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Technical Manual for Design and Construction of Road Tunnels - Civil Elements
Chapter 4 - Geotechnical Reports
Conventionally, for typical roadway and bridge projects, the geotechnical engineer prepares "geotechnical reports" that serve to summarize the subsurface investigations performed, interpret the existing geological conditions, establish the geotechnical design parameters for the various soil and rock strata encountered, provide geotechnical recommendations for design of the proposed foundations and/or geotechnical features, and identify existing conditions that may influence construction. The term geotechnical report is often used generically to include all types of geotechnical reports, e.g. geotechnical investigation report, geotechnical design report, landslide study report, soil report, foundation report, etc. (FHWA, 1988). The concept is the geotechnical report is only used to communicate the site conditions and design and construction recommendations to the roadway design, bridge design and construction personnel. It may or may not be made available to prospective contractors; and when provided, they are generally only included as a reference document and may typically include disclaimers stating that the report is not intended to be used for construction, and that there is no warranty regarding the accuracy of the data or the conclusions and recommendations of the report; contractors must make their own interpretation of the data to determine the means and associated costs for construction.
Although this approach is commonly used, and may still be applicable for cut-and-cover and immersed tunnel projects, it is not appropriate for mined and bored tunnel and other underground construction projects. Underground projects entail great uncertainty and risk in defining typically complex geological and groundwater conditions, and in predicting ground behavior during tunneling operations. Even with extensive subsurface investigations, considerable judgment is required in the interpretation of the subsurface investigation data to establish geotechnical design parameters and to identify the issues of significance for tunnel construction. This situation is further complicated for tunneling projects since the behavior of the ground during construction is typically influenced by the contractor's selected means and methods for tunnel excavation and type and installation of tunnel supports.
Using conventional geotechnical reports for tunnel projects would essentially assign the full risk of construction to the contractor since the contractor is responsible for interpreting the available subsurface information. Although this approach appears to protect the owner from the uncertainties and risks of construction, experience on underground projects has demonstrated that it results in high contingency costs being included in the contractors' bids, and does not avoid costly contractor claims for additional compensation when subsurface conditions vary from those that could reasonably be anticipated.
Current practice for tunnel and underground projects in the U.S. seeks to obtain a more equitable sharing of risks between the contractor and the owner. This approach recognizes that owners largely define the location, components and requirements of a project and the extent of the site investigations performed, and therefore should accept some of the financial risk should ground conditions encountered during construction differ significantly from those anticipated during design and preparation of the contract documents, and should they negatively impact the contractor. The overall objectives of this risk sharing approach are to:
Contracting practices for underground projects in the U.S. have evolved and currently include a number of measures to help achieve the above objectives. These measures vary somewhat between projects, depending on specific project conditions and owner preferences, but typically consist of the following fundamental elements:
This chapter focuses on the three types of geotechnical reports (GDR, GDM, and GBR) noted above for bore/mined tunnel projects, will discuss the specific purposes and typical contents of these reports, and provide guidelines for their preparation. Related topics, including subsurface investigations for tunnel projects and provisions for dispute resolution, are addressed in other chapters of this manual. Additional information on geotechnical reports for underground projects is provided by ASCE (1989, 1991, 1997, and 2007), Brierley (1998), Essex (2002), and Edgerton (2008).
4.2 Geotechnical Data Report
The Geotechnical Data Report (GDR) is a document that presents the factual subsurface data for the project without including an interpretation of these data. The purpose of the GDR is to compile all factual geological, geotechnical, groundwater, and other data obtained from the geotechnical investigations (Chapter 3) for use by the various participants in the project, including the owner, designers, contractors and third parties that may be impacted by the project. It serves as a single and comprehensive source of geotechnical information obtained for the project.
The GDR should avoid making any interpretation of the data since these interpretations may conflict with the data assessment subsequently presented in the Geotechnical Design Memorandum or other geotechnical interpretive or design reports, and the baseline conditions defined in the Geotechnical Baseline Report. Any such discrepancies could be a source of confusion to the contractors and open opportunities for claims of differing site conditions. In practice, it may not be possible to eliminate all data interpretation from the GDR. In such case, the data reduction should be limited to a determination of the properties obtained from that individual test sample, while avoiding any recommendations for the geotechnical properties for the stratum from which the sample was obtained.
The GDR should contain the following information (ASCE, 2007):
Table 4-1 presents a typical outline for a GDR, modified from Brierley (1998).
The GDR would include the logs of all borings performed for the project, but should not present a subsurface profile constructed from the borings since such a profile requires considerable judgment and interpolation of the borehole records to show inferred strata boundaries. As illustrated in the outline, the text of the GDR provides background information and a discussion of the subsurface investigations performed, while the specific data are presented in appendices to the report. The introduction provides a general project description and notes the purpose and scope of the report. The section on background information should identify other sources of geotechnical information that may have been obtained by others at or near the project site, and may include subsurface investigation data and records from previous construction activities. If such additional information is limited in volume, consideration should be given to including these data in an appendix to the report.
Background information should also include a discussion of the regional and local geologic setting, since such information will be invaluable in the assessment of the limited amount of factual data obtained from site investigations. It is recognized that a description of geological conditions requires interpretation of information in the literature and an understanding of the geological processes controlling the formation and properties of soil and rock deposits; however, since an understanding of the geological setting is fundamental to a successful tunneling project, such information is considered an essential component of the GDR.
The report section on field investigations should include a brief description of the type of investigations performed, references to applicable standards for performing the investigations, the method of obtaining and handling samples, and discussion of any special procedures used for the investigations. If specialty work, such as geophysical investigations, is performed by others, the report prepared by the specialty firm can be included as an appendix to the GDR and simply referenced within the text of the GDR.
The section on laboratory testing should document the number of each type of test performed, the name and location of the testing laboratory, the specific standards used to perform each test, and other information pertinent to the testing program.
The attachments and appendices would present the field and laboratory test records, and may also include helpful summary tables and plots that summarize the factual data obtained from the investigations.
4.3 Geotechnical Design Memorandum
For tunnel projects, one or more interpretive reports may be prepared to evaluate the available data as presented in the GDR, address a broad range of design issues, and communicate design recommendations for the design team’s internal consideration. These interpretive reports are also used to evaluate design alternatives, assess the impact of construction on adjacent structures and facilities, focus on individual elements of the project, and discuss construction issues. The current guidelines recommend referring to such design reports as Geotechnical Design Memoranda (GDM), instead of Geotechnical Interpretive Report (GIR) (ASCE, 2007).
GDM, or GIR may be prepared at different stages of a project, and therefore may not accurately reflect the final design or final contract documents. Hence, preparation of such interpretive reports in the course of the final design is superfluous, and is strongly discouraged to avoid a potential source of confusion and conflict.
Since GDMs are used internally within the design team and with the owner as part of the project development effort, it is not appropriate to include GDMs as part of the contract documents. Thus, GDMs should be clearly differentiated from the Geotechnical Baseline Report (GBR) (Section 4.4). The GBR should be the only interpretive report prepared for use in bidding and constructing the project. The GBR must supersede – take precedence over any other Geotechnical Report(s). However, in the interest of "full disclosure" to prospective bidders, GDMs are often made available "for information only." In such instances, the GDM must include a disclaimer clearly noting the specific purposes of the report and stating that the information provided in the report is not intended for construction. The GDM must also clearly state that the contract documents, including the GDR and GBR, are the only documents to be considered by contractors when assessing project requirements and determining their bid price for the work. A sample outline for a GDM, similar to the previously recommended outline for GIR, is presented in Table 4-2.
The GDM should include other disclaimers to highlight the interpretive nature of the report. Following are several issues that are commonly addressed by disclaimers:
In evaluating the engineering properties of the soil and rock materials, it is appropriate for the GDM to note the likely ranges for these properties and to recommend a value, or range of values, for use in design. The report should document the basis for selecting these parameters and discuss their significance to the design and construction of the proposed facilities. As an interpretive report, it is appropriate and useful to discuss the reasoning and judgment associated with these and other design recommendations presented in the report.
Presenting a range of parameters, along with a discussion of their consequences on the design, helps the owner and the design team understand and quantify the inherent uncertainty and risk associated with the proposed underground project. Such information allows the owner to determine the level of risk to be accepted, and the share of the risk to be borne by the contractor. An example of this decision process would be a case where a tunnel must be constructed through relatively low strength rock that contains intrusive dikes of very hard igneous rock of unknown frequency and thickness. Based on limited geotechnical investigations, the geotechnical engineer determines that the amount of hard rock may range from 10 to 30 percent of the total length of the tunnel. This range, and possibly a best estimate percentage, would be reported in the GDM. During subsequent preparation of the GBR and other contract documents, a specific baseline value would be determined and referenced for contractual purposes and reflected in the design. If the owner, in an effort to get lower bid prices, is willing to accept the greater risk of cost increases during construction, a value closer to the lower end of the range would be selected as the baseline. However, if the owner wishes to reduce the risk of cost extras during construction, a value closer to the conservative end of the range would be selected. However, in choosing this second option, the owner needs to recognize that it will result in higher bid prices.
In preparing a GDM it is acceptable to use ambiguous terms, such as "may," "should," "likely," etc. in discussing the various technical issues. Such terms reflect the reality of our uncertainty in defining subsurface stratigraphy and the engineering properties of natural materials, and in predicting the behavior of these materials during construction.
The GDM should reference the Geotechnical Data Report (GDR) as the source of information used to develop the conclusions and recommendations of the GDM. The GDM should also identify any other sources of information that may have influenced the findings of the GDM, including technical references, reports, and site reconnaissance observations, among others.
The GDM should include generalized subsurface profiles developed from an assessment of the available geotechnical and geological information. These subsurface profiles greatly facilitate a visualization and understanding of the existing subsurface conditions for design purposes. However, it must be recognized that such definition of subsurface conditions is highly dependent on the quantity and quality of the available geotechnical investigation data, and the judgment of the geotechnical engineer in interpreting these data and the relevant geological information. Accordingly, the report must emphasize that the profiles are based on an interpolation between widely spaced borings, and that actual subsurface conditions between the borings may vary considerably from those indicated on the profiles.
In addition to providing recommendations for design, the GDM should also address construction issues, including the general methods of construction considered appropriate for the existing site conditions and proposed facilities. However, the engineers preparing the report must recognize that the contractor is responsible for selecting the specific equipment, means and methods for performing the work, thus must avoid any detailed recommendations on these issues accordingly. For example, for a proposed subaqueous tunnel through highly permeable sand deposits, it is appropriate to state that a closed face Tunnel Boring Machine (TBM) consisting of either an Earth Pressure Balance (EPB) Shield or Slurry Shield TBM should be used, but it is inappropriate to recommend a specific TBM model, horsepower, etc.
It is also particularly important for the GDM to identify and discuss all potential hazards that may be encountered during construction, and discuss possible measures to mitigate these hazards. A thorough discussion of such issues should help both the design team and the contractor to anticipate and avoid problems that could cause major cost and schedule impacts. For example, for a tunnel to be excavated in mixed face conditions, the GDM should note that typical problems may include: a), large water inflow at the contact between the soil and rock that will be difficult to fully dewater, b), steering problems for the TBM, and c), ground loss, and corresponding surface settlement due to excavation of the soil in the upper part of the tunnel heading at a faster rate than the rock in the lower part of the heading. For this example, the report should also note mitigating measures, such as grouting the soil to reduce seepage and ground loss, and facilitate steering of the TBM; drilling drainage holes horizontally from the tunnel heading; providing an articulated TBM to facilitate steerage corrections; etc.
In summary, the GDM is written by the engineers solely for use by the design team in developing the design for the proposed facilities. It provides an interpretation of the available subsurface information to determine likely subsurface conditions for design purposes. Depending on its specific purpose and the time of its preparation, the GDM may not reflect the final design shown on the contract drawings. An important element of the GDM is a general discussion of the appropriate methods of construction and the potential hazards that may be encountered during construction, as well as the possible measures that can be considered to mitigate these hazards. The GDM is not intended to be a definitive representation of the actual ground conditions, and is not to be used as a baseline for contractual purposes.
4.4 Geotechnical Baseline Report
4.4.1 Purpose and Objective
As discussed in Section 4.1, a fundamental principal in current U.S. contracting practices for tunnel projects is the equitable sharing of risk between the owner and contractor, with the objectives of reducing contingency fees in contractor bids, achieving lower total cost for the project, and streamlining resolution of contractor claims for changed conditions during construction. Over the years, various forms and names have been given to the interpretive geotechnical report to be incorporated into the Contract Documents for underground projects in order to achieve the aforementioned objectives. Originally, this was called the Geotechnical Design Summary Report (GDSR). However, since 1997 and continuing with the current (2007) "Geotechnical Baseline Reports-Suggested Guidelines" the industry has determined that the incorporated report be called GBR (Geotechnical Baseline Report).
The primary purposes of the GBR are:
Although it reflects the findings of the geotechnical investigations and design studies, a GBR is not intended to predict the actual geotechnical and geological conditions at a project site, or to accurately predict the ground behavior during construction. Rather, it establishes the bases for delineating the financial risks between the owner and the contractor.
ASCE (1997) also noted the secondary purposes of the GBR as listed below:
A common misconception of the GBR is that it represents a warranty of the existing site conditions by the geotechnical engineer and designer. Based on this understanding, the owner of the project may believe they are entitled to compensation by the designer should actual conditions be found less favorable than the conditions defined in the GBR. However, since it principally serves as a contractual instrument for allocating risks, the GBR is not intended to predict or warranty actual site conditions. If the GBR were to become a warranty, it is reasonable to expect that the geotechnical engineer and designer would more conservatively define subsurface conditions and ground behavior, resulting in a higher cost for the project, a consequence clearly contrary to the primary motivation for adopting a risk-sharing approach to tunnel construction contracts.
It is also important to clearly differentiate the GBR from other interpretive reports may be prepared by the design team to addressing a broad range of design issues for the team's internal consideration. As discussed in Section 4.3, such reports should be referred to as Geotechnical Design Memoranda (GDM). The GBR should be the only final report prepared for use in bidding and constructing the project. The GBR should be limited to interpretive discussion and baseline statements, and should make reference to, rather than repeat or paraphrase, information contained in the GDR, drawings, or specifications (ASCE, 2007).
4.4.2 General Considerations
The various elements of the construction contract documents each serve a different purpose. The GDR provides the factual information used by the designer for designing the various components of the project, and by the contractor for developing appropriate means and methods of construction. The contract plans and specifications detail the specific requirements for the work to be performed, without providing an explanation or background information. The GBR is based on the factual information presented in the GDR as well as input from the owner regarding risk allocation, and provides an explanation for the project requirements as presented in the contract plans and specifications. The baseline information presented in the GBR must be coordinated with the GDR, contract plans and specifications, and contract payment provisions to assure consistency throughout the contract. However, the GBR should not repeat or paraphrase statements made in these other contract documents since even minor rewording of a statement may cause confusion or an unintended interpretation of the statement. Any inconsistency or confusion in the contract documents could lead to a successful contractor claim for additional compensation during construction since these are usually judged against the owner as the originator of the contract.
The contract General Provisions or Special Provisions should clearly define the hierarchy between the various parts of the contract documents to help resolve any conflicts that may inadvertently remain after issuing the documents. The GBR takes precedence over the GDR and any and all other geotechnical report prepared for any reason.
Most often, there is a possible baseline range that can be established for a given set of geologic and construction conditions. As a consequence, where the baseline is set determines the risk allocation for the project. When an adverse baseline is adopted; 1), more risk is assigned to the Contractor who will bid higher, 2), less risk and reduced potential for change orders accrue to the owner, and, 3), higher costs accrue to the owner due to paying for the contingency of encountering the adverse condition(s). Conversely, when a less adverse baseline is adopted: 1) Contractor bids less due to less risk and contingency, 2) higher risk and potential for change orders accrue to the owner, 3) owner pays more if adverse conditions are encountered but less if they are not encountered. In either case, the cost of site conditions remains with the Owner.
4.4.3 Guidelines for Preparing a GBR
The GBR translates facts, interpretations and opinions regarding subsurface conditions into clear, unambiguous statements for contractual purposes. Items typically addressed in a GBR include:
A general checklist for a GBR is presented in Table 4-3. This checklist assumes that the Geotechnical Data Report contains the information noted in Section 4.2
Following are general guidelines that should be followed for preparation of a Geotechnical Baseline Report:
Individuals who prepare the GBR must be highly knowledgeable of both the design and construction of underground facilities, with construction experience particularly important for the necessary understanding of construction methods, equipment capabilities, ground behavior during tunnel excavation, and the potential hazards associated with the different ground conditions and methods of construction. In addition, these individuals must be experienced in the preparation of a GBR and clearly understand its role as a contract document establishing reference baseline conditions. In general, to achieve greater consistency in the contract documents, the individuals preparing the GBR should belong to the same organization that prepares the contract plans and specifications.