Critical state and dilatancy characteristics of an unsaturated kaolin

Abstract:

This study investigates the critical state characteristics of kaolin across a wide suction range and clarifies their role in governing stress-dilatancy behavior. A series of constant-suction triaxial tests covering suctions from 0 to 367.54 MPa were performed to examine the evolution of the critical state stress ratio M, the critical state line, and the stress-dilatancy curves (D-η, where D is the dilatancy rate and η is the stress ratio). A normalization approach (Dη/M) was further employed to unify the stress-dilatancy relationships under different suction conditions. The results reveal that when suction exceeds a certain threshold, the critical state, shear strength, and stress-dilatancy behavior of kaolin cease to evolve with increasing suction. Below this threshold, M increases significantly with suction, while the slope of the critical state line in the v-p′ plane is only marginally affected. With respect to dilatancy, suction and net mean stress exert little influence on the slope of the D-η curves; instead, the principal effect of suction arises from its control of M, which enables normalization of the stress-dilatancy relationship as Dη/M. The findings demonstrate that the evolution of the critical state in unsaturated kaolin exhibits a distinct threshold feature and that the influence of suction on stress-dilatancy behavior can be consistently interpreted through changes in M. Building on this understanding, a modified stress-dilatancy equation within the framework of the Modified Cam-Clay model is proposed, which provides reliable predictions of suction effects and offers new experimental evidence and theoretical support for the development of constitutive models of unsaturated soils over a wide suction range.