Abstract: Influenced by the continuous cumulative rainfall of 350.6 mm in the previous 14 days, a large-scale reactivation of a giant basalt ancient landslide in Yanyuan Boli Village occurred on July 19, 2018. The resurrected volume was 1 390×104 m3 and 186 houses were damaged, causing significant economic losses. Based on field investigation, aerial survey of drone, drilling, physical and mechanical experiments, and numerical simulation, the factors and reactivated mechanism of the landslide were identified on the basis of geological structure and characteristics of the landslide. The results show that fractured rock-soil mass and topography are the fundamental causes for reactivation, and continuous rainfall and the rising of groundwater level are the triggering factors of reactivation. The landslide can be divided into two areas with different failure modes: main slide area and lateral collapse area. The seepage field in the landslide has significantly changed due to rainfall, and the pore water pressure has increased. As a result, the ancient landslide began to deform on July 13, and the stability factor gradually decreased. Constrained by microtopography, the main slide area is characterized by multiple stages and retrogressive failure. The front edge of the lateral slope is controlled by the main slide at the toe of the slope. During the movement of the main slide, the slope displacement and maximum shear strain increment in the lateral collapse area gradually increase, and the plastic zone expands, showing a certain relevance and lag with the main slide. This analysis indicates that due to the long-term influence of groundwater, the strength of the sliding mass gradually weakens. Rainfall leads to intensification of slope seepage and a decrease in shear strength, which induces landslide reactivation.

Key words: ancient landslide, compound slide, reactivated mechanism, stability, Yanyuan-Lijiang Platform margin geotectogene