Mechanism of shear strength deterioration of soil-rock mixture after freeze–thaw cycles

The growth and melting of internal ice crystals reorganizing the pore structure and internal skeleton of soil-rock mixture (S-RM), which leads to the deterioration of the S-RM shear strength after freeze–thaw cycles and its strength characteristics after the cycles are different from the normal temperature one. Due to unclear S-RM strength deterioration mechanism after freeze–thaw cycles, stability of S-RM cutting slopes in cold regions cannot be effectively evaluated. In this paper, particle flow code (PFC) simulation as well as direct shear and nuclear magnetic resonance (NMR) tests were carried to study strength degradation behaviors and pore structure changes of S-RMs containing various amounts of rock after freeze–thaw cycles. The results show that the shear strength and internal friction angle of S-RM are no longer positively correlated with the rock content after the cycles, and the strength parameters will decrease at rock contents of above 45%. Based on the simulation test, shear band thickness variation after the cycles was quantitatively evaluated, and an innovative method to obtain the fluctuation value of shear failure surface was proposed. It is found that the change laws of shear band thickness and failure surface fluctuation value with rock content after the cycles are consistent with that of shear strength, and they all reach the maximum value when the rock content is 45%. Fractal theory was introduced for quantitative evaluation of changes in S-RM pore structure after the cycles, and combined with the strength parameter attenuation, the strength deterioration mechanism of S-RM was revealed: The deterioration of shear strength in samples containing low rock content is mainly due to changes in the contact form between particles caused by internal inclusion structure formation after the cycles. The deterioration of S-RM with rock content of 55% and 65% is mainly due to attenuation of the internal skeleton effect caused by the appearance of overhead structures. The internal pores and skeleton structure of the sample with 45% rock content have little change, so the attenuation changes of the strength parameters and the undulation value of the failure surface are minimal.