Failure modes and slope stability of expansive soil canal under wet-dry cycles

Abstract: Expansive soil is rich in clay minerals such as montmorillonite and illite and it is very sensitive to change of the environment. Aiming at the problem of slope instability and damage caused by seasonal water flow in expansive soil canals in northern Xinjiang, the deformation and failure characteristics of expansive soil canal slopes under wet and dry cycles were studied by establishing a centrifugal model test that simplified geological conditions. The failure mode of expansive soil canal slopes was caused by canal water flow and water stoppage; on this basis, the influence of different fissure distribution on seepage characteristics and stability of expansive soil canal slopes was analyzed by using GeoStudio software. The results showed that the cracking of the expansive soil in the canal foundation during the operation of the canal was the decisive factor for the shallow instability of the canal slope. In the centrifugal model test, the shallow damage caused by the expansive soil canal slope during the process of passing through and stopping the water was caused by the mixing and superposition of the 2 failure modes. At the beginning of construction, the integrity of the soil on the slope of the canal was high, and the initial slope had no initial cracks. With the increase of the canal operation time, the shallow soil underwent the wet-dry cycle effect. Under the gradual cracking, the "sub-soil blocks" generated on the surface gradually began to spall, and at the same time, the cracks gradually developed into the slope in the vertical direction when the depth of the cracks reached the slope. After the critical depth of the surface, its propagation path was deflected. At this stage, the cracks gradually passed through the shallow slope of the canal slope along the slope surface with the increase of the "soil mass" exfoliation degree on the slope surface. As shown in the canal during the last stage of water stoppage, the overburden pressure of the shallow soil firstly decreased due to the decrease of the water level of the canal and the exfoliation of the "sub-soil mass". The internal fissures in the soil were developed to a high degree. After the water was stopped, the internal canal water in the shallow soil flew out of the soil through the preferential path formed by the fissures and converged at the foot of the slope. This drying process was extended again, eventually forming a fissure parallel to the canal slope sliding band in the internal canals. Under the combined effect of the above effects, the shallow soil layer in the canal slope slid along the fissure slip zone, eventually causing the shallow failure of the expansive soil canal slope. The existence of cracks aggravated the pore pressure fluctuation of the surface soil on the slope surface, which easily caused the surface "sub-soil blocks" to peel off. With the increase of the crack depth of the canal slope, the soil deeper from the slope surface was more susceptible to the fluctuation of the canal water level, making the shallow soil on the canal slope more vulnerable to damage under the action of tension cracks at the trailing edge. Comparing the safety factor of the canal slope at different depths of the trailing edge cracks in the model, the shallow failure of the slope was mainly determined by the "sub-soil block" exfoliation pattern, and the extension of the trailing edge tension cracks promoted the slope instability.