非饱和高岭土的临界状态与剪胀特性

Critical state and dilatancy characteristics of an unsaturated kaolin

关键词:  非饱和土  剪胀特性  临界状态  持水特征  剪切行为

Key words:  unsaturated soil  dilatancy characteristic  critical state  water retention characteristic  shear behavior

摘要:

为揭示高岭土在广吸力范围内的临界状态特征及其对应力剪胀关系的影响机制,开展了覆盖0~367.54 MPa的常吸力三轴试验,分析了临界状态应力比M、临界状态线及应力剪胀曲线（Dη线,D为剪胀系数,η为应力比）的变化规律,并采用归一化方法（Dη/M）统一了不同吸力条件下的应力剪胀关系。试验结果表明:当吸力超过一定阈值后,高岭土的临界状态、强度和应力剪胀关系均不再随吸力增大而演变；当吸力小于该阈值时,非饱和高岭土的临界状态应力比M随吸力显著增大,而吸力对比体积有效应力空间（vp′面）中临界状态线的斜率影响较小；在剪胀特性方面,吸力和平均净应力对高岭土的应力剪胀关系,即Dη曲线的斜率影响有限；吸力对应力剪胀关系的主要作用来源于其对临界状态应力比M的影响,其可通过将应力剪胀关系表示为Dη/M关系进行归一化。研究结果表明,非饱和高岭土在广吸力范围内的临界状态演化有明显的阈值特征,吸力对应力剪胀关系的作用可通过M的变化来统一描述。基于上述结果,对修正剑桥模型的应力剪胀方程进行了改进,该方程能够合理预测吸力效应,为广吸力范围内非饱和土本构模型的发展提供了新的实验依据与理论支持。

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.