To investigate the disaster-causing mechanisms, dynamic evolution, and post-disaster stability trends associated with the reactivation of the Huangci No. 2 landslide in Heifangtai, Gansu Province, on December 10, 2025, field geological investigations and the transient electromagnetic (TEM) surveys were conducted to characterize the deep structure of the landslide.

The entire three-dimensional dynamic sliding process was back-analyzed using the Massflow numerical model, and the three-dimensional limit equilibrium method was employed to quantitatively evaluate the post-event deposit and the stability of the high and steep rear slope and to predict their potential kinematic responses.

(1) The reactivation of the landslide resulted from the coupled effects of slope-toe excavation, long-term irrigation, and winter freezing conditions. The freezing-induced water-retention and pore-pressure buildup effect acted as the direct trigger: surface freezing blocked seepage pathways, causing pore-water pressure to accumulate at depth and triggering a bedding-parallel slide along the loess–mudstone contact. (2) The entire sliding process lasted 22 h 10 min, with a cumulative displacement of 310 m. The Massflow simulation reproduced four stages of the dynamic evolution: creep, accelerated sliding, deceleration and deposition, and stagnation and compaction. The intersection over union (IoU) of the simulated and observed deposition morphologies reached 85.85%. (3) Quantitative calculations indicate that the current deposit has a factor of safety greater than 1.15, suggesting a relatively stable state characterized by settlement and consolidation. However, under extreme saturation conditions, the potential failure volume of the high and steep rear slope could reach 40.9×10⁴ m³, with a maximum sliding distance of approximately 640 m.

The reactivation of the landslide was controlled by freezing-induced water retention and pore-water pressure buildup, together with multiple external disturbances. Although the main landslide body has generally stabilized, the high and steep rear slope remains highly susceptible to secondary failure. A long-term dynamic monitoring system should therefore be established to mitigate the risk of high-elevation secondary hazards.

This study quantitatively demonstrates freezing-induced water retention and pore-water pressure buildup as a distinctive triggering mechanism for winter loess landslides, extending the conventional understanding of loess landslide initiation beyond single hydrodynamic factors such as rainfall and irrigation. The findings provide direct technical references and theoretical support for winter landslide prevention and mitigation, emergency monitoring deployment, and early identification of geological hazards in Heifangtai and other irrigated loess tablelands, thereby contributing to the protection of local communities and their property.