1571 GG Brown
The geotechnical engineer has always been faced with the problem of characterizing near-surface materials. The near-surface region is often within 10 to 100 m of the ground surface. Traditionally, field exploration programs involved boring, sampling, and penetration testing. In the 1960s, in-situ geophysical measurements began to be employed in geotechnical engineering. This work primarily involved seismic (stress wave) measurements at small strains which were adapted from exploration geophysics. These measurements were used to characterize geotechnical sites (e.g. layering, top of bedrock, depth to water table) and geotechnical materials (e.g. small-strain stiffnesses in shear and compression). The demand for seismic measurements grew out of the need to evaluate the dynamic properties of near- surface soils for use in soil dynamics and geotechnical earthquake engineering.
Today, however, in-situ seismic measurements are used in many more applications because they:
- Have a strong theoretical basis
- Can be performed in the field and laboratory, thus forming an important link between field and laboratory conditions
- In recent developments involving surface waves, are noninvasive which makes them very cost effective.
Applications of these techniques are discussed and recent studies are presented to highlight some of their strengths and limitations. These studies include:
- Siting large tunnel shafts to avoid potential liquefaction problems,
- Evaluating density of alluvium beneath an earthen dam,
- Monitoring construction of a 27-m thick fill at a nuclear power plant, and
- Predicting settlements of shallow foundations on granular soil.
Most recently a field method to evaluate the nonlinear characteristics in shear of shallow, hard-to- sample soils has been developed. This method is illustrated using measurements of the liquefaction potential of loose sands in Christchurch, New Zealand.