Objective New tectonic movement in the Bailong River Basin is active, the geological environment is fragile, and high-level long-distance landslides occur frequently. The deformation and failure modes of such landslides are complex, and the failure mechanism is unclear, which threatens the regional security.

Methods Taking the Beishan landslide in Zhouqu Lijie as the research object, field investigations, high-precision in situ monitoring instruments of deep displacement and pore water pressure, and three-dimensional numerical simulation methods were used to systematically reveal the deformation and failure mode of landslides under rainfall. Numerical simulation was used to analyse the motion characteristics of landslides after instability, and the landslide risk was scientifically evaluated.

Results The results reveal that under the action of single rainfall events, the landslide experienced leading edge traction sliding, and the deformation area gradually expanded backward with increasing rainfall intensity. When the rainfall intensity reaches 100 mm/d, the sliding zone forms a penetrating plastic zone. Overall, the strata above the landslide depth of 26.7 m are active, and the maximum surface displacement is 148.3 mm. Moreover, with the accumulation of pore water pressure and shear extrusion, the sliding zone at a burial depth of 18 m gradually destroyed. After instability occurs, the landslide transforms into a debris flow because of the cutting and diversion of the slope topography. The peak velocity is 44.6 m/s, and the coverage area at the foot of the slope is 0.27 km². This finding impacts the right bank of the Bailong River and blocks the river to form a barrier dam with a length of 558.4 m and an average thickness of 7.4 m.

Conclusions The deformation and failure mode of the Beishan landslide are affected by both internal and external forces. The deformation gradually expands from the leading edge to the back, showing a composite failure mode of leading edge traction and back pushing. After the overall instability of the landslide, there is a potential risk of the disaster chain blocking the river, which is a serious threat. The results provide a scientific theoretical basis for monitoring, early warning and disaster prevention of high-level remote landslides in the Bailongjiang River Basin.