Abstract: To investigate how the degradation of physical and mechanical parameters of moraine soils under freeze-thaw cycles affects the stability of glacial lake moraine dams, this study focused on Nuquegou in Shannan City on Qinghai-Xizang Plateau. Field investigations were first conducted on high-outburst-risk glacial lakes and moraine dams, during which moraine sandy soil and moraine silty soil samples were collected on-site. Subsequently, eight evaluation factors were selected using the analytic hierarchy process (AHP) to assess the hazard risk of 18 glacial lakes in the Nuquegou watershed. Laboratory standard triaxial tests and falling-head permeability tests were then performed on both soil types subjected to varying numbers of freeze-thaw cycles to analyze the evolution patterns of moraine dam stability. Finally, the Massflow numerical simulation software was employed to reconstruct the complete dynamic disaster chain process, spanning moraine dam failure, glacial lake outburst, flooding, and subsequent debris flow. The results indicate the following: 1) In the early stages of freeze-thaw cycles (0-5 cycles), the cohesion of both moraine sandy and silty soils exhibited a pronounced degradation trend, with degradation rates of 12.21% and 7.89%, respectively. Meanwhile, the internal friction angle fluctuated slightly as the number of cycles increased, whereas the permeability coefficient increased continuously, indicating that freeze-thaw cycles exert a sustained effect on soil microstructure and pore distribution. 2) The stability coefficient of the moraine dam experienced its maximum attenuation rate (－0.022 2) during the early stages  of freeze-thaw cycles. This attenuation pattern aligned with the degradation of the strength parameters of the moraine soils, confirming that soil strength characteristics are a critical factor governing moraine dam stability. 3) Under the worst-case scenario of complete failure of the BH06 moraine dam, the simulated glacial lake outburst triggered a flash flood that subsequently transformed into a debris flow. The maximum flow velocity reached 5.1 m/s, resulting in a final debris flow deposition thickness of 4.5 m and an accumulation area of 4 220 m², which partially blocked the main channel of the watershed. In summary, the significant strength degradation of moraine soils during the early stages of freeze-thaw cycles drives a rapid decline in moraine dam stability, potentially triggering glacial lake outbursts and cascading disaster chains.

Key words: freeze-thaw cycle, moraine soil, moraine dam, instability, glacier lake outburst, catastrophic evolution, Qinghai-Xizang Plateau