Multi-Scale Damage Evolution of Soil-Rock Mixtures Under Freeze–Thaw Cycles: Revealed by Electrochemical Impedance Spectroscopy Testing and Fractal Theory

The response of the microscopic structure and macroscopic mechanical parameters of SRM under F–T cycles is a key factor affecting the safety and stability of engineering projects in cold regions. In this study, F–T tests, EIS, and uniaxial compression tests were conducted on SRM. The construct equivalent model of different conductive paths based on EIS was constructed. A peak strength prediction model was developed using characteristic parameters derived from the equivalent models, thereby revealing the mechanism by which F–T cycles influenced both microscopic structure and macroscopic strength. The results showed that with increasing cycles, both RCP and RCPP exhibited an exponential decreasing trend, whereas CDSRP and Df increased exponentially. Peak strength and peak secant modulus decreased exponentially, but peak strain increased exponentially. The expansion and interconnection of pores with different radii within CPP and CP caused smaller pores to evolve into larger ones while generating new pores, which led to a decline in RCPP and RCP. Moreover, this expansion enlarged the soil–rock contact area by connecting adjacent gas-phase pores and promoted the transformation of CSRPP into DSRPP, enhancing the parallel-plate capacitance effect and resulting in an increase in CDSRP. Moreover, the interconnection increased the roughness of soil–soil and soil–rock contact surfaces, leading to a rising trend in Df. The combined influence of CDSRP and Df yielded a strength prediction model with higher correlation than a single factor, providing more accurate predictions of UCS. However, the increases in CDSRP and Df induced by F–T cycles also contributed to microscopic structure damage and strength deterioration, reducing the load-bearing capacity and ultimately causing a decline in UCS.