Shear stiffness and damping ratio of unsaturated soil

Abstract

In geotechnical practice, unsaturated soil condition is commonly encountered. Both shear stiffness and damping ratio are the fundamental parameters for dynamic study in geotechnical engineering. These have garnered more interest in the research to determine dynamic soil properties using laboratory tests like the small strain shear modulus (Gmax), Young's modulus (Emax) and the damping ratio (ξ). In this project, the effect of the saturation on shear stiffness and damping ratio of the soil will be investigated.

Triaxial compression and bender element test were performed using two types of soil, namely, kaolin and Changi sand. A modified triaxial cell with bender element test system was employed to carry out wave propagation test in soil specimen to determine the compression (P) and shear (S) wave velocities of the different soil types. The relationship between shear stiffness and strain as well as the shear strength were determined by conducting constant water content triaxial test. It was found that shear stiffness increased as the soil desaturate. Higher initial matric suction also resulted in higher initial shear stiffness and greater shear strength of the soil.

On the other hand, damping ratio of the soil will be estimated using three methods. They are, namely, Logarithmic Decrement Method (LDM), Half-power Bandwidth Method (HPBM) and Spectral Ratio Method (SRM). Silicon rubber specimen chosen as the standard material to calibrate the bender element set as well as obtaining the reference wave signals which were used in LDM. Experiments were conducted and found that damping ratio generally decreases as the soil desaturate. Furthermore, damping ratio was higher for coarse-grained soil than that of fine-grained soil. SRM is found to be the most reliable among the three methods that were employed in this project.