Quantitative Analysis of the Microstructures of Deep Silty Clay Subjected to Two Freezing–Thawing Cycles under Subway Vibration Loading

The bypass of subway tunnels in soft soil areas is usually constructed by the artificial freezing method due to the waterproof property and high strength of frozen soil. The thawing settlement and deformation of silty clay under subway vibration loading is becoming a major issue. The Shanghai subway lines cross vertically and horizontally, of which the deepest station reaches 33.1 m. With the rapid development and utilization of underground space, the depth of subway excavation will become increasingly deeper. Under subway vibration loading, the dynamic characteristics and microstructures of deep clay should be investigated. In this paper, a series of scanning electron microscopy (SEM) tests and mercury intrusion porosimetry (MIP) tests were carried out to study the microscopic parameters of deep silty clay subjected to two freezing–thawing cycles under cyclic loading. The freezing–thawing soil produced higher axial strain than that of the unfrozen soil with the same loading. After freeze–thaw cycling, the soil particles show the phenomenon of agglomeration. As the pore diameter increases, the pore shape coefficient decreases gradually from 0.45 to 0.85. Under similar vibration loading, the anisotropy of the frozen–thawed clay increases as the freezing temperature decreases. The fractal dimension obtained by SEM tests is between 1.25 and 1.33. There is an approximate linear relationship between the axial strain and the fractal dimension. In MIP tests, there exists a bottleneck effect in the process of mercury intrusion. The pore volume and pore diameter of the freezing–thawing soil increase with decreasing freezing temperatures and freezing–thawing cycles, respectively. The thermal fractal model is applicable to the fractal characteristics of silty clay. The results can provide a reference for the design and construction of deep-buried subway tunnels in soft soil areas.