Investigation of cyclic response in unsaturated soils: Including hydraulic and mechanical hystereses

Abstract

Monotonic and cyclic responses of unsaturated soil have been investigated through an extensive program of laboratory testing. Based on the observations made a fully coupled hydro-mechanical flow and deformation model has been presented including hydraulic and mechanical hysteresis, and stress path dependency of the flow rule. In addition, a new mapping rule is proposed to facilitate simulation of complex stress reversals for practical application. The effect of hydraulic hysteresis on the variation of effective stress in unsaturated soils during drying-wetting cycles and in transition from drying to wetting was studied through a novel experimental program. It was shown that upon suction reversal, i.e. from drying to wetting, the effective stress parameter decreased with decreasing suction until it reached a point of minimum, beyond which it increased with reducing suction. The stress increment directionality of plastic flow was investigated through a comprehensive programme of stress probing in which identical samples of clay and clayey sand, with different initial stress history and saturation levels were subjected to stress perturbations in different directions. The results showed that there was a strong stress increment direction dependency of plastic strains. It was also found that suction had no discernible effect on the flow rule provided. The model was presented in an effective stress space. A series of constant water content monotonic and cyclic loading tests were conducted on samples of loose sand with different initial degrees of saturation to examine the effect of suction on the potential of liquefaction/instability in loose granular soils. The results indicate that even with the slightest unsaturation, the potential for instability in loose granular materials is reduced. Baseline experiments were also conducted for a range of controlled suction and constant water content conditions to examine the basic concepts advocated in the constitutive modelling of unsaturated soils and characterise the test materials. In addition to the original laboratory testing program performed, the key theoretical contributions made in the thesis include: a revised model for the effective stress parameter for unsaturated soils including hydraulic hysteresis, a new stress direction dependent flow rule for saturated as well as unsaturated soils, a new mapping rule for cyclic analysis of geo-materials and a fully coupled hydro-mechanical model for cyclic analysis of unsaturated soils.