Numerical Simulations of Failure Mechanism for Silty Clay Slopes in Seasonally Frozen Ground

The freezing and thawing (F–T) of soil will lead to changes in its physical and mechanical properties, and foreign scholars have conducted a lot of research on the mechanical properties of soil under freeze–thaw conditions. In 2011, Zhu [1] investigated the dynamic properties of frozen clay in the perennial permafrost roadbed of Beiluhe of the Qinghai–Tibet Railway. It was found that the maximum dynamic shear modulus of frozen clay decreased with the increase in temperature and the decrease in surrounding pressure, and the reference shear strain value decreased with the increase in temperature and surrounding pressure and the decrease in water content. In 2012, Tang et al. [2] studied the variation rule of the freezing strength and temperature of Shanghai mud clay; in the early stage of freezing, the strength of the frozen mud clay increased to the maximum value with time, and it increased with the decrease in temperature. In 2015, Ling [3] conducted a low-temperature cyclic triaxial test on frozen compacted soil in Nehe, Heilongjiang Province, and found that the freeze–thaw process had a significant effect on the dynamic shear modulus and damping ratio of the soil. In 2019, Lv et al. [4] analyzed the effects of the water content, freezing temperature, and freeze–thaw cycle time on shear strength by using cohesion and the internal friction angle as the shear strength indexes, and they analyzed the effects of the water content, freezing temperature, and freeze–thaw cycle time by combining them with the grey theory, and the results showed that the water content has the greatest effect on the cohesion and the angle of internal friction, and the angle of internal friction and the cohesion within the range of the plastic limit to the liquid limit decrease with the increase in the water content. Chen et al. [5] investigated the strength characteristics of unsaturated powdery clay after cooling and a single freeze–thaw cycle using a GDS triaxial test system after controlling the initial matrix suction, and the results showed that the initial matrix suction freezing process can enhance the shear strength of the soil body, so that the stress–strain curves of the soil body show certain strain-hardening characteristics at different temperatures. Hu et al. [6] found that the stress–strain curves of the original soil specimens were characterized by strain hardening with shear shrinkage by conducting a freeze–thaw cycle and consolidation-drainage triaxial tests on pulverized clay, and that the perimeter pressure during consolidation and the triaxial loading stages attenuated the adverse effects of the freeze–thaw cycle. Chen et al. [7] found that the freezing temperature and the number of freeze–thaw cycles had a significant effect on the axial strain of the pulverized clay under cyclic loading. In 2022, Zhou et al. [8] considered the effect of F–T to carry out static mechanical property tests on basalt-fiber and basalt-powder-modified powdered clay; after 30 cycles of F–T, the shear strength of the two kinds of modified powdered clay were higher than that of the unamended powdered clay, and the effect of basalt fibers on the reinforcement of this increase was generally better than that of basalt powder, which provides some theoretical references to the practical engineering in seasonal permafrost areas. Xu et al. [9] carried out electron microscopy (SEM) and CT scanning on the pulverized clay after freeze–thaw cycles. After continuous F–T, the internal microstructure of the pulverized clay became looser, the pores and connections between soil particles deteriorated, and the shear strength, elastic modulus, and cohesion decreased and stabilized with the increase in the number of cycles.

The research on seasonal permafrost in China has developed relatively rapidly. Song et al. [10] conducted freeze–thaw cycle tests on loess in Lanzhou, and the test results showed that with the increase in the temperature gradient under the same dry bulk weight, the decrease in the consolidation pressure in the early stage of the soil after multiple freezes and thaws gradually decreased, and the decrease in cohesion increased. Wang [11] and others conducted compression tests and drainage shear consolidation tests on in situ soils and unfrozen soils under different freeze–thaw conditions, respectively. The results show that the compression ratio of clay after F–T is higher than that of in situ soil and that F–T has a bi-directional effect on remolded soils with different dry densities, decreasing the compressibility of loose lumpy soils with low densities, and increasing the compressibility of dense, high-density soils. Qi [12] believes that the strength theory of thawed soil is used in the study of permafrost strength, it is difficult to reflect the compression and thawing phenomenon of soil under high stress, and the establishment of the ontological relationship of permafrost is mainly based on empirical formulas and focuses on the study of creep, which needs to be further improved. Based on the theory of significance analysis, Chang et al. [13] investigated the significant effects of the number of freeze–thaw cycles, freezing temperature, enclosing pressure, and the interactions among the factors on the mechanical properties of pulverized sandy soils. The results of the study show that the perimeter pressure and the number of F–T cycles have a strong effect on the mechanical properties of chalky sand, while the freezing temperature has little effect, and the combined interaction of freezing temperature with the number of F–T cycles has a great effect on the mechanical properties of chalky sand. Zheng [14] studied the changes of soil particles and soil pores in the process of a freeze–thaw cycle using the relevant mechanical tests of soil, and found that the particles of soil will be gradually ruptured after many times of F–T, which will lead to a gradual increase in the specific surface area of the soil and the bounding water content. Hu et al. [15] conducted a triaxial shear test in order to elucidate the effect of cooling temperature on the freeze–thaw cycle effect of soil. The results showed that freezing and freeze–shrinkage coexisted during soil freezing, and the deformation of both increased with the decrease in the cooling temperature. The lower the cooling temperature, the smaller the range of variation in damage strength with the number of freezes and thaws and the number of freezes and thaws required to reach a new steady state, and the weaker the cumulative effect of the freeze–thaw cycles.

The study of slope stability has always been a more complex problem, and research scholars at home and abroad have conducted more detailed studies on slope failure and instability problems, and have achieved many fruitful results. However, for the study of landslides on powdery clay slopes in the Quaternary freezing zone, relying only on traditional analytical methods can no longer meet requirements due to the complex coupling of the hydrothermal multiphysical fields involved. The damage problem of seasonal permafrost zone slopes mainly originates from the freeze–thaw damage caused by the cold environment, and its thaw–sliding mechanism is different from that of the non-permafrost-zone slopes, and the resulting sliding surfaces are not the circular sliding surfaces in the traditional sense, but rather in the form of shallow landslides. Shallow landslides are distributed in many parts of the world, and Table 1 summarizes some of the representative research literature on shallow landslides [16,17,18,19,20,21,22,23,24], which shows that the research on shallow landslides started late and the research power is weak. Scholars have given different definitions of shallow landslides according to different research disciplines and research purposes, and generally defining the rapid destruction of shallow slopes as soil sliding, earth flow, and shallow advection is also more commonly used. However, no matter which definition, these landslides have the following common characteristics: small volume of landslides, a depth of the slip surface between 1 and 5 m, and some scholars define shallow as within 10 m. In northeastern China, due to the spring temperature rise, they began to enter the thawing stage, the slope surface’s soil began to melt, while the bottom of the slope soil is still in a frozen state, the unthawed soil layer plays a similar role as a water blocker; the water flow effect, cannot be ejected in a timely way from the slope, reducing the strength of the shallow soil on the slope surface, leading to the slope having its own gravity for thawing and sliding, and the slide surface is mostly of a flat and straight type.