Subsurface Exploration

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Subsurface Exploration

• Identify Type, Depth and Location of Underlying
  Soil Layers
• Determine Physical Characteristics of Soil Layers

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Purpose of Subsurface Exploration

• Locate Water Table
• Evaluate Load Bearing Capacity
• Predict Lateral Earth Pressures
• Select Type Depth of Foundation
• Estimate Settlements
• Determine Potential Problems
• Establish Construction Methods

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Exploration Program

• Collect Existing Structure Information
• Collect Existing Subsoil Information
• Reconnaissance of Proposed Site
• Detailed Site Investigation

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Existing Structure Information

• Type of Structure and Future Use
• Local Building Codes
• Column Loads
• Load-Bearing Wall Loads
• Span Length for Bridges
• Pier Abutment Loads

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Existing Subsurface Information

• Geologic Survey maps (USGS)
• County Soil Maps (USDA-SCS)
• DOT Soils Manuals
• Soil reports for nearby structures

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Field Reconnaissance

• Visual inspection of site and surrounding areas
• Vegetation types
• Site accessibility
• Nature of surface drainage
• Open cuts in nearby areas
• Inspection of nearby structures

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Detailed Site Investigation

• Test borings to collect disturbed and/or
  undisturbed samples for visual and laboratory
  investigation
• Number and depth of borings based on project
  type, depth of stress influence

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Types of Borings

• Auger Borings
• Rotary Drilling
• Wash Borings

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Auger Boring
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Rotary Drilling

• Rapidly rotating drilling bits attached to bottom
  of drill rod
• Water or drilling mud forced down rod to bit,
  return flow carries cuttings to surface
• Borehole from 50 to 200 mm diameter
• Rotary drilling used in sand, clay and rock

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Wash Borings
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Common Sampling Methods

• Standard Split Spoon
• Standard Penetration Test (SPT)
• Thin Wall Tube
• Piston Sampler

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Split Spoon Sampler
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Thin Wall Tube Sampler
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Piston Sampler
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Sample Disturbance

• Ar()100 x (D2o-D2i) / D2i
• Undisturbed if Ar lt 10
• Standard Split Spoon Ar 110
• Shelby Tube Ar 13.8

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SPT Correlations

• Consistency of Clays
• Relative Density of Sands
• Drained Friction Angle of Granular Materials

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Table 8.3
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Table 8.5
N (deg) 27.1 0.3Ncor - 0.00054N2cor Eq 8.9
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Other In Situ Tests

• Vane Shear Test
• Cone Penetration Test
• Borehole Pressuremeter (PMT)
• Dilatometer Test

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Rock Coring
Recovery Ratio
Rock Quality Designation, RQD
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Boring Logs

• Project ID
• Thickness, location and description of soil
  layers
• Location of water table
• Sampling depths
• In situ/lab test results
• Drillers notes

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Questions ?