Geotechnical Site Investigations for Underground Projects: Volume 1 (1984)

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1. Introduction and Executive Summary INTRODUCTION In the construction of recent subsurface projects, particularly transit systems, it has been apparent that costs were often greater than first expected. Although there are a number of reasons for this, including periods of high inflation, one particular phase of the process of design and construction of underground structures has become the target of se- rious questions. This target is the exploration phase, specifically subsurface geology and site investigations. Essentially, geologic conditions constitute the greatest source of unknowns prior to actual construction of a project. These unknowns usually exist in inverse pro- portion to the amount, nature, and quality of the geotechnical investi- gations. Moreover, data indicate that differing site conditions are the alleged basis for many costly construction changes and claims*. In order to evaluate the relationship between geology and construc- tion costs, the u.s. National Committee on Tunneling Technology formed a special subcommittee to study geotechnical site investigations for un- derground design and construction. It was decided at the outset that the subcommittee should not prepare a manual on site investigations, because such manuals already exist. Moreover, manuals tend to becane increasingly out of date as the state-of-the-art of design and construc- tion progresses. Instead, therefore, a decision was made to collect data from completed projects and have the data examined by a body of ex- perienced engineers, geologists, contractors, designers, owners, and attorneys. The product of the study would then be an accessible reposi- tory of pertinent case history data and a statement or written discus- sion of the subcommittee's interpretation of the case histories. *In this report, •claim• is a shorthand expression that may encanpass not only an assertion of differing site conditions but also any extra payment made as a result of an unexpected subsurface situation. For example, some owners may settle all subsurface-related overruns as •contract modifications• and, therefore, the word •claims• would not ap- pear in their records. we use the word to cover all geotechnically re- lated requests for extras. 1 

The two volumes resulting from the study are intended for a broad and disparate audience in terms of perspectives, needs, and goals. Therefore, Volume 1 begins with overview chapters presenting the basic rationale and legal considerations for exploration programs. It becomes increasingly specific with chapters discussing particular problems and projects, and then proceeds to evaluation of selected cases and inter- pretation of case histories. The final chapter reports the conclusions and recommendations, both analytical and judgmental, developed through- out the course of the study. For the convenience of readers desiring additional information, Volume 1 also includes suggested formats for Geotechnical Design Reports, a Selected Bibliography, and a Glossary. Volume 2, supplemented by the data matrixes provided separately as Plates 1 and 2, presents raw data in abstract form for 87 projects re- ported as case histories and a computer program for managing the data to suit several purposes. The subcommittee believes that the information presented in these volumes will result in improvements in the planning of site investiga- tions and in the securing of geotechnical data that are specifically needed by a variety of users. These potential users include: owners developing new projects and related cost estimates; designers of new structures; contractors estimating project costs and selecting construc- tion methods and equipment; and equipment manufacturers seeking to pro- duce tools and machines which are more efficient and less costly to operate. The ultimate result should be improvements in the economics, efficiency, and effectiveness of underground construction, thereby bene- fiting projects for whict. it is the only alternative as well as those where it may prove an advantageous alternative to surface construction. EXECtn'IVE SlHIARY The geotechnical site investigation must provide data to foster a safe and economical design; to assist the contractor in analyzing the feasi- bility, costs, procedures, and equipment for construction; to enable the owner to prepare contract and bid documents that accurately reflect and provide equitable methods for resolution of potential areas of contin- gent costs (which are a function of the known and unknown geotechnical information and data). The geotechnical site investigation must answer, or assist in answering, the designer's question of the loads for which the structure must be designed; the contractor's questions of what type of ground is to be excavated, how it will behave during construction, what method is suitable to build the structure, and how much it will cost; and the owner's questions of whether the budget is adequate and the schedule can be met. The ultimate goal of the geotechnical investigation must be an un- derstanding of the behavior of the soil and rock. Elucidating those behavioral characteristics is the essence of the geotechnical investiga- tion; raw data and the identification and classification of materials are not enough. The analysis and recommendations of specialists expe- rienced in the acquisition, interpretation, and presentation of the data 2 

are vital to the successful design and construction of underground proj- ects. The geotechnical site investigation is not an isolated part of the design and construction processes, nor is it only an early part of basic feasibility decision making. Rather, it must and should serve as a con- tinuous resource throughout the design/construct/operation processes. The format and content of the geotechnical site investigation must be oriented toward the owner, designer, and contractor. Conclusions • It is in the owner's best interests to conduct an effective and thorough site investigation and then to make a complete disclosure of it to bidders. • Disclaimers in contract documents are generally ineffective as a matter of law, as well as being inequitable and inexcusable in most cir- cumstances. • Contracting documents and procedures can provide for resolution of uncertain or unknowable geological processes or conditions before and during construction, rather than afterwards. • On major projects especially, it is important that (a) the owner employ a multi-disciplined team including engineering geologists, engi- neers, and a construction specialist to develop subsurface data and evaluate their impact on design and construction; (b) designers and geol- ogists possess a thorough working knowledge of construction methods and equipment so that the proper geotechnical data are secured and design is consistent with construction systems1 and (c) contractors employ geolo- gists experienced 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. • Site investigations have to proceed through, but should not al- ways end with, completion of the feasibility/alignment setting/final design programs. • Procedures for logging, documenting, and preserving samples from boreholes require improvement. • Geophysical methods can be used to advantage, especially in coor- dination with boreholes. • Groundwater and its effects on the subsurface materials merit greater attention in exploration programs. • Laboratory testing of the subsurface materials generally needs to be increased. • Exploratory adits and shafts are generally justified only when absolutely essential to obtain critical design data or when a substan- tial benefit to construction is indicated. • Maintenance of technical data obtained during design and con- struction of underground projects often is not pursued by owners or demanded of their consultants and contractors. 3 

Recc:.aendations • Expenditures for geotechnical site exploration should be in- creased to an average of 3.0 percent of estimated project cost, for better overall results. • 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. • 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. • All geologic reports should be incorporated as part of the con- tract documents. • Designers of mined tunnels should compile a •Geotechnical Design Report, • which should be bound into the specifications and be available for use by bidders, the eventual contractor, and the resident engineer. • Monitoring of ambient conditions prior to construction should be undertaken to establish a baseline of information for comparison during and after construction. • Pre-bid conferences and site tours should be conducted to ensure that all bidders have access to the maximum amount of project informa- tion. • Geologic information from preconstruction explorations and as- built tunnel mapping and construction procedures should be compiled in a report detailing project completion. • Investigation methods and predictions should be improved for three specific conditions: in-situ stress, stand-up time, and ground- water. • Improved horizontal drilling techniques should be developed that can recover rock core and penetrate long distances without wandering from line and grade. • Research and development should be conducted to expand the capa- bilities of geophysical or other remote sensing methods for obtaining geotechnical data between boreholes and from the surface down to depths too great for boreholes. 4