Freeze-thaw cycle effects on granite and the formation mechanism of long-runout landslides: insights from the Luanshibao case study in the Tibetan Plateau, China

The Tibetan Plateau is susceptible to large-scale long-runout landslides during freeze-thaw cycles. The Luanshibao (LSB) long-runout landslide in Litang County, China, serves as a notable example. This study investigates the granite mechanical properties under freeze-thaw cycles effects in the LSB landslide initiation zone, by the methods of mechanical tests, NMR (nuclear magnetic resonance) tests, triaxial rock mechanics tests, and FLAC 3D simulations. Results show that the granite’s internal structure damage intensifies with increasing freeze-thaw cycles. Single rock wave velocity notably decreases from 3.60–4.09 km/s (0 cycles) to 2.12–3.11 km/s (30 cycles). freeze-thaw cycles enlarge fissure size and scope, evident in increased FID peak values from 1223–1162 (no freeze cycles) to 1307–1447 (freeze-thaw cycles). Granite’s compressive strength, elastic modulus, peak strength, residual strength, and volume strain gradually decrease with more freeze-thaw cycles, while Poisson’s ratio remains relatively stable. Numerical simulations demonstrate decreased slope stability with increasing freeze-thaw cycles, resulting in an arc-shaped shear penetration surface at the top of the LSB landslide when reaching 50 cycles. These findings suggest that freeze-thaw cycles weaken the mechanical properties of granite, thereby establishing the fundamental conditions for the occurrence of long-runout landslides. Additionally, external forces like earthquakes can trigger a chain reaction: earthquake → high-position rock or ice avalanche → landslide collapse and hit the ground → high-speed debris flow. These findings contribute to a better understanding of the impact of freeze-thaw cycle intensity on the formation of long-runout landslides worldwide.