Study on mechanical properties and physical mechanism of expansive soil under different cyclic modes

The complex environmental factors in the cold and arid area of northern Xinjiang are the important causes for the deterioration of expansive soil canal slope, which may induce the sliding failure of expansive soil canal slope. In order to further study the deterioration mechanism of expansive soil canal slope in cold and arid areas, the direct shear, compression, infiltration and scanning electron microscope tests of expansive soil under three different circulation modes were carried out to analyze the mechanical properties and reveal its physical mechanism from the macro-micro perspective. The test results show that: (1) Direct shear test: the cohesion of expansive soil under different cyclic modes decreases gradually with the increase of the cycles. The decrease magnitude shall be the most significant for the dry-wet-freeze-thaw cycle, and the next is the dry-wet cycle, followed by the freeze-thaw cycle. The internal friction angle changes little and is basically stable. After the cycles of expansive soil, the increased cracks may destroy the bonds of particles, decrease the cementation between particles, and reduce the shear strength. (2) Compression test: with the increase of the cycles, the stable void ratio tends to decrease, indicating that the compressibility of the soil may increase; under the same conditions, the change of compressibility index shall be the most evident for the dry-wet-freeze-thaw cycle, and its change shall be the least for the freeze-thaw cycle. Under different cyclic modes, the dry-wet cycle causes serious damage to the particles and the decrease in the cementation force of the soil particles, resulting in the irrecoverable change of the soil particle skeleton and the increased compressibility. The freezing of free water in the sample during the freeze-thaw cycle may increases the pores, and the soil particles are squeezed by the ice crystals to form a new skeleton structure. Though the void ratio decreases, the overall change is not significant. The microscopic porosity and particle orientation frequency are two main factors affecting the compression index. (3) Permeability test: The change of permeability coefficient in the process of cyclic process is divided into three stages, namely slow stage, rapid stage and stable stage. The change magnitude of permeability coefficient is the least for the freeze-thaw cycle, while it is shall be the most for the dry-wet-freeze-thaw cycle. Under different cyclic modes, in the process of dry-wet cycle, water migrates in the soil skeleton and dissolves the soil aggregate so that a new seepage channel may be formed. The frost heave from the freeze-thaw cycle causes the deformation of the soil skeleton and the destruction of the pore structure, resulting in a channel for seepage. Among those influencing factors, the microscopic porosity is the main factor affecting the permeability coefficient. © 2023 Editorial Office of China Civil Engineering Journal. All rights reserved.