Interpretation of shear wave velocity in unsaturated soils

Abstract

Shear wave velocity measurements in soil can provide estimates of small strain stiffness and are used for seismic site classification. In comparison to saturated soils, there has been relatively little research on the behavior of shear wave velocity in unsaturated soils. Current research was conducted to understand the behavior of shear wave velocity in unsaturated soil. The objectives of this research were to: 1) Investigate the effect of various soil properties on the behavior of shear wave velocity in unsaturated soils. This included studying the effect of soil type, moisture content, suction, confining stress, wetting-drying hysteresis, density, and soil structure. 2) Compare and analyze shear wave velocity measurements taken in the field and the laboratory. 3) Investigate the effect of seasonal changes in moisture content on shear wave velocity measurements from the Seismic Cone Penetrometer (SCPT), and the potential impact on seismic soil properties and site classification. And finally, 4) develop a model that uses the physical and mechanical properties of the soil to estimate the shear wave velocity for different soil types under various saturation conditions. To achieve the objectives of this study the following tasks were completed: 1) Performed shear wave velocity laboratory testing on soil samples undergoing the vapor equilibrium suction control for wetting and drying paths using the bender elements method with and without confining stress. 2) Performed SCPTu shear wave velocity measurements at twelve sites and recorded seismic velocity measurements at 1 m depth intervals. 3) Performed shear wave velocity testing in the laboratory on soil samples from the field under stress conditions similar to the field. 4) Investigated proposed relationships in the literature and based on them, develop a new model for predicting shear wave velocity that includes the desired properties and stress conditions of the soils. The major contributions of this research include: 1) Shear wave velocity was tested under various conditions where soil type, water content, suction, density, confining stress, and soil structure were varied and the combined effect of changing these parameters was assessed. 2) An investigation of shear wave velocity with the seismic cone penetrometer in the field at nine test sites during wet and dry seasons demonstrates the importance of considering changes in shear wave velocity due to changes in moisture content and suction in the active zone of the soil profile. This has important implications for the determination of soil dynamic properties and seismic site class based on field measurements of shear wave velocity in unsaturated soils. 3) A systematic comparison of shear wave velocity determined using SCPT in the field and bender element testing in the laboratory on Shelby tube samples obtained on the same day was accomplished for three different test sites. The study revealed that field measurements were nearly the same as lab measurements made under similar confining stress. These results indicate that measurements of shear wave velocity, whether in the field or lab, are robust and reliable. 4) Existing power models for predicting shear wave velocity with effective stress were examined and a new linear model was developed and found to provide better predictive capability than the power model.