Effect of Hysteresis on the Critical State Behavior of an Unsaturated Silty Soil

Abstract

In this paper the effect of hysteresis on the critical state behavior of unsaturated soils is investigated through conducting a number of controlled suction triaxial tests on samples of an unsaturated silty soil. The slurry method was used for preparing the samples for the main tests. The tests were carried out in a double-walled triaxial cell. In the experiments the samples were consolidated isotropically to virgin state at suctions of 0, 100, 200, 250, and 300 kPa on drying and wetting paths of soil water characteristic curve. Then they were sheared under constant suction at various constant cell pressures. The results of the drained triaxial tests were used to determine the effect of the hysteresis phenomenon on the characteristics of the critical state framework. The obtained data were examined in terms of mean net Bishop’s stress (p∗) (by including degree of saturation) or p¯ (i.e., the mean total stress in excess of pore–air pressure), deviator stress (q), suction (s), and specific volume (v) as state variables. The results show that the critical state lines (CSLs) for the dry and wet paths are not parallel for different suctions in the q:p* or q:p¯ space. The slopes and intercepts of the CSLs in this space are functions of suction. In addition, the critical state lines in the v:Ln p∗ or v:Ln p¯ plane are not parallel for drying and wetting paths and the slope and intercept of them are also functions of suction. The results also indicated that two frameworks showed similar trends of critical state parameters but the framework based on p∗ is more reliable than the one based on the p¯.