National Academies Press: OpenBook

Manual on Subsurface Investigations (2019)

Chapter: Chapter 3. Subsurface Investigation Processes

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Suggested Citation:"Chapter 3. Subsurface Investigation Processes." National Academies of Sciences, Engineering, and Medicine. 2019. Manual on Subsurface Investigations. Washington, DC: The National Academies Press. doi: 10.17226/25379.
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Suggested Citation:"Chapter 3. Subsurface Investigation Processes." National Academies of Sciences, Engineering, and Medicine. 2019. Manual on Subsurface Investigations. Washington, DC: The National Academies Press. doi: 10.17226/25379.
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Suggested Citation:"Chapter 3. Subsurface Investigation Processes." National Academies of Sciences, Engineering, and Medicine. 2019. Manual on Subsurface Investigations. Washington, DC: The National Academies Press. doi: 10.17226/25379.
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Suggested Citation:"Chapter 3. Subsurface Investigation Processes." National Academies of Sciences, Engineering, and Medicine. 2019. Manual on Subsurface Investigations. Washington, DC: The National Academies Press. doi: 10.17226/25379.
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Suggested Citation:"Chapter 3. Subsurface Investigation Processes." National Academies of Sciences, Engineering, and Medicine. 2019. Manual on Subsurface Investigations. Washington, DC: The National Academies Press. doi: 10.17226/25379.
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Suggested Citation:"Chapter 3. Subsurface Investigation Processes." National Academies of Sciences, Engineering, and Medicine. 2019. Manual on Subsurface Investigations. Washington, DC: The National Academies Press. doi: 10.17226/25379.
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Suggested Citation:"Chapter 3. Subsurface Investigation Processes." National Academies of Sciences, Engineering, and Medicine. 2019. Manual on Subsurface Investigations. Washington, DC: The National Academies Press. doi: 10.17226/25379.
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Suggested Citation:"Chapter 3. Subsurface Investigation Processes." National Academies of Sciences, Engineering, and Medicine. 2019. Manual on Subsurface Investigations. Washington, DC: The National Academies Press. doi: 10.17226/25379.
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Suggested Citation:"Chapter 3. Subsurface Investigation Processes." National Academies of Sciences, Engineering, and Medicine. 2019. Manual on Subsurface Investigations. Washington, DC: The National Academies Press. doi: 10.17226/25379.
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Suggested Citation:"Chapter 3. Subsurface Investigation Processes." National Academies of Sciences, Engineering, and Medicine. 2019. Manual on Subsurface Investigations. Washington, DC: The National Academies Press. doi: 10.17226/25379.
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Suggested Citation:"Chapter 3. Subsurface Investigation Processes." National Academies of Sciences, Engineering, and Medicine. 2019. Manual on Subsurface Investigations. Washington, DC: The National Academies Press. doi: 10.17226/25379.
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Suggested Citation:"Chapter 3. Subsurface Investigation Processes." National Academies of Sciences, Engineering, and Medicine. 2019. Manual on Subsurface Investigations. Washington, DC: The National Academies Press. doi: 10.17226/25379.
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Suggested Citation:"Chapter 3. Subsurface Investigation Processes." National Academies of Sciences, Engineering, and Medicine. 2019. Manual on Subsurface Investigations. Washington, DC: The National Academies Press. doi: 10.17226/25379.
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Suggested Citation:"Chapter 3. Subsurface Investigation Processes." National Academies of Sciences, Engineering, and Medicine. 2019. Manual on Subsurface Investigations. Washington, DC: The National Academies Press. doi: 10.17226/25379.
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Suggested Citation:"Chapter 3. Subsurface Investigation Processes." National Academies of Sciences, Engineering, and Medicine. 2019. Manual on Subsurface Investigations. Washington, DC: The National Academies Press. doi: 10.17226/25379.
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Suggested Citation:"Chapter 3. Subsurface Investigation Processes." National Academies of Sciences, Engineering, and Medicine. 2019. Manual on Subsurface Investigations. Washington, DC: The National Academies Press. doi: 10.17226/25379.
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Suggested Citation:"Chapter 3. Subsurface Investigation Processes." National Academies of Sciences, Engineering, and Medicine. 2019. Manual on Subsurface Investigations. Washington, DC: The National Academies Press. doi: 10.17226/25379.
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Suggested Citation:"Chapter 3. Subsurface Investigation Processes." National Academies of Sciences, Engineering, and Medicine. 2019. Manual on Subsurface Investigations. Washington, DC: The National Academies Press. doi: 10.17226/25379.
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Suggested Citation:"Chapter 3. Subsurface Investigation Processes." National Academies of Sciences, Engineering, and Medicine. 2019. Manual on Subsurface Investigations. Washington, DC: The National Academies Press. doi: 10.17226/25379.
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Suggested Citation:"Chapter 3. Subsurface Investigation Processes." National Academies of Sciences, Engineering, and Medicine. 2019. Manual on Subsurface Investigations. Washington, DC: The National Academies Press. doi: 10.17226/25379.
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Suggested Citation:"Chapter 3. Subsurface Investigation Processes." National Academies of Sciences, Engineering, and Medicine. 2019. Manual on Subsurface Investigations. Washington, DC: The National Academies Press. doi: 10.17226/25379.
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Suggested Citation:"Chapter 3. Subsurface Investigation Processes." National Academies of Sciences, Engineering, and Medicine. 2019. Manual on Subsurface Investigations. Washington, DC: The National Academies Press. doi: 10.17226/25379.
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Suggested Citation:"Chapter 3. Subsurface Investigation Processes." National Academies of Sciences, Engineering, and Medicine. 2019. Manual on Subsurface Investigations. Washington, DC: The National Academies Press. doi: 10.17226/25379.
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12 C H A P T E R 3 Subsurface Investigation Processes Introduction This chapter presents processes for developing the scope for the subsurface investigations that are typically conducted to collect subsurface data for planning, designing, constructing, and operating transportation facilities. The specific motivations may include selecting appropriate foundation types, determining if specialty construction means and methods are required, or estimating bid quantities. The scope of a subsurface investigation is typically governed by the types and amount of data that need to be collected to develop an appropriate ground model for the project to address these requirements. The scope is optimized by selecting the most appropriate and efficient methods and equipment for collecting and evaluating the required data. The topics covered in this chapter include the following: 1. Identifying the types of data required to address the anticipated geotechnical risks and performance issues 2. Determining the required appropriate type of data 3. Selecting the most appropriate investigation equipment for the anticipated site conditions 4. Selecting the appropriate scope for geophysical testing and the appropriate geophysical methods 5. Selecting the appropriate scope for in situ testing and the appropriate in situ tests 6. Selecting the appropriate sampling equipment and borehole advancing methods for the anticipated subsurface conditions 7. Selecting the appropriate type of measurements to evaluate groundwater conditions 8. Selecting the appropriate scope for laboratory testing and the appropriate laboratory tests AASHTO R 13 (ASTM D420) contains complementary guidance on developing the scope of a geotechnical site investigation and selecting the appropriate geophysical, in situ, and laboratory test methods. Types of Data Required for Subsurface Investigations Determining the types of data required for an investigation is a critical step of investigation planning because the data drives the scope as well as the cost and schedule of the investigation activities. If inappropriate types of data are obtained or appropriate information is missed, data gaps will occur. The data gaps may require remobilizing to the field to collect the missing information, which will add to the investigation cost and could delay a project. An understanding of the primary factors that influence the types of information needed for an investigation is critical to developing a sound investigation plan. The remainder of this section discusses the three primary factors that govern the types of data that need to be collected and analyzed for an investigation: • Subsurface investigation objectives • Information that can be obtained from available records • Information that can be obtained from a site reconnaissance

13 Reviewing the available records and conducting site reconnaissance provides information that will aid in understanding the local geology, potential variability in subsurface conditions, likely performance issues that need to be addressed by the investigation, available pertinent data, and the data gaps that need to be filled to satisfy the investigation objective(s). 3.2.1 Subsurface Investigation Objectives The objective(s) of a subsurface investigation must be clearly defined and understood for the project features being investigated to correctly determine the data requirements. Otherwise the type of data collected may not be appropriate. Investigation objectives can be broadly grouped into the following four categories: 1. Project planning 2. Design and construction 3. Performance monitoring during construction and operation 4. Forensic evaluations 3.2.1.1 Project Planning Investigations Investigations related to project planning are typically conducted during the initial stages of project development when multiple corridors and alignments within each corridor are being evaluated. This is so project planning investigation results can be factored into selecting the project corridor and alignments. The primary objective of a geotechnical investigation during the planning stage is to identify geologic or man- made constraints that can significantly affect major planning considerations (e.g., public safety during and after construction, environmental protection, construction costs, operation costs, project delivery schedule). Acquiring subsurface information to identify geologic or man-made constraints may or may not require an extensive or costly field effort during the planning stage. Examples of geologic and man-made constraints that could affect project planning are presented in Table 3-1. To aid in determining the types of information that may be required during a planning investigation, Table 3-1 also includes the types of information that are typically required to evaluate each potential constraint. Table 3-1. Geologic and man-made constraints Geologic or Man- Made Constraint Geotechnical Performance Issue Evaluation Methods Information Requirements Subsidence (mass displacement, open voids in roadways) Foundation support, settlement, and ground improvement Mapping, geophysical methods, in situ testing, and drilling and sampling Depth, geometry, areal extent, stratigraphy Landslides in soil and rock Slope stability, lateral earth pressure and excavation support, dewatering, and permanent groundwater control Remote sensing, mapping, geophysical methods, in situ testing, and drilling and sampling Geometry of mass, definition of driving stresses, shear strength, unit weight, hydraulic conductivity, groundwater conditions, and stratigraphy Unstable soil and rock (e.g., soft or weak soils, Foundation support, settlement, ground improvement, Mapping, geophysical methods, in situ testing, Stratigraphy, shear strength, groundwater conditions, moisture

14 Geologic or Man- Made Constraint Geotechnical Performance Issue Evaluation Methods Information Requirements degradable rock, expansive soils) permanent groundwater control, soil shrink and swell, use of materials excavated from the project, and frost penetration and freezing and drilling and sampling content, Atterberg limits, organic content, hydraulic conductivity, unit weight, sensitivity, coefficient of consolidation, and compression index Chemical properties Corrosion of buried metals and use of materials excavated from the project Known detrimental geologic formations, mapping, drilling, petrographic, and laboratory testing Stratigraphy, depth, geometry, and areal extent, pH, resistivity, and mineralogy Abandoned landfills Foundation support, settlement, corrosion of buried metals, and slope stability Remote sensing, mapping, reviewing records, geophysical methods, in situ testing, and drilling and sampling Depth, geometry, areal extent, contents of the landfill, and chemical properties of the landfill materials Contaminated sites Deterioration of buried structural components, constructability hazards, and disposal of materials excavated from the project Mapping, reviewing available records, geophysical methods, in situ testing, and drilling and sampling Depth, geometry, areal extent, and chemistry of waste materials Flooding, scour, and erosion Foundation support, settlement, slope stability, lateral earth pressure, and excavation support Mapping, reviewing available information, geophysical methods, in situ testing, and drilling and sampling Estimation of erosion susceptibility and determination of level or path of flow Rock structure Slope stability, foundation support, settlement, dewatering, and permanent groundwater control Mapping, drilling, sampling, testing, and reviewing available information Orientation and spacing of rock discontinuities, rock classification, shear strength, elastic modulus, and unit weight Groundwater conditions Impacts most geotechnical performance issues Reviewing available records, geophysical methods, in situ testing, monitoring wells, and piezometers Aquifer and aquitard characteristics, groundwater levels, direction and gradient of groundwater flow Stratigraphy Impacts most geotechnical performance issues Geophysical methods, in situ testing, and drilling and sampling Depth, thickness, and classification of each strata Sources: AASHTO and FHWA (2002)

15 3.2.1.2 Design and Construction Investigations Design and construction investigations are typically conducted after the project alignment and grade has been selected and the locations of structures have been established. The objectives of the design and construction investigations are to collect data to aid with the following: • Identifying geotechnical performance issues of concern • Identifying areas of concern and mapping their three-dimensional (3D) extent (areal and depth) • Understanding the potential geologic variability of the site • Developing design recommendations and specifications Table 3-2 provides a summary of information to aid with selecting which performance issues to evaluate for each type of project feature and the types of data that may be needed. The data required to identify the stratigraphy and groundwater conditions is required for most of the project features because these are important to identifying the geotechnical performance issues of concern, areas of concern, and quantifying the geologic variability of a site. Table 3-2. Performance issues and required design properties and parameters Project Feature Performance Issues Data Required Bridges and viaducts Foundation support, settlement, lateral earth pressure, seismic evaluations, dewatering, corrosion or decay potential, and construction impacts on adjacent structures Index properties, shear strength (drained and undrained), unit weight, coefficient of consolidation, compression index, preconsolidation pressure, chemical properties of soil and rock, lateral stress coefficient, hydraulic conductivity, artesian conditions, rock structure, durability of rock, shear modulus, shear damping, shrink and swell, stratigraphy, and groundwater conditions Retaining structures Lateral earth pressure, foundation support, settlement, permanent groundwater control, seismic evaluations, corrosion, and construction impacts on adjacent structures Index properties, lateral stress coefficient, shear strength (drained and undrained), elastic modulus, unit weight, coefficient of consolidation, hydraulic conductivity, chemical properties of soil and rock, rock structure, shear modulus, shear damping, stratigraphy, and groundwater conditions Cuts and embankments Slope stability, heave potential, permanent groundwater control, ground improvement, use of materials excavated from the project, evaluation of material sources, and construction impacts on adjacent structures Lateral stress coefficient, shear strength (drained and undrained), elastic modulus, unit weight, coefficient of consolidation, compression index, preconsolidation pressure, hydraulic conductivity, chemical properties of soil and rock, rock structure, durability of rock,

16 Project Feature Performance Issues Data Required stratigraphy, and groundwater conditions Pavements Soil shrink and swell, frost penetration and freezing, permanent groundwater control, ground improvement, use of materials excavated from the project, and evaluation of material sources Index properties, compaction characteristics, resilient modulus, CBR, resistance R-value, hydraulic conductivity, stratigraphy, and groundwater conditions Tunnels and underground structures Slope stability, heave potential, dewatering, permanent groundwater control, ground improvement, corrosion, and construction impacts on adjacent structures Index properties, lateral stress coefficient, shear strength (drained and undrained), elastic modulus, unit weight, coefficient of consolidation, hydraulic conductivity, chemical properties of soil and rock, rock structure, shear modulus, shear damping, stratigraphy, and groundwater conditions Culverts and pipes Lateral earth pressure, excavation support, dewatering, foundation support, settlement, heave potential, corrosion, use of materials excavated from the project, and evaluation of material sources Lateral stress coefficient, shear strength (drained and undrained), index properties, elastic modulus, unit weight, coefficient of consolidation, compression index, preconsolidation pressure, chemical properties of soil and rock, rock structure, durability of rock, stratigraphy, and groundwater conditions Poles, masts, and towers Foundation support, corrosion, and lateral earth pressure Index properties, shear strength (drained and undrained), unit weight, coefficient of consolidation, compression index, preconsolidation pressure, chemical properties of soil and rock, lateral stress coefficient, hydraulic conductivity, artesian conditions, rock structure, durability of rock, shear modulus, shear damping, shrink and swell, stratigraphy, and groundwater conditions Sources: AASHTO, FHWA (2002), and FHWA (2017) 3.2.1.3 Performance Monitoring During Construction and Operation During construction, the investigation objective is usually to monitor performance and guide construction. For example, settlement plates and piezometers are typically installed while constructing embankments on soft soils to measure the amount of settlement and monitor dissipation of pore pressures;

17 this information is used to determine when to place the next lift of the embankment fill or when to terminate waiting periods. During operation, the investigation objective is to collect information that can help with evaluating the existing conditions, predicting long-term performance, and identifying geotechnical assets with impending elevated risk of failure or deficient performance. This information can be used to optimize operation and maintenance budgets and establish rehabilitation priorities. The type of information that can be collected during construction and operation include pore pressures, vertical and horizontal deformations, tilt, crack widths, loads, strains, and changes in groundwater elevations during dewatering. Additional information regarding performance monitoring during construction and operation of geotechnical project features is presented in Appendices A and B. 3.2.1.4 Forensic Evaluations The investigation objective of evaluating failure is to identify the factors that contributed to the failure; the investigation is usually directed and focused on specific location(s) where failure has occurred. The type of information that needs to be collected during forensic evaluations depends on the type of failure, but it is similar to information required for design and construction and could include performance monitoring. 3.2.2 Review of Available Records Once the objectives of an investigation are defined, Tables 3-1 or 3-2 can be used to preliminary select the types of geotechnical issues that may need to be evaluated and the types of data that may need to be collected. While Tables 3-1 and 3-2 provide a good starting point, the information is not site specific, and additional screening is necessary to obtain refined site-specific information. Fortunately, there are many readily available data sources that can be used to identify such things as the major geologic processes that have affected a project site and history of land use at the site and surrounding areas that can reveal potentially problematic past activities (e.g., mining, waste disposal). For example, historical aerial photographs can be used to identify areas that have experienced landslides, locations of past mining activities, and past industrial activities. Reviewing available information from multiple sources is critical to developing a sound investigation plan because it helps narrow the relevant site-specific geotechnical issues that need to be evaluated. Reviewing available records also establishes the types and amount of information that is available which helps identify the data gaps that need to be filled by the investigation. Table 3-3 provides a listing of the types of documents that contain information relevant to geotechnical investigations, sources of information included in these documents, and the types of information that can be found in each type of document. Table 3-3. Documents and sources of available information Types of Documents Sources of Information Type of Available Information Comments Topographic Maps United States Geological Survey and state geological survey agencies Site topography, physical features, and good index map of site area Maps can be used to evaluate access issues for field equipment and identify areas susceptible to slope instability.

18 Types of Documents Sources of Information Type of Available Information Comments Digital elevation models United States Geological Survey 3D Elevation Program (3DEP), United States Interagency Elevation Inventory (USIEI), and state and regional data clearinghouses Raster grids of elevation values representing ground surface topography and terrain Geographic information systems (GIS) can use a digital elevation model for 3D surface visualization, contouring, slope calculations, etc. Soil survey reports National Resource Conservation Service and local soil conservation agencies Soil classifications using AASHTO and Unified Soil Classification System methods, moisture contents, Atterberg limits, organic contents, chemical properties (e.g., pH), permeability of soils, climate, stratigraphy, and groundwater conditions Available information is for shallow depths (6 ft or less). It can be useful in identifying near-surface problematic soils (e.g., soils susceptible to swelling and shrinkage) or identifying potential borrow sources. Geologic maps and reports and sinkhole and karst maps United States Geological survey and state geological survey agencies Soil and rock formations (rock types, fracture, orientation and approximate age), groundwater flow patterns, and bedrock contours that provide approximate estimates of rock depths, and potential geologic hazards These documents can be used to identify areas susceptible to landslides, subsidence, and others. Aerial photographs Google EarthTM, Google MapsTM, National Agriculture Imagery Program (NAIP), and aerial survey companies Man-made structures, geologic and hydrogeologic information, current and past land use, borrow sources, and potential geologic and man-made hazards Photographs can track site changes over time to identify potential problematic past land use activities or geologic events such as landslides. Existing subsurface investigation reports for nearby projects State and local DOTs, United States Geological survey, state and local environmental agencies, and United States Environmental Protection Agency Soil and rock classification, stratigraphy, groundwater conditions, engineering parameters-shear strength, unit weights, elastic modulus, coefficient of consolidation, compression index, hydraulic conductivity, Atterberg limits, moisture and organic contents, potential hazards, and locations of landfills and Superfund sites Existing subsurface reports can be useful in identifying geotechnical performance issues of concern. Hydrogeological and well maps and well logs United States Geological survey, state natural Hydrogeological features (e.g., springs), groundwater hazards, Well maps and logs can be useful in evaluating the need for construction

19 Types of Documents Sources of Information Type of Available Information Comments resources and soil survey agencies stratigraphy, and groundwater depths dewatering and permanent groundwater control. Utility maps Utility companies and local government agencies Locations of buried utilities Utility maps are very useful in identifying potential locations for in situ testing, drilling, and sampling to avoid impacting utilities or creating unsafe working environment. Also, useful in mapping potential equipment access routes to drilling and testing locations. Flood insurance maps Federal Emergency Management Agency, USACE, United States Geological survey, State and local government agencies 100- and 500-year floodplains, data for evaluating scour potential This information can be used to ensure that the final alignment does not go through 100- and 500- year floodplains. Sanborn fire insurance maps Library of Congress, state and university libraries, and Sanborn Company Environmental hazards and historical land use Sanborn maps are available for urban areas. Sources: AASHTO, FHWA (2002), and FHWA (2017) 3.2.3 Site Reconnaissance Site reconnaissance provides a firsthand account of the actual conditions on the ground and often will reveal information that would most likely be overlooked without it. Site reconnaissance minimizes the potential for encountering major unexpected problems during the investigation. It also offers an opportunity to confirm and further explore issues identified from reviewing available records. Additional objectives for site reconnaissance include the following: • Identify and confirm the geotechnical and man-made constraints relevant to the investigation • Select the appropriate equipment required for geophysical testing, in situ testing, and drilling • Select locations for geophysical testing, in situ testing, and drilling • Select viable access routes to testing and drilling locations • Provide good estimation of the time that will be required to complete the field investigation • Identify field personnel safety needs Site reconnaissance activities need to be conducted systematically to avoid missing critical pieces of information. Table 3-4 includes information that can aid with planning and executing an effective site reconnaissance.

20 Table 3-4. Items that need to be evaluated during field reconnaissance Item Things to Note Comments Access Rank access using one of the following criteria: (1) easy, (2) accessible by four- wheel drive, (3) dozer and grading required, and (4) inaccessible. Evaluating access helps determine the types of equipment that will be required. Utilities Existing overhead lines, marked gas lines, manholes, sewer outfalls, and power substations. Utilities information helps select appropriate in situ testing, drilling, and sampling locations. Surface soils Presence of fill, debris, pollutants, slope instabilities, heave, subsidence, and scour Evaluating surface soils can reveal evidence of abandoned landfills, Superfund sites, subsidence, and flooding. Subsurface materials Visual soil and rock classifications, loose cobbles, boulders, rock outcrops, rock joint patterns, faults, discontinuities, weathering, planes of weakness, talus, karst and sinkholes, and caves Subsurface materials can provide evidence for subsidence, landslide activity, unstable soil and rock, and stratigraphy. Surface drainage Swampy, ponds, lakes, streams, and rivers Surface drainage information provides indications of the groundwater conditions, hydraulic conductivity of the underlying materials, potential for flooding, and others. Subsurface drainage Major aquifers, water wells, and pumping from deep wells Subsurface draining information provides indication of the groundwater conditions. Terrain Rank terrain in terms of (1) level ground, (2) sloping, (3) hummocky, (4) rolling hills, and (5) mountainous. Evaluating terrain helps with selecting appropriate field equipment, assessing the need for slope stability investigations, and others. Past investigations Existing boreholes, core holes, and evidence of past blasting operations Past investigations can provide information regarding stratigraphy and groundwater conditions. Source: AASHTO and FHWA (2002) After the field reconnaissance study is complete, a field reconnaissance report should be prepared for use in developing the investigation plan. This report should include the following: 1. A summary of the geologic framework of the site area 2. A summary of the geotechnical issues of concern 3. A stratigraphic listing of soil and rock units expected to be encountered 4. Locations, numbers, and depth ranges for the recommended subsurface investigation methods 5. Locations or areas requiring special attention 6. Opinion relating to potential use of materials excavated from the project and the probability of locating significant quantities of borrow materials near the project 7. A sketched reconnaissance map

21 Developing a Subsurface Investigation Plan While Section 3.2 primarily focused on providing guidelines for selecting the appropriate types of information to collect for each of the four investigation objectives, this section focuses on providing guidelines for developing subsurface investigation plans only for the planning, design, and construction objectives. The other two objectives—performance monitoring during construction and operation, and forensic investigations—are not included because they are usually not part of the typical subsurface investigation programs conducted for transportation projects. Information on geotechnical instrumentation for performance monitoring and forensic investigations is presented in Appendices A and B. As discussed in Chapter 2, a subsurface exploration program is a means to identify and reduce the uncertainties regarding soil and rock conditions at the project site. In general, confidence in design parameters increases with increasing quantity and quality of measurements. Design parameters derived from small number of measurements will have large uncertainty and may be inappropriate regardless of the methods used for testing or interpretation (Christian 2004). The rest of this section provides guidelines for developing the scope for a sound, cost-effective investigation plan that has one or more of the following components: 1. Developing a geophysical testing plan 2. Selecting the number and locations for in situ tests, drilling, and sampling 3. Determining the minimum depth of investigation at each location 4. Determining the required types of samples and the sampling frequency 5. Developing an in situ and laboratory testing plan 6. Developing a plan for evaluating groundwater conditions The type and overall scope of the project will dictate which of the components are required to develop an appropriate ground model. 3.3.1 Developing a Geophysical Testing Plan Geophysical investigations are used to estimate the physical properties of the subsurface by measuring, analyzing, and interpreting seismic, electrical, electromagnetic, gravitational, and magnetic fields measured at the ground surface or within boreholes. Surface geophysical methods are well suited to use 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 survey(s). A geophysical investigation followed by a targeted program of in situ tests, drilling and sampling, and laboratory tests is a robust approach to developing an accurate conceptual site model for a project. More information is presented in Chapter 4 regarding the selection and use of geophysical methods for a geotechnical site investigation. 3.3.2 Selecting the Number and Locations for In Situ Tests, Drilling, and Sampling There are primary factors that influence the selection of the number and locations for in situ tests and boreholes for obtaining samples at a site: • Objective of the investigation • Project feature(s) being targeted by the investigation • Subsurface conditions • Anticipated uncertainty and variability of the subsurface conditions • Access constraints

22 Investigations conducted for design and construction objectives are much more detailed than planning objectives and, therefore, require more investigation locations than investigations for planning objectives. Data required for planning objectives is needed to evaluate global geotechnical issues that can affect the selection of the project corridor and alignment. This type of information can generally be obtained from reviewing existing data and conducting a site reconnaissance. The primary purpose of field investigations done for planning is to verify information obtained from reviewing existing records and site reconnaissance and collecting data that can be used for preliminary designs, so the investigation locations tend to be farther apart, which reduces the number of locations required. The guidelines presented in the remainder of this section apply to design and construction investigations. The required minimum number of locations for design and construction investigations depends on the types of project features being targeted by the investigation. For example, the typical maximum spacing between investigation locations for structures such as bridges and earth-retaining structures is usually smaller than for the roadway features such as embankments and roadway cuts; therefore, for identical lengths, the number of required investigation locations for structures is usually greater than for roadway features. Column 2 of Table 3-5 provides guidelines for determining the minimum number of investigation locations for each type of project feature. The number of investigation locations may be increased or decreased depending on the expected variability of subsurface conditions and other project-specific considerations. Table 3-5. Guidelines for selecting number of investigation locations and depths of investigation Project Feature Minimum Number of Investigation Locations Minimum Depth of Investigation Bridge - shallow foundations One location per pier if width of pier is less than 100 ft (30 m) Two locations per pier if width of pier is greater than 100 ft (30 m) Additional investigation locations should be included if uncertain or highly variable subsurface conditions are encountered. 2B for L ≤ 2B 3B for 2B ≤ L ≤ 5B 4B for L ≥ 5B Should extend below any soft compressible material into competent material. Should extend 10 ft (3 m) into competent rock if rock is encountered before the above criteria is met. Bridge - deep foundations One location per pier if width of pier is less than 100 ft (30 m) Two locations per pier if width of pier is greater than 100 ft (30 m) Additional investigation locations should be included if uncertain or highly variable subsurface conditions are encountered. At each shaft location for rock socketed shafts. In soil: Extend below the anticipated pile tip elevation a minimum of 20 ft (6 m) or 2x the maximum group dimension whichever is greater. Piles on rock: Extend below anticipated pile tip elevation a minimum of 10 ft (3 m). Shafts on or in rock: Extend below anticipated shaft tip elevation a minimum of 10 ft (3 m) or 3x shaft diameter for isolated shafts or 2x maximum group dimension whichever is greater.

23 Project Feature Minimum Number of Investigation Locations Minimum Depth of Investigation Retaining structures A minimum of one location for each wall. If wall is greater than 100 ft (30 m), spacing should be at intervals of 100 to 200 ft (30 to 60 m) with locations alternating from in front to behind the wall. Anchored walls: Additional locations in the anchorage zone spaced at intervals of 100 to 200 ft (30 to 60 m) Soil nail walls: Additional locations behind the wall at a distance of 1 to 1.5x the wall height. Spacing should be at intervals of 100 to 200 ft (30 to 60 m). Temporary structures: The same recommendations generally apply to temporary structures. However, the scope of the subsurface investigation for temporary structures is usually within the purview of the contractor. Extend below bottom of the wall 2x the wall height or 10 ft (3 m) into hard rock. Should extend below any soft compressible material into competent material. Roadway - embankment foundations Spacing of 200 ft (60 m) in uncertain or highly variable conditions to 400 ft (120 m) in uniform conditions along centerline embankment. At critical locations (maximum height or maximum depth of soft strata): a minimum of three locations along the transverse direction. Bridge approach embankment: a minimum of one location per abutment. Depth of 2x the embankment height unless a hard stratum is encountered above this depth. If soft strata are encountered extending to a depth greater than 2x embankment height, extend below the soft strata into competent material. Roadway cuts Spacing of 200 ft (60 m) in uncertain or highly variable conditions to 400 ft (120 m) in uniform conditions along centerline of cut. At critical locations (maximum cut depth or maximum depth of soft strata): a minimum of three locations along the transverse direction. For cut slopes in rock, perform geologic mapping along the length of the cut slope. Minimum depth of 15 ft (4.5 m) below lowest cut elevation unless a hard stratum is encountered before the minimum depth is achieved. If soft strata are encountered extend depth of investigation to a competent layer. If base of cut extends below groundwater level, extend depth of investigation to determine the depth of the underlying pervious strata Pavements Spacing of 100 to 300 ft (30 to 90 m) depending on the subsurface conditions. Closer spacing for uncertain or highly variable conditions Minimum depth of 10 ft (3 m) from the proposed top of subgrade elevation.

24 Project Feature Minimum Number of Investigation Locations Minimum Depth of Investigation and longer spacing for uniform conditions. Culverts and pipes One boring at each end of the culvert. Additional borings between the end of culvert spaced at 100 to 300 ft (30 to 90 m) depending on the variability of the subsurface conditions For culvert extensions, one boring every 50 to 100 ft (15 to 30 m) with a minimum of one boring. Same criteria as for bridge foundations for large culverts. Small culverts: Minimum of 10 ft (3 m) below anticipated invert elevation Poles, masts and towers One boring at each foundation location. 30 ft (9 m) below the anticipated top of foundation in soil or 10 ft (3 m) of rock coring whichever is shallower. Source: FHWA (2002), FHWA (2017), New York State DOT (2013), and South Carolina DOT (2010) B: Footing width L: Footing length The nature of subsurface conditions also influences the number of in situ tests and borings that can be conducted within the available schedule and budget for the site investigation. For example, if ground conditions are suitable for using CPTs, it is likely possible to conduct a larger number of tests than for sites where extensive rock coring is needed to obtain samples for laboratory testing. The anticipated uncertainty and variability in subsurface conditions is another consideration for selecting the number of investigation locations. A larger number of locations should be planned for sites with uncertain or highly variable subsurface conditions to reduce the knowledge uncertainties and to more accurately estimate the natural variability associated with soil and rock conditions at the project site. Finally, access constraints at a site drive the equipment selection. Costs associated with mobilizing and using equipment are a large component of the investigation cost. For a project with a fixed budget, if the site has easy access and as such a lower equipment cost, the budget can include a larger number of investigation locations. For example, drilling over water is typically much more expensive than drilling on land because it requires using barges. Mobilization costs for barges are generally high, and set up times and time required to move between investigation locations on water are much longer than drilling on land; so drilling over water is a much slower investigation process than drilling on land. This slower process means that for the same budget and schedule, fewer investigation locations will be selected when drilling over water. Table 3-6 presents information that can aid with selecting the appropriate equipment for the anticipated access conditions. Additional information on equipment for conducting borings and soundings is presented in Chapters 5 and 6. Table 3-6. Investigation equipment and their applications Rig Type Application Truck-mounted drill rigs Areas with easy access All-terrain vehicles drill rigs Sites with soft ground and rugged terrain Track-mounted drill rigs Sites with swampy and very soft ground

25 Skid drill rigs Sites with steep terrain Wireline drill rigs Rock sampling Hydraulic direct-push rigs Fast, continuous sampling, cleaner (no spoils) Sonic rigs Continuous sampling of soil and rock Barges – regular Over water drilling for shallow water depths (10 ft [3 m] or less) Jack up platform barges Over water drilling for areas with deep water (up to 40 ft [12 m]) Source: Australian Drilling Industry Training Committee (2015) 3.3.3 Determining the Minimum Depth of Investigation at Each Location The minimum required depth of investigation at each location usually depend on (i) the type of anticipated subsurface conditions and (ii) the type of feature being investigated. Column 3 of Table 3-5 provides guidelines on the criteria that should be used to select the minimum depth of investigation for each type of project feature. The criteria for bridge foundations works best in projects where a preliminary investigation has been completed and there is some site-specific subsurface information that can be used to select the most likely foundation type and approximate depth to the bottom of the foundation element. If a preliminary investigation has not been completed, some assumptions regarding the subsurface conditions need to be made based on available records and knowledge of the local geology to start the investigation; adjustments should be made during the investigation in consultation with the geotechnical design engineer when the actual site conditions differ from the assumed conditions. 3.3.4 Determining the Required Types of Samples and Sampling Frequency Once the investigation locations and the depths of investigation at each sampling location have been selected, the next step includes determining the required types of samples (disturbed and undisturbed) and the required sampling frequency. Disturbed samples are used to conduct index tests that are required for (i) classifying soil and rock, (ii) developing stratigraphy, and (iii) identifying problematic soil or rock conditions. Disturbed samples and index tests are more economical, so they are typically used more frequently. Undisturbed samples are used to run performance tests that are required to measure specific design parameters for fine-grained soils. Undisturbed samples and performance tests are more expensive, so they are used less frequently than disturbed samples. The required sampling frequency for each type of sample varies depending on the variability of the subsurface conditions at the site, the type of project feature being investigated, and the required design properties. In general, a lower sampling frequency is required for sites with more uniform subsurface conditions. An additional consideration for undisturbed samples is difficult sampling conditions that can induce sample disturbance. In these situations, the sampling frequency should be higher to offset the samples that may be unsuitable for laboratory testing due to a high degree of disturbance. FHWA (2002) recommends the following minimum sampling frequencies for disturbed and undisturbed samples: • Two disturbed samples per 5-ft (1.5-m) interval in the top 10 ft (3 m) • One disturbed sample per 5-ft (1.5-m) interval for depths from 10 to 100 ft (3 to 30 m) • One disturbed sample per 10-ft (3-m) interval for depths greater than 100 ft (30 m) • One undisturbed sample in each layer of fine-grained soil • For layers of fine-grained soil thicker than 10 ft (3 m), one undisturbed sample for every 10 to 20 ft (3 to 6 m)

26 These guidelines should be considered as a starting point and may need to be adjusted during the investigation depending on the results of the investigation. If the results show that the subsurface conditions are uncertain or highly variable, additional samples will likely be required. 3.3.5 Developing an In Situ and Laboratory Testing Plan Once the types of samples and sampling frequency have been determined, the next step is selecting the combination of in situ and laboratory tests that will provide reliable measurements of design parameters. Both types of tests have advantages and disadvantages. Laboratory tests usually offer the opportunity to make direct measures of design parameters (e.g., strength, compressibility), and it is often possible to control the boundary conditions precisely (e.g., confining pressure, drainage). However, laboratory tests are conducted on small specimens that may not be representative of the properties of the entire stratum in the field, and the test results will be affected by sample disturbance. On the other hand, in situ tests often involve minimal disturbance and, by definition, measure soil and rock properties at the in situ stress state. Some in situ tests, such as the CPT, allow for continuous profiling with depth. The disadvantage of many in situ tests is that they make an indirect measurement of design parameters. The desired parameter must be obtained from empirical correlations or via the solution of an appropriate boundary value problem. This introduces an additional source of uncertainty, which Phoon and Kulhawy (1999) called transformation uncertainty, due to the method being used to estimate the design parameter. In situ and laboratory tests complement one another well. In situ tests are often a large component of the subsurface exploration program because tests can usually be conducted at lower cost. Laboratory tests provide direct measurements of design parameters that can either be used for design or to evaluate the accuracy of the values estimated from in situ tests. On larger projects, sufficient laboratory tests can be conducted to develop site-specific empirical correlations that can reduce transformation uncertainties associated with in situ tests. 3.3.5.1 In Situ Testing In situ testing programs are designed to provide the information required to accomplish the following: 1. Classify soil and rock 2. Develop stratigraphy 3. Establish variability of the subsurface conditions 4. Identify problematic soils and rock 5. Provide measurements of the required parameters for design and construction Because it is possible to collect abundant data via in situ tests, they allow for a more comprehensive definition of soil strata, zones, layering, and stratigraphy, as well as the identification of lenses, weak zones, and inclusions. In situ tests also allow an investigation of vertical and horizontal variability to evaluate the heterogeneity across a site. The selection of the most appropriate in situ tests for a site depends on the (i) anticipated subsurface conditions, (ii) ability of the in situ tests to provide reliable estimates of the required design parameters, and (iii) cost. Anticipated subsurface conditions tend to rule out certain in situ tests. For example, in situ test methods that require direct push into the ground are usually not appropriate for sites with hard soil deposits or rock, while in situ tests that requires a borehole may be appropriate for a wide variety of subsurface conditions. Table 3-7 provides a summary of information that should aid in selecting the appropriate in situ tests for the anticipated subsurface conditions and data requirements. Additional information pertaining to in situ tests is provided in Chapter 5.

27 Table 3-7. Summary of in situ tests and associated design parameters In Situ Tests Design Parameters Estimated Advantages Disadvantages SPT Drained shear strength of sands and over- consolidated clays Most widely used in situ test, economical, can be conducted in a wide variety of materials including partially weathered rock, and allows recovery of samples SPT is unreliable for soils containing course gravels, cobbles, boulders, silts, or soft sensitive clays. Piezocone Penetrometer Test Drained and undrained shear strength, coefficient of consolidation, and hydraulic conductivity; compression and shear wave velocity can be measured via a seismic downhole test using the piezocone penetrometer test Fast and continuous profiling (excellent for stratigraphy), test is fast (economical and productive), results are not operator dependent Piezocone penetrometer test is unsuitable for very stiff to hard clays, gravel, boulder, and boulder deposits, and there is no sample recovery. Dilatometer Elastic modulus and lateral stress coefficient Good for predicting elastic settlements; can be used in sands, silts, and clays; can be run with either drilling or direct-push equipment A dilatometer may be difficult to push in very stiff to hard clays and very dense sands, and there is no sample recovery. Vane shear test Undrained shear strength and sensitivity Most accurate in situ test for determining undrained shear strength for soils with an undrained shear strength of 500 pounds per square foot (24 kilopascals) or less This test is not applicable for stiff and hard clays and there is no sample recovery. Pressuremeter Elastic modulus, shear modulus, lateral stress coefficient, and drained and undrained shear strength Can be used for both soil and weathered rock, can be run with either drilling or direct-push equipment, excellent for design of shallow foundations and evaluating lateral capacity of deep foundations This is a slow test that requires a very experienced operator. The results are affected by quality of borehole and there is no sample recovery. Rock PLT Elastic modulus of rock mass and lateral stress coefficient Measures rock mass properties and is good for evaluating settlement This test requires a specialized operator. Field rock DS test Drained shear strength Can be used along joints and shear planes to measure strength of discontinuities, good for slope stability analysis This test requires a specialized operator.

28 In Situ Tests Design Parameters Estimated Advantages Disadvantages Rock dilatometer Elastic modulus of rock mass Good for predicting settlement of rock mass This test requires a specialized operator. Rock borehole shear test Drained shear strength Robust, measures shear strength directly, and is good for evaluating slope stability and foundation design of drilled shaft socketed in rock This test requires a specialized operator and may not be readily available. Acoustic and optical televiewer Location and orientation of rock joints and other discontinuities Economical method for evaluating rock mass properties This test requires a specialized operator. Source: Clayton et al. (2008) and FHWA (2002) Information pertaining to stratigraphy and groundwater conditions is needed for most situations (Tables 3-1 and 3-2). One in situ test that is very efficient in developing stratigraphy and determining the approximate depth to groundwater is the piezocone penetrometer test (CPTu). Therefore, if the subsurface conditions are suitable for CPTu testing, it may be advantageous to conduct CPTu tests first and follow up with a more targeted in situ testing and sampling program. CPTu tests will help (i) establish the uniformity or heterogeneity of the subsurface conditions, which should help develop the scope of the sampling program (i.e., if uniform conditions are encountered, the required number of samples may be reduced), (ii) identify the types of soils in terms of coarse or fine grained, and (iii) quantify the consistency of fine-grained soils in terms of soft to hard, which should help with selecting the layers that require more targeted in situ testing and acquiring undisturbed samples. This information will also aid with determining the most appropriate in situ test(s). For example, if very soft to soft fine-grained soil layers are identified and shear strength is one of the required design parameters, vane shear tests (VSTs) can be conducted in those layers. The performance of geotechnical features founded in rock or constructed in rock depends on the composite strength of the rock mass rather than the strength of the intact rock. Therefore, in addition to obtaining cores for laboratory testing, field mapping and evaluating rock discontinuities should be included in the in situ evaluation program. Field mapping includes collecting information such as spacing of the discontinuities, continuous lengths of discontinuities, alignment of the discontinuities relative to the direction of loading, condition of the discontinuities in terms of roughness, and hardness. This information is needed to classify the rock mass and calculate its properties. Appropriate in situ tests on rock should be included to supplement the field mapping and evaluation efforts. 3.3.5.2 Sampling Equipment and Methods A summary of information pertaining to available sampling equipment for subsurface investigations, along with their applications, advantages, and disadvantages, is included in Table 3-8 to aid with selecting the most appropriate sampling equipment for an investigation. Additional information is presented in Chapter 6. Most of the sampling equipment presented in Table 3-8 requires a borehole. Therefore, selecting sampling equipment also requires selecting an appropriate borehole advancing method. To facilitate the selection of the most appropriate borehole advancing method for the anticipated subsurface conditions, information pertaining to borehole advancing methods and their applications is summarized in Table 3-9. Chapter 6 also contains information on methods for soil borings and rock corings.

29 Table 3-8. Sampling equipment and their applications Sampling Equipment Applications Advantages Disadvantages Split barrel Obtaining disturbed soil samples and partially weathered rock Robust and economical Results in poor or no recovery in loose sands, gravels, and cobbles; cannot obtain undisturbed samples. Sonic Obtaining continuous disturbed samples of soil and rock Excellent for stratigraphy, fast, and produces less spoils These cannot obtain undisturbed samples. Vibracore Obtaining continuous disturbed samples of soil at the bottom of a body of water, requires a minimum of 20 ft (6 m) of water above the mudline Excellent for stratigraphy These cannot obtain undisturbed samples. Shelby tube Obtaining undisturbed samples of soft to stiff silt and clay soils in cased boreholes Can obtain high-quality undisturbed samples Tubes are easily damaged if very stiff clays are encountered; cannot sample granular materials (e.g., sands, gravel). Laval Obtaining undisturbed samples of soil in cased boreholes Obtains very high- quality samples due to its large diameter, can sample a wide variety of soils These are more expensive than Shelby tubes. Piston Obtaining high-quality undisturbed samples in uncased boreholes Ability to sample in uncased boreholes These are complicated, time consuming, and costly. Osterberg Obtaining high-quality undisturbed samples of soft and potentially sensitive soils in uncased boreholes Excellent for sampling in swampy areas and areas with difficult access due to its portability, ability to sample in uncased boreholes These are unsuitable for sampling hard, dense, or gravelly soils. Denison Obtaining high-quality undisturbed samples of sand soils, gravel soils, hard clays, partially Excellent sample recovery These are unsuitable for sampling loose sands and soft clays.

30 Sampling Equipment Applications Advantages Disadvantages cemented soils, and partially weathered rock Core barrel Obtaining high-quality rock core samples Can sample a wide variety of rock materials These are unsuitable for sampling badly fractured rock. Source: Clayton et al. (2008) and FHWA (2002) Table 3-9. Borehole advancing methods and their applications Borehole Method Application Comments Solid flight auger Good for advancing soil borings to shallow depths (20 ft [6 m] or less) Sampling can only be done in soils (e.g., stiff clays, silts) that can allow the borehole to remain open (no cave in). Hollow-stem auger (HSA) Good for advancing deep soil borings (depths can exceed 100 ft [30 m]), may not work for sites with shallow depths to groundwater The augers may get stuck in the ground at sites with groundwater at shallow depths. Rotary drilling Good for advancing shallow and deep soil borings, especially good for sites with groundwater at shallow depths This is the most common exploration method (economical) and works for most materials. Test pits and trenches (excavated by backhoes) Good for mapping stratigraphy, determining depth to rock, presence of faults, degree of weathering, and groundwater inflow This is expensive but excellent for evaluating slope stability in projects with major cuts and where the geologic structure controls slope stability. It allows recovery of large block samples for laboratory testing. Exploratory shafts Good for mapping rock structure to adequately assess nature, elevation, and spacing of rock discontinuities This is expensive but excellent for obtaining design information for design of tunnels and underground structures. Manual methods (hand probes and hand augers) Used for shallow depth exploration in wetland areas and areas with very soft soils that are difficult to access with equipment, excellent for mapping thicknesses and lateral extent of soft clays and compressible organic soils These methods are slow (low productivity) but can be economical for small projects. Source: Clayton et al. (2008) and FHWA (2002) In planning and executing an in situ testing and sampling program, it is very important to minimize the number of equipment mobilizations and demobilizations to control cost and schedule. Therefore, in developing an in situ testing and sampling program, it is prudent to try to group together activities that can be efficiently conducted with the same equipment. For example, if in situ tests that require a borehole are a

31 part of the in situ testing program, they should be conducted during sampling and in the same borehole if possible. Also, it is important to provide regular updates to the geotechnical design engineer to facilitate adjustments in the testing and sampling program, especially if unanticipated subsurface conditions are encountered. For example, if CPTu test results identify fine-grained soil layers prone to consolidation, CPTu pore-pressure dissipation tests may need to be added to the testing program to evaluate time rate of consolidation parameters; if pore-pressure dissipation tests are necessary, the tests should be conducted before demobilizing the CPTu equipment. 3.3.5.3 Laboratory Testing Program Laboratory tests offer the capability to systematically characterize the behavior of soil and rock in a controlled environment. This capability allows laboratory tests to be used to model existing in situ conditions as well as conditions that will exist at various stages of project development. A typical laboratory testing program usually includes index and performance tests. Index tests provide general information regarding the material and include such tests as grain size distribution, Atterberg limits, moisture content, and organic content. Performance tests measure specific material parameters that are required for design and assessment of constructability, such as shear strength, coefficient of consolidation, compression index, and elastic modulus. The objective in developing the scope of a laboratory testing program is to select the types and quantities of laboratory tests that should be conducted to provide reliable estimates of the required design parameters. The selection of the requited types of tests is usually governed by (i) anticipated performance issues that need to be addressed, (ii) predominant material types anticipated at the site (i.e., coarse-grained soil, fine- grained soils, or rock), and (iii) anticipated subsurface conditions (e.g., soft, hard). Determining the quantities of each type of test that should be conducted is usually based on the anticipated variability of the subsurface conditions (e.g., a site with highly variable subsurface conditions would require more tests than a site with more uniform conditions) and the scope of the planned in situ testing program (e.g., if the scope of the in situ testing program is small, a large number of laboratory tests may be required). Table 3-10 provides some guidelines for selecting the appropriate laboratory tests for the required parameters. Additional information pertaining to laboratory testing is included in Chapter 8. Table 3-10 Guidelines for selecting laboratory tests Geotechnical Issue Pertinent Parameters that Can Be Obtained from Lab Tests Applicable Lab Tests Foundation support Shear strength, particle size distribution, unit weight, and durability of rock Index tests DS test (drained shear strength of soil and shear strength of rock) UU Triaxial (undrained shear strength) CU Triaxial (undrained and drained shear strength) CD Triaxial (drained shear strength) Triaxial or uniaxial tests on rock (compressive strength) Point-load strength test (shear strength index of rock) Slake durability test (durability of rock) Settlement Elastic modulus, coefficient of consolidation, compression index, preconsolidation pressure, Index tests One dimensional consolidation test to obtain coefficient of consolidation, compression index, preconsolidation pressure

32 Geotechnical Issue Pertinent Parameters that Can Be Obtained from Lab Tests Applicable Lab Tests particle size distribution, Atterberg limits, moisture content, and organic content Triaxial or uniaxial tests on rock (elastic modulus) Seismic evaluations Shear modulus, shear damping, particle size distribution, and Atterberg limits Index tests Resonant column (shear modulus and material damping ratio vs. shear strain) Cyclic triaxial (cyclic strength for liquefaction evaluations) Dewatering Hydraulic conductivity Index tests Flexible-wall or rigid-wall permeameter tests (hydraulic conductivity) Permanent groundwater control Hydraulic conductivity Index tests Flexible-wall or rigid-wall permeameter tests (hydraulic conductivity) Corrosion of buried metals pH and resistivity pH Resistivity Soil swell and shrink Particle size distribution, moisture content, and Atterberg limits Index tests 1D consolidation (swell potential) Use of excavated material Particle size distribution, Atterberg limits, maximum dry unit weight, optimum moisture content, modulus, shear strength, moisture content, and organic contents Index tests Compaction tests Resilient modulus test (estimating resilient modulus of subgrade soils for pavement design) CBR (estimating strength of subgrade soils for pavement design) Resistance R-value (estimating strength of subgrade soils for pavement design) Source: information from AASHTO (2018), ASTM Standards, Clayton et al. (2008), and FHWA (2002) Notes: UU: Unconsolidated-Undrained CU: Consolidated-Undrained CD: Consolidated-Drained The predominant material type(s) anticipated at a site plays a significant role in determining the types of laboratory tests that may be required. If the anticipated predominant material type happens to be granular soils, performance laboratory tests on undisturbed samples may not be required, and the laboratory testing program may consist of conducting laboratory index tests to confirm stratigraphy and performance tests on compacted disturbed samples to obtain parameters needed for pavement design. If the anticipated predominant material type is fine-grained soils, performance tests on undisturbed samples may be required when the consistency of the materials is in the range of very soft to stiff. Furthermore, index tests and performance tests on compacted disturbed samples may be required to confirm stratigraphy and facilitate evaluating the use of excavated material, the shrink and swell potential, and the parameters needed for

33 pavement design. If the anticipated predominant material is rock, the focus of the laboratory testing program should be on obtaining parameters that are needed for evaluating rock mass properties (e.g., the strength of intact rock specimens, durability of rock). The quantities of samples that should be tested for each type of required test is typically finalized after the field testing and sampling program is completed. This provides an opportunity to review all the available in situ test results and visually classify the collected samples prior to determining the final quantity of the samples that need to be tested. Reviewing and evaluating the in situ test results and samples can assist with developing a preliminary stratigraphy that can be used to (i) quantify the variability of the subsurface conditions, (ii) identify soil strata of concern that should be targeted for laboratory testing, and (iii) assess the appropriateness and adequacy of the completed in situ testing program to identify any data gaps that should be filled by the laboratory testing program. 3.3.6 Developing a Plan for Evaluating Groundwater Conditions Hydrogeology plays a significant role in the geotechnical analysis, design, and performance of the subsurface features of the transportation infrastructure. Thus, a hydrogeologic characterization to gain an understanding of the distribution, thickness, composition, and continuity of the lithologic units that influence groundwater flow is critical to most subsurface investigation programs. Results of hydrogeologic characterizations are specifically used to address issues such as slope failures, landslides, piping erosion, subsidence, subgrade pumping, heave in excavations, and uplift of structures due to buoyancy. A comprehensive hydrogeologic characterization should include information on (i) geology and hydrogeology, (ii) aquifer characteristics, (iii) aquitard characteristics, (iv) groundwater levels, and (v) the direction and gradient of groundwater flow. Knowledge of the site geology and stratigraphy is essential to define the hydrogeologic framework and identify pathways for groundwater flow. It is necessary to define water-bearing zones that allow groundwater movement (i.e., aquifers) and zones that may restrict movement of groundwater (i.e., aquitards), as well as any geologic features that may affect groundwater movement, such as faults, folds, fractures, buried channel deposits, or solution features. Aquifer characteristics that should be evaluated include the (i) hydraulic conductivity, (ii) porosity, (iii) permeability, (iv) transmissivity, and (v) storage coefficient. Aquitards are confining layers that limit the vertical movement of water. Some aquitards have poor integrity due to secondary permeability in the form of fractures, rootlets, or other features, and thus the hydraulic conductivity may vary considerably. Therefore, characterizing aquitards is a critical element of site investigations. Groundwater levels (i.e., potentiometric information) may be obtained from soil borings, monitoring wells, and piezometers and used to estimate the direction and gradient of groundwater flow. Additional information pertaining to hydrogeological site characterization for evaluating groundwater conditions is presented in Chapter 7.

34 Chapter 3 References AASHTO. 2018. Standard Specifications for Transportation Materials and Methods of Sampling and Testing, and AASHTO Provisional Standards. American Association of State Highway and Transportation Officials, Washington, DC. Australian Drilling Industry Training Committee. 2015. The Drilling Manual - 5th Edition. CRC Press/Taylor & Francis, Boca Raton, FL. ISBN 9781439814208. Clayton, C.R.I., N.E. Simons, and M.C. Matthews. 2008. Site Investigations, Second Edition. Halsted Press, Technology & Engineering, London. Christian, J.T. 2004. “Geotechnical Engineering Reliability: How Well Do We Know What We Are Doing?” Journal of Geotechnical and Geoenvironmental Engineering, ASCE, Vol. 130, No. 10, pp. 1556–1571, DOI: 1061/(ASCE) 1090- 0241(2004)130:10(985). 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. FHWA. 2002. Evaluation of Soil and Rock Properties. Publication No. IF-02-034. Federal Highway Administration, U.S. Department of Transportation, Washington, DC. New York State DOT. 2013. Geotechnical Design Manual. Albany, New York. Phoon, K.K., and F.H. Kulhawy. 1999. “Evaluation of Geotechnical Property Variability.” Canadian Geotechnical Journal, Vol. 36, pp. 625–639. South Carolina DOT. 2010. Geotechnical Design Manual. South Carolina Department of Transportation, Columbia, South Carolina.

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TRB's National Cooperative Highway Research Program (NCHRP) Web-Only Document 258: Manual on Subsurface Investigations provides an update to the American Association of State Highway Transportation Officials (AASHTO) 1988 manual of the same name. This report reflects the changes in the approaches and methods used for geotechnical site characterization that the geotechnical community has developed and adopted in the past thirty years. The updated manual provides information and guidelines for planning and executing a geotechnical site investigation program. It may also be used to develop a ground model for planning, design, construction, and asset management phases of a project.

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