Shear properties and pore structure characteristics of soil–rock mixture under freeze–thaw cycles

Soil–rock mixtures (S-RMs) are widely distributed in the shallow surfaces of cold regions that experience frequent freeze–thaw cycles, considerably threatening the stability of the soil–rock slopes on both sides of the road. We examine the S-RM shear strength and deformation characteristics under different freeze–thaw cycles through experiment and simulation. Nuclear magnetic resonance (NMR) was used to analyze the influence of freeze–thaw cycles on the internal pore structure of S-RM and establish the internal relationship between pore structure and shear characteristics under freeze–thaw cycles. Results show that the shear strength parameters of S-RM first decrease, then increase, and finally decrease as the number of freeze–thaw cycles increases; besides, the vertical expansion range of cracks gradually increases during shearing with the number of cycles. Porosity first increases, then decreases, and increases again with the number of freeze–thaw cycles. NMR results showed that S-RM pores had fractal characteristics. According to the fractal dimension, the soil particles inside the sample formed larger aggregates after the first freeze–thaw cycle. After the second–third freeze–thaw cycles, the agglomerated soil particles gradually disintegrate and tend to homogenize. The soil particles cover the surface of the gravel under the skeleton deformation to form an inclusion structure. The subsequent increase in the number of freeze–thaw cycles causes the soil particles attached to the gravels to gradually peel off.