Experimental study on crack evolution and strength attenuation of expansive soil under wetting-drying cycles

Expansive soil is characterized by high dilatability and low permeability, particularly sensitive to the change of external environment, due mainly to the fact that this kind of high plastic clay is rich in hydrophilic clay minerals. Crack initiation and expansion often occur, as a result of repeatedly uneven dilatancy and shrinkage of soil after rainfall and evaporation under the exposure of rain and sunshine. The crisscrossing network of cracks has harmed the integrity of soil structure. Infiltration of surface water via crack channels has caused a great threat to the anti-seepage barrier facilities, such as earth or rock dams, river reservoirs, and channels. Especially in the frequent occurrence of the rainy season, the rainwater can easily infiltrate through cracks, resulting in the softening of internal soil and the sharp decline in shear strength. As such, it is very easy to induce instability and collapse of a shallow layer of expansive soil slope, leading to the destruction of irrigation and water conservancy facilities, even aggravation of floods and droughts to the whole ecological environment. Therefore, the soil strength degradation in the cycling load of dry and wet has become an important factor endangering the long-term stability of expansive soil engineering in an extremely seasonal climate. This study aims to reveal the evolution characteristics of expansion and shrinkage cracks in the expansive soil under the wetting-drying cycle, and thereby determine the perturbation of soil structure strength. The wetting-drying cycle tests were carried out on the weak expansive soil sampled from Hefei, Anhui Province, China. The crack parameters were extracted from the crack images of soil samples by image processing technology. The shear strength test was carried out under low stress. The experiment results show that: 1) There were three stages: the gestation, the rapid development, and the stable development of cracks. The growth of the crack index focused mainly on the rapid development of cracks. 2) Under the action of wetting-drying cycles, the variation in the internal friction angle of the expansive soil was less than 2.2°, indicating little influence from the number of wetting-drying cycles. The decrease of soil strength was due mainly to the decrease of cohesive force. The cohesion of expansive soil decreased rapidly at first and then slowly, where the attenuation rate of cohesion during the first four wetting-drying cycles was 50.97%-66.92%. The attenuation rate was also related to the amplitude of wetting-drying cycles. 3) In grey correlation analysis, there were obvious correlations between the attenuation of cohesive force and the crack area ratio, the total length, and the average width of the crack trend. Specifically, the maximum correlation between the crack area ratio and the attenuation of cohesive force was 0.785-0.832 with the weights of 0.461-0.472, indicating a good linear relationship. The minimum correlation between the attenuation of cohesive force and the average width of crack was 0.392-0.414, where the weight was only 0.228-0.240. The findings can provide an insightful theoretical basis to further reveal the catastrophic mechanism of expansive soil slope induced by a drastic change in temperature and humidity.