Manual on Subsurface Investigations (2019)

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1 C H A P T E R 1 Introduction Purpose The primary purpose of geotechnical subsurface investigations in transportation projects is to collect data that will help transportation engineers and planners identify, assess, and address risks associated with subsurface conditions; select appropriate design parameters; and monitor performance during construction and operation phases of an asset’s life cycle. Geotechnical risks generally pose the greatest risk to transportation projects because adverse subsurface conditions can have significant impacts on public safety, project schedules, life cycle costs, and environmental sustainability. A sound subsurface investigation program can mitigate these risks and yield significant dividends for state transportation agencies in terms of cost savings and timely completion of projects. Thus, it is prudent for transportation agencies to include geotechnical investigations as a part of their project development process. This manual provides information and guidelines to help geoprofessionals (i.e., geotechnical engineers, geological engineers, geologists, and engineering geologists) plan and execute a sound geotechnical site investigation program; use the results to develop a ground model for planning, design, construction, and asset management phases of a project; and report and document the results in a manner that facilitates peer review, communication with stakeholders, and potential future uses. Organization of the Manual The organization of the manual mirrors how the subsurface investigations are typically performed: • Planning the investigation (Chapters 2 and 3) • Executing the investigation (Chapters 4 through 8) • Interpreting the results of the investigation (Chapters 9 and 10) • Reporting and presenting the results of the investigation (Chapter 11) Supplemental information is presented in appendices that are broadly grouped into two categories— investigative and administrative. Appendices A through C contain information regarding investigations that are not typically conducted as part of the routine subsurface investigations but are conducted either during construction or operation to monitor performance and assess the condition of existing geotechnical features. Appendices D through H contain information pertaining to administration functions, such as geotechnical data management, quality assurance/quality control (QA/QC), and best practices for health and safety. The following is a synopsis of the content included in each of the chapters and appendices. 1.2.1 Planning the Investigation Chapter 2 describes the two types of uncertainty related to subsurface conditions—natural variability and knowledge uncertainty—and summarizes the nature of the technical and financial risks associated with a subsurface investigation that inadequately recognizes and addresses these uncertainties. General strategies for conducting a sound subsurface investigation program are also provided. Special considerations for

2 subsurface investigations performed for the AASHTO LRFD bridge design and design-building projects are also discussed. Chapter 3 presents processes for developing the scope for the subsurface investigations related to planning, designing, constructing, and operating transportation facilities including the following: • Identifying the types of data required to address the anticipated geotechnical risks and performance issues • Determining the required appropriate type of data • Selecting the most appropriate investigation equipment for the anticipated site conditions • Selecting the appropriate scope for geophysical testing and the appropriate geophysical methods • Selecting the appropriate scope for in situ testing and the appropriate in situ tests • Selecting the appropriate sampling equipment and borehole advancing methods for the anticipated subsurface conditions • Selecting the appropriate type of measurements to evaluate groundwater conditions • Selecting the appropriate scope for laboratory testing and the appropriate laboratory tests 1.2.2 Executing the Investigation Chapter 4 provides an overview of surface and borehole geophysical methods based on measuring, analyzing, and interpreting seismic, electrical, electromagnetic, gravitational, and magnetic fields. Geophysical methods are often useful during the initial phases of a site investigation program to efficiently gain an understanding of the overall subsurface conditions, including stratigraphy and the location and size of potential anomalies. The locations of subsequent borings and soundings can then be optimized to investigate areas of concern identified from the geophysical surveys. Geophysical methods are also useful to estimate some engineering properties of subsurface materials directly. Chapter 5 includes information to aid geoprofessionals with evaluating the different in situ geotechnical tests commonly used for characterizing soil and rock and with selecting the appropriate in situ test(s) for an investigation. In situ tests can efficiently collect abundant data to help define subsurface stratigraphy, evaluate vertical and horizontal variability, and obtain geotechnical engineering parameters for analysis and design. The in situ tests presented in Chapter 5 for soils include the standard penetration test (SPT), cone- penetration test (CPT), flat plate dilatometer test (DMT), vane shear, and pressuremeter; and the in situ tests for rock are plate load test (PLT, also known as plate jacking test), flat jacking test (FJT), rock dilatometer test, large field direct shear (DS) test, rock borehole shear test, and borehole cameras. Chapter 6 provides information on the variety of equipment, methods, and procedures available for drilling and sampling soil and rock to obtained disturbed and undisturbed samples: • Field equipment • Methods for advancing boreholes • Soil sampling • Rock coring methods • Logging borings • Boring closure Chapter 7 describes the methods available to perform a comprehensive hydrogeologic characterization to gather information on (i) geology and hydrogeology, (ii) aquifer characteristics, (iii) aquitard characteristics, and (iv) the direction and gradient of groundwater flow. Methods for establishing groundwater levels and measuring pressure heads (e.g., monitoring wells, piezometers) and estimating aquifer characteristics (e.g., pumping tests, slug tests, packer tests) are presented. Chapter 8 presents the laboratory testing methods commonly used to characterize soil and rock. The chapter provides references for pertinent AASHTO, ASTM International (ASTM), and other applicable standards for each of the tests discussed. The following topics are presented:

3 • QA (e.g., sample identification and tracking; transportation, storage, and handling of samples; assessment of sample disturbance) • Index tests (e.g., particle size distribution, Atterberg limits, moisture content) • Soil classification • Performance tests on soil (e.g., compaction, hydraulic conductivity, one-dimensional [1D] consolidation, shear strength, dynamic properties) • Performance tests for pavement design (e.g., resilient modulus, California Bearing Ratio [CBR]) • Tests on rock (e.g., slake durability, compressive strength, elastic moduli) 1.2.3 Interpreting the Results of the Investigation Chapter 9 provides guidance for interpreting soil properties derived from geophysical, in situ, and laboratory testing methods. The emphasis is on parameters commonly used in highway analysis and design: • Soil classification • Preconsolidation stress or effective yield stress (i.e., stress history) • Effective-stress and total-stress shear strength parameters • Lateral stress state • Modulus • Coefficient of consolidation • Hydraulic conductivity Chapter 10 provides guidance for interpreting the engineering properties of intact rock and rock masses. The specific topics presented include: • Intact rock classification • Intact rock properties (e.g., uniaxial compressive strength, shear strength) • Rock mass classification (e.g., rock mass rating [RMR], Geological Strength Index [GSI]) • Rock mass properties (e.g., shear strength, foundation capacity) 1.2.4 Reporting and Presenting the Results of the Investigation Chapter 11 provides background on the characterization of geotechnical data as either factual or interpretative and typical uses of geotechnical information. The chapter also includes some guidance on compiling, preparing, and presenting geotechnical information. The following are specific topics presented: • Presentation of factual information • Presentation of interpretative information • Geotechnical reports for conventional (i.e., design-bid-build) and alternative (design-build) project delivery methods • Contractual implications of geotechnical reports 1.2.5 Supplemental Investigative Information Appendix A contains the following information regarding geotechnical instrumentation to assist geoprofessionals evaluate the potential effectiveness of an instrumentation program: • Identifying potential failure modes where instrumentation may add significant value • Making a preliminary selection of the appropriate types of instrumentation • Acquiring a conceptual understanding of the steps involved in developing an instrumentation plan • Evaluating, presenting, and managing the data obtained from instrumentation

4 Appendix B provides guidelines for implementing a geotechnical instrumentation program for embankments, deep foundations, excavations in soil and rock, dewatering, earth retaining structures, tunnels, and grouting. Appendix B also highlights how geotechnical instrumentation can be used to resolve legal disputes and manage geotechnical assets. Appendix C provides guidelines for characterizing the integrity of the existing bridge foundation elements (condition assessment) and assessing their load carrying capacity. 1.2.6 Supplemental Administrative Information Appendix D introduces basic geotechnical data management concepts and then advances to providing guidance for developing and implementing a standardized geotechnical data management system. The following aspects of geotechnical data management systems are presented and described: • Introduction • Basic features • Implementation guidelines • Software requirements • Data sources • Business process considerations • Data interchange for geotechnical and geoenvironmental specialist (DIGGS). Appendix E establishes QA guidelines to define an agency’s standards and quality management protocols during the subsurface investigation to ensure delivery of consistent and competent subsurface investigation products and services. The topics presented include the following: • QA/QC policy • Roles and responsibilities • QA/QC plan • Prequalification and verification • Policies and procedures • Standards • Geotechnical data management • Performance monitoring Appendix F provides guidelines for planning and executing a subsurface investigation following commonly accepted safe operating practices to protect human health and the environment. The topics discussed include the following: • General health and safety guidelines • Responsibilities • Administrative requirements • Site inspection • Development of a health and safety plan • On-site activities Appendix G provides guidelines and best practices for contracting subsurface investigation services. The topics presented include the following: • Prequalification of private engineering firms • Development of scope of work • QA/QC practices • Contract administration

5 Appendix H summarizes the resources available to assist geoprofessionals plan and execute subsurface investigations, analyze and interpret subsurface investigation data, present subsurface information, and manage subsurface investigations. The following topics are presented: • Technical manuals and reports • Geotechnical websites • Available training resources • References Key Complementary Resources To complement the information in this manual, geoprofessionals responsible for planning and executing subsurface explorations, and using the results for planning, design, construction, and asset management phases of a project should consult the following complementary resources that also provide comprehensive information on geotechnical site characterization: • AASHTO R 13 (ASTM D420) on Conducting Subsurface Investigations • Article 10.4 of the AASHTO LRFD Bridge Design Specifications (AASHTO 2017) • The Federal Highway Administration (FHWA) Geotechnical Engineering Circular No. 5 on Geotechnical Site Characterization (FHWA 2017) • The reference manuals for the National Highway Institute (NHI) course on Soils and Foundations (FHWA 2006a, 2006b) • The United States Army Corps of Engineers (USACE) Engineer Manual No. 1110-1-1804 on Geotechnical Investigations (USACE 2001)

6 Chapter 1 References AASHTO. 2017. AASHTO LRFD Bridge Design Specifications. US Customary Units, 8th Edition. American Association of State Highway and Transportation Officials, Washington, DC. FHWA. 2006a. Soils and Foundations, Reference Manual – Volume I, Publication No. NHI-06-088, Federal Highway Administration, U.S. Department of Transportation, Washington, DC. FHWA. 2006b. Soils and Foundations, Reference Manual – Volume II, Publication No. NHI-06-089, Federal Highway Administration, U.S. Department of Transportation, Washington, DC. FHWA. 2017. Geotechnical Site Characterization. Geotechnical Engineering Circular No. 5, Publication No. NHI-16-072. Federal Highway Administration, U.S. Department of Transportation, Washington, DC. USACE. 2001. Geotechnical Investigations. Engineer Manual 1110-1-1804, U.S. Army Corps of Engineers, Washington, DC.