Mechanical properties and microstructure failure mechanism of gravel with prefabricated fissures under the freeze-thaw cycles

Freeze-thaw cycle is a critical factor influencing rock landslides in high-altitude mountainous regions. In this study, rock samples from the Lagangcun landslide in a high-altitude area were collected to conduct triaxial unloading tests on fractured conglomerates subjected to varying freeze-thaw cycles. The mechanical properties, energy evolution, acoustic emission characteristics, and multi-scale structural features were systematically analyzed to elucidate the freeze-thaw degradation mechanism of conglomerate with prefabricated fissures. The results demonstrate that peak strength of the samples progressively decreased with increasing freeze-thaw cycles. After 60 freeze-thaw cycles, compared to untreated samples, the peak strength of intact specimens under confining pressures of 10 MPa, 20 MPa, and 30 MPa decreased by 44%, 48%, and 70%, respectively. Under identical confining pressure and freeze-thaw cycles, samples with longer pre-existing fractures exhibited lower peak strength and greater susceptibility to failure. Both the absorbed and dissipated energy during rock failure decreased with freeze-thaw cycle, indicating that less energy was required to induce failure. As freeze-thaw cycles increased, the average acoustic emission energy per second diminished, the number of rupture points declined, and the failure duration shortened. The failure mode transitioned from X-shaped shear failure to tensile-shear conjugate failure. At the macroscopic scale, secondary cracks or fractures were closely associated with pre-existing fissures. Microscopic analysis revealed that crack propagation intensified with freeze-thaw cycles. Compared to untreated samples, 4 cm pre-fractured specimens after 40 and 60 cycles showed crack volume increases of 1.71% and 1.83%, respectively, with more pronounced cracking along pre-existing fractures, highlighting the exacerbated damage in fractured rocks. These findings clarify the strength degradation law of conglomerates under freeze-thaw conditions, providing critical insights for engineering construction and rock mass stability in alpine regions.