干湿循环过程中膨胀土的胀缩变形特征

为了了解干湿循环过程中膨胀土的胀缩变形特征,分别开展了两组干湿循环试验。在控制吸力干湿循环试验中,吸力控制范围为 0.4 ～ 262 MPa ,采用了两种吸力控制方法,分别为渗析法（吸力＜ 4 MPa ）和蒸汽平衡法（吸力＞ 4 MPa ）,当每一级吸力达到平衡时,测量试样对应的含水率和体积;在常规干湿循环试验中, 采取了两种干缩路径,分别为全干燥和部分干燥,并测量试样在每次干湿循环过程中的轴向变形及循环结束后的含水率。结果表明： 在脱湿和吸湿过程中,试样孔隙比随吸力变化可分 3 个典型阶段：大幅变化阶段（ 0.4 ～ 9 MPa ）、过渡阶段（ 9 ～ 82 MPa ）和平缓阶段（ 82 ～ 262 MPa ）; 当吸力大于 113 MPa 时,试样的胀缩变形基本可逆,而当吸力小于 113 MPa 时,试样的胀缩变形 表现出明显的不可逆性,且不可逆程度随吸力的减小而增加。试样在常规干湿循环过程中的胀缩变形随循环次数的增加逐渐趋于稳定;胀缩特征受干缩路径的影响非常明显,全干缩路径中测得的膨胀率高于部分干缩路径,膨胀速率随干湿循环次数的增加而增加;试样在干湿循环过程中的膨胀率大小在一定程度上取决于吸湿能力。

Swelling and shrinkage behaviour of expansive soil during wetting-drying cycles

Two groups of laboratory cyclic wetting-drying tests are conducted on expansive soil to study the swelling and shrinkage behaviour. In suction-controlled wetting-drying tests, two suction control methods are used: the osmotic method for low suction range (0.4 ～ 4 MPa) and the vapor equilibrium technique for high suction range (4 ～ 262 MPa). At each suction level, water content and volume of soil specimen are determined after equilibrium is reached. In conventional wetting-drying tests, soil specimen is allowed to swell , and either shrinks fully or partially to the initial height in each cycle. Simultaneously, axial deformation is measured. After each cycle, the water content of specimen is also assessed. The results show that the void ratio of specimen varies with the suction in three typical stages in both drying and wetting paths, i.e., sharp variation stage (0.4 ～ 9 MPa), transition stage (9 ～ 82 MPa) and peace stage (82 ～ 262 MPa). The reversibility in the swelling and shrinkage deformation is found when the suction is higher than 113 MPa, but significant hysteresis is observed as the suction is lower than 113 MPa and the hysteresis extent increases with decreasing suction. In conventional wetting-drying tests, it is observed that the swelling and shrinkage behaviour gradually reaches equilibrium with increasing cycles and is significantly influenced by shrinkage pattern. The measured swelling potential of specimen subjected to full shrinkage cycle is higher than that of specimen subjected to partial shrinkage cycle. Swelling velocity increase with the increase of the wetting-drying cycle is noted when the specimen fully shrinks. Generally speaking, the swelling ability of specimen in wetting-drying cycle significantly depends on water absorption capability.