Geotechnical Investigation Basics

Site Investigation

• The first step in foundation investigation is obtaining preliminary information to plan the project.
• The next step is subsurface exploration to understand soil and rock properties, impacting the foundation.

Specific Items Discussed

• Document review
• Purpose of subsurface exploration
• Borings (soil samplers, sample disturbance, field tests, boring layout, depth)
• Test pits and trenches
• Preparation of logs
• Geophysical techniques
• Subsurface exploration (geotechnical earthquake engineering)
• Subsoil profile

Prior Development

• If a site had previous development, information on fill, septic systems, leach fields, storage tanks, and man-made structures is crucial.
• This information is essential to understand possible impacts on the new development (utilities, pipelines, etc.).

Aerial Photographs and Geologic Maps

• Aerial photographs are taken from aircraft at specific altitudes.
• Engineering geologists interpret these photos, offering a three-dimensional view.
• Geologic maps provide additional reference materials.

Purpose of Soil Investigations

• Evaluate the site's suitability for the proposed project.
• Enable adequate and economical design.
• Disclose potential construction difficulties.

Phases of Soil Investigation

• Phase I: Gather information (pile plan, structure type/size, loading conditions, previous reports, maps, and clippings).
• Phase II: Reconnaissance visit, using Phase I information to compare current site conditions.
• Phase III: Detailed soil exploration, determining geological structure, groundwater conditions, and collecting samples for lab tests.
• Phase IV: Comprehensive report including soil description, exploration methods, profiles, and groundwater locations.

Soil Exploration Program

• Involves test pits or borings.
• Determine the need and extent of geophysical exploration;
• Locate/number boreholes/test pits.
• Plan depth of boreholes/test pits.
• Methods of advancing boreholes.
• Sampling instructions (number and possible location) for at least one borehole.
• Indicate requirements for groundwater observations and in situ tests.

Methods of Soil Investigation

• Geophysical methods: Seismic, electrical resistivity (variations in sound waves or electrical resistivity of soil formations). Test pits/trenches (shallow). Augers (shallow), Wash Boring, Rotary rigs (detailed soil investigations)

• Boring or Drilling: used to obtain representative samples for lab tests and assess groundwater conditions

• Geophysical Methods: Nondestructive technique providing information on soil, rock, hydrological, and environmental conditions; boring and test pits provide definite but time-consuming and expensive results.

Geophysical Techniques Indirect Methods

• Ground Penetrating Radar (GPR)
• Electromagnetic (EM)
• Magnetic
• Utility Locating
• Seismic
• Electrical Resistivity
• Gravity
• Very Low Frequency (VLF)

Advantages of Geophysical Techniques

• Quick and cheaper than borings or test pits
• Provide thorough area coverage
• Provide less definitive results

Advantages of SPT

• Low cost and easy-to-use, versatile.
• Provides simple, universally applicable testing method.
• No sophisticated equipment necessary.
• Can provide information for hard-to-sample soils.

Field Tests

• Standard Penetration Test (SPT): Used to determine density and compressibility of granular soils; used to check consistency of stiff cohesive soils and weak rocks; useful for evaluating liquefaction potential.
• Cone Penetration Test (CPT): Electric probes with a 60° tip, high-speed continuous sampling and accuracy compared to drilling, low cost, reduces laboratory sample requirements; not for hard samples, only unconsolidated sediments.

Test Methods and Standards

• Procedures for drilling, driving weight, anvil mass for SPT.
• Incremental penetration, blow counts for seating and test drive (to determine N-value).
• Total blow counts, increments of penetration.

Wash Boring

• High-pressure water jet loosens soil and carries it to the surface.
• Can be used in various soil types.

Rotary Drilling

• Used mainly for rock investigations
• Drilling tools (cutting or coring bit) which is attached to hollow rods.
• Water or drilling mud lubricates the drilling tool.