The purpose of a Geotechnical Investigation is to determine the engineering qualities of the soils or rocks at a site, and how their location and depth affects an envisioned construction project. The presence of any buried organic materials with poor engineering qualities, obstructions to earthwork, and the depth of the water table are also important to identify during a geotechnical investigation. Prior to initiating expensive field work, an initial sense of soil conditions for specific localities may be obtained through government soil surveys that have been recorded for most of the United States. Geotechnical investigations prepared for adjacent construction projects are another potential source of available site information.
An initial field assessment of soil strength properties can be obtained at the site surface through the use of a penetrometer. This is a hand-held mechanical device with a probe end that pushes into the soil and measures the soil’s resistance to insertion, which can correlate to bearing strength. A vane-type penetrometer places metal flanges vertically in the soil, and a spring measures the soil’s resistance to torque as the device is twisted, which can provide an estimation of soil shear strength. The data provided by penetrometers is helpful to quickly determine the soil’s capacity to support construction equipment and its general strength properties.
Site sampling is a process where soil or rock is retrieved from beneath the site so its characteristics can be analyzed. Samples should be taken commensurate with the type and depth of structures envisioned for the site. A different sampling program would be undertaken for a highway project which may only require samples to a depth of 12 feet versus a deep pile foundation project which might require sampling to a depth of 200 feet.
Two general categories of soil samples are “disturbed” (or representative), and “undisturbed” samples. Disturbed or representative samples are adequate if the soil is going to be used as a construction material, or for general classification such as a percentage breakdown of soil constituents. A hand auger is a typical device used to retrieve shallow (disturbed) samples. The auger blades rotate and extract soil to the surface that can be collected or studied in the field or laboratory. For deeper holes in tougher strata, bore holes can be drilled by an equipment-mounted rotary drill, or hollow-stem auger. These drilling tools create the bore holes, but are not sampling tools in themselves. Sampling tools can then be inserted through the auger stem or into the drilled bore holes, which may be uncased, or cased with pipes to provide support in weak soils. A split-spoon sampler is an enclosed metal cylinder than can be rotated or pushed into the bottom of a bore hole and then returned to the surface, containing a soil sample. The cylinder can be split open in two pieces, allowing the soil sample to be removed and analyzed. Split-spoon samplers offer varying degrees of disturbance depending on the soil qualities and method of insertion. In the recovery of rock samples, drilling bits can be swapped out for core drill bits. Hollow-core drill bits with hardened edges cut through and can recover solid rock sections from the bottom of a bore hole. Undisturbed soil samples are required for applications like deep foundation work where more exact “in-situ” soil qualities are important to determine. Undisturbed samples are provided by using a thin-walled hollow tube that can be inserted to the sample depth, and extracted with a preserved soil column within it. The tube sections can be cut in segments for transport and analysis. Piston samplers can be used to retrieve deeper undisturbed specimens. A piston sampler is a thin-walled tube section that can be placed at the bottom of a bore hole and hydraulically extended into the soil to capture an undisturbed soil core. These soil recovery methods allow laboratory staff to better determine the soil’s strength and density characteristics as they exist at depth.
Whether disturbed or undisturbed soil or rock samples are taken at the site, it is important to document each sample point with its specimens’ lab results. Most often, this is accomplished on a plan view grid that can be graphed as an overlay to the construction site plan. The plan view grid usually includes a cross-referenced vertical presentation of the soil qualities at each sample point.
Seismic surveys can be useful to define the depth of a layer of dense soil or rock beneath a softer layer of soil. Seismic surveys involve the installation of several listening devices called geophones. A small explosive charge is detonated, and the time interval from when each geophone picks up the sound is recorded. Denser objects transmit sounds quicker, and the reflected sound waves can be timed and analyzed to locate the depth of a hard layer beneath less dense soil. Electrical resistance surveys measure the soil’s resistance to electrical current. Typically, four electrodes (metal rods) are placed in the soil and the electrical resistance across the electrodes can be measured and mapped. This technique models the flow nets that water (or electricity) would use to move through the soils, and thus determines dense and less dense material locations.