Study on physical and mechanical properties of high-grade highway subgrade slate in permafrost region under freeze–thaw cycles

The subgrade crushed-rocks of Gonghe-Yushu (Gongyu) Expressway in Qinghai Province are seriously weathered, resulting in a series of pavement diseases. Among the weathered crushed-rocks, the weathering degree of slate is particularly serious, and its physical and mechanical properties, weathering resistance and applicability are not clear. Therefore, this paper takes the slate in the subgrade crushed-rocks of Gongyu Expressway as the research object, and drills the core of the slate rock block to make a cylindrical standard sample, and uniaxial and triaxial compression tests, nuclear magnetic resonance tests, and electron probe micro-analysis tests were performed on it within 50 freeze–thaw cycles (FTC) under saturated conditions. According to the test results, the mass, longitudinal wave velocity, and strength of the slate specimens all decrease with the increase of the number of FTC, the cohesion \(({\text{C}})\) $(\text{C})$ increases first and then decreases, and the change trend of internal friction angle (φ) is completely opposite to the cohesion. The FTC has an expansion effect on the pores of the slate specimens, and the microstructure of the rock particles on the specimen’s surface is removed and becomes smooth. The results of mechanical tests are used in the Hoek–Brown (H-B) strength criterion, and a unified expression of the H-B criterion suitable for slate in permafrost regions is established. The above conclusions can provide some construction reference and maintenance of high-grade highways in cold regions.