critical to the
contractor bidding the job.
One important aspect of the geotechnical investigation that is often
overlooked is monitoring of conditions prior to construction. Monitor-
ing establishes a baseline of information for comparison during and
after construction. The process can range from visual inspection of ex-
isting facilities or structures within a zone of vibration or subsidence
to long-term measurement of groundwater levels. Records of specific
data can be useful in preventing or settling disputes related to con-
struction conditions or effects, as well as in protecting both owner and
contractor from frivolous claims.
USES OF GEOTECHNICAL DATA
The needs for geotechnical data were once relatively unsophisticated,
and were keyed primarily to site selection and design. hence the meth-
ods of investigation were simple as well, because construction tech-
niques were readily adaptable to adverse conditions. Current and devel-
oping underground construction methods are not so forgiving; they demand
greater attention to the collection of geologic information to permit
their efficient and economic use as an integral part of modern and fu-
ture practice. When we find that tunneling costs escalate because of
unexpected conditions that geologic studies have overlooked, it becomes
clear that we must reevaluate our exploration programs and interpreta-
tion techniques so as to improve the detection of adverse conditions, or
else be forever plagued by cost overruns.
Site Selection
Geotechnical evaluation should play a more significant role in the sit-
ing of an underground structure. In the past, tunnel site selection was
often based principally on geographic or solely on engineering consid-
erations; occasionally, portal locations might be changed slightly to
minimize adverse geologic conditions. Now, in many instances, geologic
considerations together with engineering considerations are entering
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into the planning and site selection processes. For example, the loca-
tion of a major interstate highway tunnel may be abandoned and a longer
route around a mountain adopted due to the prediction of difficult tun-
neling conditions, or the elevation of a water transmission tunnel may
be changed drastically because of difficult geology at depth.
The cost of excavation is the most significant part of many projects
and may be a critical factor, especially if geologic conditions are ad-
verse. Controlling or reducing the cost by improving site selection is
highly desirable. In addition to the economic benefits, an appropriate
site selection process will provide important contributions toward ac-
complishing project goals and objectives for the owner, designer, con-
tractor, and ultimately the user.
Design
The traditional approach to data collection is to answer questions or
determine the parameters that the designer needs for the project e Prior
to beginning final design, the designer should be provided with all data
collected, so that the need for and type of supplemental information to
be developed are decided according to the designer's specified inter-
ests. It behooves geologists and geotechnical engineers to understand
how the data they collect and interpret are used. The following is a
list of some design uses of geotechnical data:
Rock/soil classification and rock mass characterization
Tunnel configuration selection (horseshoe, circular)
Overbreak prediction (in rock tunnels)
Wall/face stability analysis (e.g., wedge failures, slaking,
squeezing)
Support system selection and requirements (e.g., loading
values)
Shaft and station location and layout
Groundwater prediction and control
Lining requirements (need for and/or type)
Grouting requirements (e.g., location, materials)
Subsidence prediction and control
Portal location, configuration, and stability
Alignment and invert elevation adjustments
Operations and maintenance
The traditional approach to geologic interpretation should be con-
stantly updated, modified, and expanded through the use of advanced
techniques. This can be accomplished by review of available techniques
and by continuing research. Constant attention to the application of
developing techniques will lead to cost-effective exploration programs
(cost-effective in the sense that the programs should be iterative and
able to identify and interpret anticipated underground conditions,
rather than to simply produce borehole logs). For example, a study of
jointing in a granitic batholith at a tunnel location may show that the
two major petrographic facies have different joint orientation maxima
because of different cooling conditions (stress history). However, when
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individual joint orientation diagrams include measurements from both
facies, the different maxima are superimposed and effectively disguised.
A stress history, indicated in regional studies, should alert the geolo-
gist and geotechnical engineer as to the need to determine the values of
stress in the rocks by a specific test program. Results can verify or
eliminate that concern for design--clearly a cost-effective procedure.
Gaps in stratigraphic sequence along a tunnel alignment may cause sig-
nificant construction problems if they are not discovered in the explo-
ration program. Cost-effective programs would insist that pump-out
hydrologic tests be carried to equilibrium, a necessary condition for
proper interpretation. Even inspecting different sets of aerial photo-
graphs may identify important features that viewing only one set would
fail to disclose.
Bidding and Construction
The particular construction methods best suited to the project will nor-
ma~ly be selected by each bidder according to the description of the
rock/soil character and behavior, hydrologic conditions, and site loca-
tion provided in the contract documents. Field data as reported from
borings (clarified by visual inspection), laboratory and field tests,
geophysical surveys, and geologic reports from the project and any
neighboring structures have an important status. These data will assist
the contractor in identifying the best methods of excavation, choosing
the size and type of equipment needed, estimating rates of advance,
selecting methods and stages of temporary support systems, calculating
anticipated rock overbreak, establishing groundwater control measures,
developing the contingencies which should be available for control of
fluids or gases, and determining the possible uses of excavated materi-
als. In addition to selection of methods and equipment, the geotech-
nical data will be important to the contractor in determining the price
and contingencies to be added. Hence, geotechnical data are equally im-
portant to the owner.
During construction, the contractor should be able to use the geo-
technical data and interpretations to predict the limits of each method
of temporary support and loading and to anticipate the need for any spe-
cial equipment. This helps to avoid delays in job progress and to
reduce safety problems for the workers and equipment. The methods and
procedures selected during the design and bidding stages may then be
maintained or revised slightly to meet the conditions as excavation pro-
ceeds.
Post-Construction
The geotechnical information obtained prior to and during construction
does not cease to be useful on completion of the project; rather, it
should serve several purposes that benefit the owner. Unquestionably,
such data are important to effective and efficient operations, as well
as maintenance, of the completed facility. The availability of this
data assumes even greater importance considering the increasing emphasis
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on repair, rehabilitation, and expansion of existing projects. In
addition, the owner should be able to incorporate the information into
an evaluation of the project, particularly exploration, design, con-
tracting, and construction management techniques.
REPORTS OF GE:OTECHNICAL DATA
Geotechnical Report
A geotechnical report should be produced prior to construction; specif-
ically, it should be compiled prior to or during design, and should be
included in the contract documents. The designer should be able to use
the report in developing the design concept; the contractor should be
able to use it as a basis for bidding. The report should include col-
lected data, interpretations of data, predictions of ground behavior,
and recommendations to the designer. (Note: Construction contract doc-
uments should include a statement to those other than the designer that,
if recommendations or other information in the report conflict with the
designer's statements in the specifications, then the specification's
statements shall take precedence.)
The geotechnical report should contain data collected in the field,
test results from the laboratory, information on regional geology, and
historical data regarding previous and existing work in the area. Such
data should include only observations and facts, and should be clearly
distinguished from the interpretation portion of the report. The degree
of confidence in or opinions as to the validity of the individual ex-
trapolations and interpretations should be made explicit.
Field data include borehole logs, geologic surface maps, geophysical
data, water levels in wells, occurrences of springs, gases, chemicals,
etc. Field observations should note all unusual features or conditions
that the field personnel believe may have some effect on design or con-
struction. Laboratory data should include standard properties of the
tunneling medium (rock or soil) and petrographic analyses, including
representative silica content in rock. Tests should be made that help
both the designer and contractor. The report should include an overview
of regional geology, including tectonic history, and regional seismic
conditions. This information will be useful in estimating the potential
for residual or in-situ stress. Historical data should include maps and
other information on previously constructed tunnels, mines, shafts,
highway cuts, quarries, and earlier geological/engineering investiga-
tions in the area. Researchers of the above data should strive to seek
both the standard "textbook" information and any other special data that
may assist the designer or contractor.
The geotechnical report should include interpretation of in-situ
tests, evaluation of in-situ stress conditions, and geological profiles.
Estimates of stand-up time and support requirements as calculated by
established empirical methods should also be included, along with a
listing of possible trouble zones. The anticipated groundwater inflow
zones and rates should be discussed, and the basis for their selection
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provided. The level of confidence should be presented candidly, with
recognition by the owners, designers, and contractors that this is among
the most difficult aspects of subsurface investigations.
Recommendations in the geotechnical report should cover estimated
loadings, applicable geotechnical properties, sizes of the zones of in-
fluence, and suggested tunneling methods. Such recommendations should
be considered from a geotechnical point of view, yet be aimed at the de-
signer. It is the designer's responsibility to study these recommenda-
tions and other information in the geotechnical report, combine it with
information from all other aspects of the job, and then make appropriate
recommendations or statements in the specifications for the benefit of
bidding contractors.
Geotechnical Design Report*
Depending on the philosophy of the owner and subsurface specialist, the
geotechnical report (also known as a geologic report or subsurface in-
vestigation report) might or might not make predictions about construc-
tion conditions and might or might not be made available to contractors.
However, whatever the label, all such reports have one thing in common:
Due to the time frame of their compilation they may include recommenda-
tions on geotechnical design parameters, but they cannot present the
"last word," as the designer may modify the preliminary design concept
and try other alternatives for final design after the reports have been
completed.
In underground construction, the potential for critical situations
is high and changes are costly. The contractor has a right to know pre-
cisely how the anticipated subsurface conditions affected the final de-
sign and what the owner and/or designer thought about subsurface effects
on, and behavior during, construction. The designer is in ~ unique
position to provide explanations by writing a report that develops par-
allel with the design. Such a report might be called a "Geotechnical
Design Report." It should be based on (without repeating verbatim) the
information contained in earlier geotechnical reports compiled by the
investigator responsible for the exploration program. The report should
be bound into the contract specifications, thereby making it easily
available and confirming its status as an actual contract document.
Appendix D presents general outlines of both rock and earth tunnel
geotechnical design reports used by the Washington Metropolitan Area
Transit Authority. These reports have served well for at least nine
years, proving to be more suitable than previous means of apprising bid-
ders of the geotechnical ramifications for design and construction. A1-
though the outlines would have to be modified for different geographical
Geotechnical Design Reports is terminology adopted from the Washington
Metropolitan Area Transit Authority and is WMATA's shortened version of
their formal title, "Geotechnical Basis of Design and Construction Spec-
ifications." Many agencies issue this type of document under a differ-
ent title, such as "Design Rationale Report."
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areas and tunnel types, the basic idea is worthy of adoption. In addi-
tion, it may be useful to expand the content of such reports by includ-
ing one of the systems for rock classification, such as the Rock Struc-
ture Rating or RSR concept (Wickham and Tiedemann, 1974), the Rock Mass
Rating or RMR system (Bieniawski, 1974), the Q-system (Barton et al.,
1977), or the Terzaghi classification (Proctor and White, 1968~. How-
ever, these systems are not suited to use by inexperienced personnel; to
ensure proper application, the purposes and limitations of these systems
must be thoroughly understood.
Geotechnical ~As-Built. Report
Geologic information obtained during preconstruction explorations and
recorded in the log of job progress, including as-built geotechnical
tunnel mapping, should be compiled in a project completion report.
Ideally, for consistency the constructed project should be mapped by the
same group that conducted the original explorations. As an absolute
minimum, the geotechnical conditions encountered should be reviewed by
the original exploration group to see how and where their techniques and
interpretations could be improved.
Knowledge of actual construction conditions can assist in identify-
ing the cause of and proper method of correcting problems encountered
during the operational life of a tunnel. The existence and availability
of an as-built report would prove invaluable if another project were to
be constructed in the same area. Such a report would also be useful in
transferring experience to projects in other areas with similar geotech-
nical considerations. For example, as-built geotechnical data are re-
quired as part of safety analysis reports submitted in accordance with
licensing procedures for nuclear power facilities.
Barton, N., R. Lien, and J. Lunde. 1977. "Estimation of Support Re-
quirements for Underground Excavation," Design Methods in Rock- Mechan-
ics (Proceedings of the 16th U.S. Symposium on Rock Mechanics). New
York, New York: American Society of Civil Engineers.
Bieniawski, Z.T. 1974. "Geomechanics Classification of Rock Masses and
its Application in Tunneling," Advances in Rock Mechanics, Volume IIA
(Proceedings, 3rd Congress of the International Society for Rock Mechan-
ics). Washington, D.C.: National Academy of Sciences.
Proctor, R.V., and T.L. White, eds . 1968. "Introduction to Tunnel
Geology," Rock Tunneling with Steel Supports (revised edition).
Youngstown, Ohio: Commercial Shearing and Stamping Company.
Wickham, G.E., and H.R. Tiedemann. 1974. Ground Support Prediction
Model (RSR Concept) (Report for the U.S. Bureau of Mines under Contract
HO 220075, ARPA Program). Springfield, Virginia: National Technical
Information Service.
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EFFECT OF GEOLOGIC FACTORS ON COSTS
In the bidding phase of a project, a method of accomplishing the work is
determined by the contractor. Specific manpower, equipment, and mate-
rial requirements are calculated and then costed. Finally, margin or
fee (with contingencies) is added to produce a total cost to the owner.
The adopted methods will be either conservative, middle-of-the-road, or
optimistic, depending on the perceived importance of various factors.
For an underground project the factors are usually manifold. They may
include such considerations as availability and quality of labor, proj-
ect location, geology, how well the project lends itself to the applica-
tion of various types of equipment, the general rate of economic infla-
tion, and the reputation of the owner. Contingency may or may not be an
element of estimated cost, depending on how many uncertainties exist.
As the project moves into the execution phase, costs will vary with con-
ditions encountered. In this respect, geologic conditions are the most
significant factor for every underground project.
With regard to geology, during the bidding or estimating phase the
contractor either has no knowledge (the rare case), or a little, or a
reasonable amount. When nothing is known, the chosen method often will
be conservative and costs will be high; there will be a number of un-
knowns, so a contingency will probably be added, increasing costs even
more. Moreover, it is likely that no innovative equipment will be as-
sumed in the bid preparation, because high capital costs would result
with no reasonably assured benefit. Production will usually be set low
because there is no reason to set it high, and support requirements will
be literally guessed. All in all, the owner will be penalized greatly
for failure to provide an adequate rationale for bidding. Bids may be
significantly higher than the estimate; in the extreme, bids may not be
submitted at all.
When only a little of the geology is known and conditions look dif-
ficult, the owner will still pay, for the same reasons noted above.
Similarly, when a little is known and conditions look favorable, the
owner might well reap benefits in the form of lower bids but pay later
in claims. However, most competent contractors will tend to discount
the importance of what is known when it is based on a small sampling.
The result will be reflected in the bids in the form of reduced produc-
tion, non-innovative equipment, conservative support, and probably con-
tingency as well.
The pre-bid condition to strive for is that in which a reasonable
amount of raw data and interpretation is available. Then {and only then)
does the contractor have a rational basis for preparing a bid and the
owner have a rational basis for evaluating bids. If the data look prom-
ising, then the owner will properly reap the benefits of good, solid,
competitive bids with almost no contingency (assuming the existence of a
changed conditions clause). If the data look unfavorable, then the cost
will properly reflect the conditions; bids will be neither overly pessi-
mistic nor overly optimistic.
Just as knowledge of predicted geologic conditions is reflected in
cost during the bidding stage, so too are geologic facts reflected in
actual costs during construction of the project. As the geologic facts
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come in, they may present welcome or unwelcome surprises, or no
prices, when compared to pre-bid predictions. Unwelcome surprises
the most usual. ~ ~
, ,
sur-
are
water inflow, heavily fractured
ground, and too-hard ground are commonplace in the industry.
Cases of excessive
It is worth noting a few of the generally overlooked effects of such
_ _ _ , _
, .
.
surprises. First, management and supervisory personnel may have been
· . · . .
selected for a project because particular conditions were assumed to
exist. In the underground business there are Good ground" specialists
and "bad ground" specialists, and it is usually wise to keep them within
their respective areas of expertise. A manager accustomed to working
slowly in the face of adverse ground conditions may slow down needlessly
Conversely, a
respective areas ot expertise
in favorable conditions because of training and habit.
"good ground" manager usually is able to proceed much more rapidly, and
this can be disastrous when in adverse ground. Thus, in the case of un-
expected ground conditions--particularly adverse conditions--a mismatch
of management talent may be expensive for the owner or the contractor.
Second, if favorable conditions are assumed at bid time, highly spe-
cialized, high-production equipment might well be selected. By its
nature, specialized equipment requires a particular type of ground in
order to be effective. The lack thereof will usually render the eauin-
_ _ it, ~ ,
ment ineffective, many tomes to the point of complete change. Thus, the
cost of the initial equipment selection is lost, and the additional cost
of new equipment has to be incurred. Again, either the owner or con-
tractor bears the expense.
Last, schedule slippage resulting from adverse and unexpected geo-
logic conditions delays commencement of service life of the structure.
This will cost the public, as well as the owner and contractor.
As it is, even welcome geologic surprises can be of no value e For
example in the case of equipment, the use of a tunnel boring machine
(TBM) may have been eliminated based on assumed adverse geology. When
the ground is found to be much better than expected 25 percent of the
way through the job, it is usually too late to secure a TOM. Someone
pays the penalty of lost opportunity in terms of production rates.
Sometimes no affordable _
conditions. However, only when an adequate amount of pre-bid
investigation is undertaken are surprises usually eliminated.
instances, no surprises are the only "good" surprises.
~ ,
.
· . .
. . . . . .
_
amount of investigation will forecast adverse
geologic
In most
15
Representative terms from entire chapter:
geotechnical report
