Effect of Freeze–Thaw Cycles on Mechanical Strength and Microstructure of Silty Clay in the Qinghai–Tibet Plateau

Freeze–thaw (F–T) cycles exert a significant effect on engineering activities in cold regions, thus attracting increasing attention around the world. In this study, a set of triaxial compression tests, scanning electron microscopy (SEM), and X-ray computerized tomography (X-ray CT) tests were conducted on silty clay samples with various moisture contents after different number of F–T cycles. The mechanical parameters of silty clay, such as peak shear strength and resilient modulus, and the microstructure evolution before and after F–T cycles were obtained. The results show that all the stress–strain curves of silty clay samples exhibit a strain-hardening behavior, and the higher the confining pressure is, the higher the hardening degree will be. The peak shear strength, resilient modulus, and cohesion of the silty clay samples are reduced after being exposed to F–T cycles. These parameters reach a minimum around the fifth cycle and gradually stabilize after 10 cycles. However, the change in internal friction angle induced by F–T cycles is trivial. With increased water content, the resilient modulus and cohesion decrease linearly. SEM and CT results show that the inner microstructure of silty clay sample becomes looser, and the pores and connections between soil particles deteriorate as the number of F–T cycles increase. The porosity of sample after five F–T cycles is higher than that before exposure to F–T cycles. The microstructural results agree well with the deterioration in the strength of silty clay.