Dynamic behavior of unsaturated cohesionless soil.

Abstract

This dissertation describes the results of an experimental investigation of capillary effects on dynamic shear modulus of cohesionless soils in a partially saturated condition. The overall objective of the investigation was to study the influence of the following effects on the dynamic characteristics: (1) void ratio, (2) confining pressure, (3) grain shape, and (4) grain size distribution. In addition, an empirical method was developed to predict the maximum shear modulus ratio, G$\sb{\rm max}$/G$\sb{\rm dry}$, and the optimum degree of saturation, (S$\sb{\rm r}$)$\sb{\rm opt}$. Laboratory test results were obtained by performing resonant column tests. It was found that capillary stresses significantly increase the shear modulus of unsaturated cohesionless soil. These effects are more pronounced for soils with low void ratios and confining pressures. The optimum degree of saturation increases with increasing void ratio of a soil. There is a linear relationship between the maximum shear modulus ratio and void ratio. The slope of this relationship is not affected by confining pressure and grain size distribution; it only depends on the soil grain shape. The grain size distribution is an important factor affecting the dynamic behavior of unsaturated cohesionless soils. If the smallest grains of a soil are larger than #400 sieve size, the grain size distribution does not affect the optimum degree of saturation, and only affects the maximum shear modulus in a partially saturated condition. The content of the minus #400 sieve size fraction may affect the values of both the maximum shear modulus and the optimum degree of saturation for unsaturated cohesionless soils. Finally, it was found that the soil grain shape also affects the values of both the maximum shear modulus and the optimum degree of saturation. A new method was proposed to predict the maximum shear modulus and the optimum degree of saturation based on the test results and theoretical analyses. Hardin's Equation, Wu's Function and the method proposed herein for calculating the maximum shear modulus and the optimum degree of saturation of unsaturated cohesionless soils together constitute a complete method for predicting the shear modulus of cohesionless soils in either a dry or partially saturated condition.