Experimental Study on Macroscopic Mechanical Characteristics and Microscopic Pore Structure Evolution of Soil-Rock Mixture under Repeated Freeze-Thaw Cycles

The response characteristics of the mesostructure and macro-characteristics of the soil-rock mixture under repeated freeze-thaw action have an important influence on the safety and stability of the dump slope in low-temperature environments. In order to further understand the multi-scale response behavior of a soil-rock mixture under freeze-thaw cycles, this paper carried out indoor freeze-thaw cycles, uniaxial compression, and electrochemical impedance spectroscopy tests on a soil-rock mixture taken from a graphite mine dump in Jixi City, Heilongjiang Province, China. Combined with the simulation calculation of discrete element numerical software (PFC2D 7.0), the effects of freeze-thaw cycling on electrochemical impedance spectrometry (EIS) mesoscopic parameters, uniaxial compressive strength, and crack propagation of soil-rock mixtures were analyzed. The intrinsic relationship between mesoparameters and macroscopic mechanical properties was established. The results showed that as the number of freeze-thaw cycles increases from 0 to 15, the mesopores inside the soil-rock mixture gradually increase, and the angular similarity of distribution characteristics increases by 5.25%. The uniaxial compressive strength and the peak secant modulus increase exponentially with the increase in the number of freeze-thaw cycles, the uniaxial compressive strength decreases by 47.62%, and the peak secant modulus decreases by 75.87%. The peak strain and pore compaction stage showed an exponential increase and an increasing trend, respectively, and the peak strain increased from 2.115% to 4.608%. The failure mode was basically similar in different cycles; the failure cracks extended from the corners to the middle and lower parts before the failure finally occurred. The types of failure cracks were mainly tensile cracks, followed by tensile shear cracks and the fewest compression shear cracks. The similarity and uniaxial compressive strength conformed to a good linear relationship with the number of freeze-thaw cycles, with the uniaxial compressive strength decreasing linearly with the increase in similarity.