INTEGRATED GEOPHYSICAL METHODS FOR SUBSURFACE GEOTECHNICAL INVESTIGATIONS

This research report was funded through New Hampshire's State Planning and Research (SP&R) funds and it summarizes the New Hampshire Department of Transportation's attempt to supplement conventional test borings and exploration techniques through the use of geophysics on several geotechnical projects. In many of the Departments geotechnical investigations the depth to bedrock or unstable soils is quite variable over short lateral distances. Test borings are point specific and are capable of missing sudden depth changes and variations in the soil and rock properties. Additional borings can quickly exhaust available time and money and can sometimes lead to a more puzzling, instead of less confusing, subsurface interpretation. A project's site conditions or highly conductive soil properties can limit the use of the Department's ground pentrating radar unit, but through the implementation of additional geophysical techniques the Department has enhanced their capabilities. These new capabiltites have helped to allieviate some of the uncertainties when making subsurface interpretations solely through conventional exploration activities. This paper presents both successful and less successful case histories on resisitivity imaging and seismic refraction used in conjunction with test borings and ground penetrating radar on geotechnical projects to characterize a project's subsurface conditions. Included are the methods employed for using these geophysical techniques, the geophysical results and how these results were calibrated and verified.