Analysis of a retrogressive landslide with double sliding surfaces: a case study

In 2014, a cutting slope was excavated in Guizhou Province in China during the construction of the YanDe highway when a previously unknown ancient landslide was encountered. It has subsequently been named the ‘Longjiayan ancient landslide’. Large surface cracks and severe building damage occurred, as well as apparent ground subsidence. This work reports the results of a comprehensive investigation performed to identify the mechanism and process of reactivation of the ancient landside. In situ surveys and mapping activities were first carried out to obtain a preliminary delineation of the landslide surface, and 19 boreholes were set up in the reactivation area to determine the landslide structure, as well as create a picture of the sliding surfaces involved. Then, 41 surface cracks and deep displacements in eleven boreholes were monitored for over 40 days. A numerical 2D analysis was also undertaken to back analyze the landside. A conceptual model is proposed for interpreting the mechanisms responsible for reactivation of ancient landslides based on the Longjiayan landslide. Thereafter, based on the formation mechanism of the landslide, detailed reinforcement measures were proposed to control it. The results reveal that the landslide was a retrogressive soil-mass slide involving two sliding surfaces. These surfaces have different depths: one is shallow and located in a silty clay layer; the other is deep and lies in a completely weathered shale layer. The soil mass primarily slid towards the free surface along the deep sliding surface. Up to three failure stages have been identified in the landslide which was triggered by excavation in the toe region. Rainfall and adverse dip direction of weak intercalations further promoted the development of the landslide. During this process, the shallow landslide played a key role in the formation of the deep landslide. Appropriate use of anti-slide piles and water interception/drainage is an effective approach to controlling such landslides.