Ground Characterization using Geophysical Techniques for Natural Hazard Assessment

Catastrophic landslides and failures of natural and man-made slopes can result in significant social and economical damages as well as loss of lives. Advanced geophysical techniques can be employed to identify weak soil layers and unstable ground conditions by providing vital information regarding subsurface soil properties. This study presents the application of shear wave velocity and electrical resistivity geophysical measurements for characterizing various soil types. These measurements are carried out on laboratory samples under controlled stress and density conditions. Laboratory samples of loose, medium-dense and dense silica sand are prepared in a large cylinder and subjected to different stress conditions. Confining pressure is applied through hydraulic stress around the specimen while a different axial stress is applied via a hydraulic piston implemented at the bottom of the sample to simulate in-situ stress condition. Shear wave signals are generated and transmitted through soil specimens using a pair of piezo-electric bender elements and the velocity of the shear wave is determined based on the tip-to-tip distance between the bender elements and the peak-to-peak time difference between the incipient and transmitted shear wave signals. Shear wave velocity provides stiffness and shear modulus of soil which is an indirect index of soil strength. Empirical correlations are also developed for estimating the density (porosity) of soil samples with electrical resistivity measurements. These engineering properties (shear modulus and density) are directly related to the susceptibility of a soil to failure and in particular liquefaction and flowslides. Combined correlations and charts are presented for determining the susceptibility of sands to liquefaction failure and slope instability.