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.