Development and Applications of In-Hole Seismic Method to Measure Shear Wave Velocity of Subsurface Materials

The dynamic stiffness of the ground is one of the key parameters in earthquake-resistant designs involving local ground response and in assessment of geotechnical structures subjected to various man-made vibrations. Its use is being extended even to static settlement and deformation problems. Its importance and extended usage require a more manageable method than conventional seismic techniques such as crosshole, downhole test, and suspension logging. In general, conventional borehole seismic methods are expensive and technically complex. This paper presents a new method which has been developed to reduce testing cost and to improve the easiness in its use. An in-hole probe has been developed to be fit in three-inch boreholes with functional improvements including a pair of mechanical couplings driven with two servomotors, and a servomotor triggering device. The in-hole seismic method was adopted at various sites such as compaction fills of railroad subgrade, bedrocks of tunnel construction sites, tunnel-face rock mass, and abutment of rock fill dam. In-hole seismic test results were verified by comparing with reliable crosshole results. Presently, the method is very cost effective and easy to use in residual soils, compaction fills and bedrock where uncased boreholes can stay open.