干湿循环对非饱和膨胀土抗剪强度影响的试验研究

膨胀土是一种气候敏感性土体，研究在干湿循环过程中膨胀土剪切强度的变化，对了解在自然界周期性蒸发和降雨作用下原位膨胀土体工程性质的变化以及由此导致的地质灾害发生过程具有重要意义。文中以重塑非饱和膨胀土为研究对象，模拟了3次干湿循环过程，对每次干燥路径中的试样进行了直剪试验，重点分析含水率、正压力及干湿循环次数对膨胀土剪切强度的影响，得到如下主要结果：（1）在干燥过程中，随着含水率的减小，试样的刚度、脆性、抗剪强度值（峰值剪切应力）、抗剪强度指标（黏聚力、内摩擦角）及抗剪强度损失（峰值强度与残余强度之差）均呈增加趋势；（2）正压力越高，试样的剪切强度和残余强度越大，而破坏后的峰值强度损失越小，破坏韧性增加；（3）在3次干燥过程中，试样的剪切强度及黏聚力呈先增加后减小的趋势，在第二次干燥过程中达到峰值，但内摩擦角受干湿循环的影响无明显规律；（4）试样经历多次干湿循环后，其剪切特性越来越类似于超固结土，脆性显著增加；（5）干燥过程和干湿循环对试样残余剪切强度的影响都不明显，残余剪切强度基本都在100 kPa附近变化；（6）非饱和膨胀土在干湿循环及干燥过程中剪切强度的变化除了与吸力有关外，还与其微观结构调整和裂隙发育状态密切相关，需要综合非饱和土力学和土质学理论对试验现象进行分析。

Shear strength of unsaturated expansive soil during wetting-drying cycles

Expansive soil is highly sensitive to climate change. Understanding the response of engineering properties of expansive soil under wetting drying cycles is therefore significant for revealing the disaster mechanism as natural expansive soil is subjected to periodic evaporation and infiltration. In this investigation, a series of shear tests were conducted on unsaturated remolded expansive soil which was subjected to three wetting drying cycles. The influence of drying process, normal stress and wetting drying cycles on shear strength behavior were analyzed and discussed. The results show that (1) during drying, specimen stiffness, shear strength, shear strength parameters (cohesion and internal friction angle), failure brittleness and peak strength loss increase with decreasing water content, while the shear displacement decreases accordingly; (2) higher normal stress gives rise to higher shear strength as well as residual shear strength, but lower peak strength loses, and fracture toughness of the specimen increases generally with increasing normal stress applied during shearing; (3) the shear strength behavior of unsaturated expansive soil is significantly influenced by wetting drying cycles, during which both shear strength and cohesion increase first and then decrease, and reach their peak values at the second drying path; however, the internal friction angle is slightly influenced by wetting drying cycles; (4) after the specimens were subjected to multiple wetting drying cycles, the shear characteristics are similar to that of over consolidated soil, and the specimen presents more brittleness at failure; (5) the residual shear strength seems to be independent of drying process and wetting drying cycles as the obtained values are generally kept around 100 kPa; and (6) shear strength of unsaturated expansive oil during wetting drying cycles is significantly conditioned by suction, microstructure and cracking development; it is necessary to combine unsaturated soil mechanics and soil science to make an accurate evaluation.