Mechanical properties and strength deterioration mechanism of soil in inner dump of open-pit coal mine under the action of freeze-thaw cycles

In order to investigate the influence mechanism of freeze-thaw cycles on the structure and mechanical properties of soil, the typical clay soil in the inner dump of Yuanbaoshan open-pit coal mine in Inner Mongolia was taken as the research object, and the simulation analysis of the internal temperature, moisture and stress fields of the soil was carried out through laboratory freeze-thaw cycle tests and MatDEM numerical simulation. The results show that: freeze-shrinkage occurs to the soil in the dump after the freeze-thaw cycles, and the stress-strain curve shows the trend of strain softening at low confining pressure for shear failure, and the trend of strain hardening at the increased confining pressure for dilative failure. The deterioration of shear strength was most significantly affected by the first three freeze-thaw cycles, and reached the lowest value in the third cycle. The temperature transfer process can be divided into four stages: rapid temperature drop, slow phase transition, continuous cooling and temperature stabilization. Moisture migration mainly occurs in the first two stages of temperature transfer, with more in the second stage. The overall radius of the particles decreases and the dispersion increases after freeze-thaw. The expansion and contraction of particles caused by temperature transfer and moisture migration, the ice-water phase transition and the formation of cryostructures lead to the repeated changes in particle size, position, connection state and interparticle stress, which together drive the irreversible structural damage to the soil, resulting in strength deterioration. Moisture migration not only provides water conditions for the formation of cryostructures, but also produces dissolution and erosion damage to them, which together with the phase transition of moisture is the main reasons for the structural adjustment and stress field change of soil under the action of freeze-thaw cycle. The simulation of freeze-thaw cycle by discrete element method is helpful to understand the mechanical properties and strength degradation mechanism of soil, and provides a reference for the stability study of dump slopes and other engineering constructions in open-pit coal mine in frozen areas.