Manual on Subsurface Investigations (2019) / Chapter Skim
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From page 12... ...
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.
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From page 13... ...
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)
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From page 14... ...
14 Geologic or ManMade 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)
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From page 15... ...
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)
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From page 16... ...
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)
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From page 17... ...
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.
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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)
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From page 19... ...
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.
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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)
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From page 21... ...
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.
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From page 22... ...
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.
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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)
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From page 24... ...
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.
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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]
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From page 26... ...
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.
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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 overconsolidated 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.
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From page 28... ...
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.
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From page 29... ...
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.
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From page 30... ...
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.
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From page 31... ...
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.
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From page 32... ...
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.
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From page 33... ...
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)
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From page 34... ...
34 Chapter 3 References AASHTO.
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