Interpretation of Large-Strain Seismic Cross-Hole Tests

Abstract

At sites in earthquake-prone areas, a reliable expression of the nonlinear dynamic stress-strain behavior of soil for the different depths of interest is needed for earthquake-response analyses. Most currently used geophysical seismic tests generate only small amplitude waves, which are in the linear stress-strain range, and the nonlinear behavior is inferred from laboratory tests. A seismic cross-hole test has been developed in which large dynamic forces are applied at different depths in a borehole. Velocity sensors located in three additional boreholes at various distances from the source hole measures the particle velocity and the time it takes the shear wave generated at the source borehole to travel horizontally to each of the receiver boreholes. The generated shear strains are well into the nonlinear stress-strain range. This paper provides a systematic interpretation scheme for the data from these large-strain geophysical crosshole tests. Use is made of both the measured particle velocities at each sensor and the travel times to each borehole to develop modulus degradation curves. The interpretation procedure is applied to a well-documented case study.