The Impact of Freeze–Thaw Cycles on the Shear and Microstructural Characteristics of Compacted Silty Clay

The shear strength characteristics and weakening effect of soils under freeze–thaw (FT) cycling are the key problems that should be solved to ensure the integrity of infrastructure construction in seasonally frozen soil areas. Thus far, however, the research on the mechanism of strength deterioration resulting from microstructural changes induced by FT cycles remains insufficiently comprehensive. To investigate the deterioration characteristics of the shear strength of seasonally frozen soils in FT cycles, a series of laboratory experiments were conducted using compacted silty clay subjected to a maximum of five closed-system FT cycles. The stress–strain curve, secant module, shear strength, and microscopic structure were measured for specimens before and after the FT cycles. The stress–strain curves of the unfrozen and thawed specimens demonstrated a strain-hardening behavior, indicating an increase in resistance to deformation. Moreover, the shear strength and secant modulus of the unfrozen specimen surpassed those of the thawed specimen significantly. As the number of FT cycles increased, there was a gradual decline observed in the strength, stiffness, cohesive properties, and internal friction angle of the thawed specimen. The nuclear magnetic resonance technique was employed to interpret the experimental findings. It was demonstrated that the micro-pores undergo continuous enlargement and transformation into medium-sized and large-sized pores, leading to FT deterioration. Based on the experimental results, a modified Duncan–Chang model was developed to simulate the mechanical behavior of compacted silty clay while considering the influence of FT cycles.

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