Failure evolution and mechanisms of vertically heterogeneous landslide dams under coupled seepage and overtopping

Natural landslide dams generally exhibit strong vertical heterogeneity in soil composition due to the complex source materials and emplacement processes of landslides. There is insufficient study on the failure evolution and mechanism of vertically heterogeneous landslide dams under coupled seepage and overtopping. In this study, the failure evolution process, breaching characteristics, and failure mechanism of landslide dams with different vertical heterogeneous structures were investigated by a series of physical model tests. The test results showed that the vertical heterogeneous structure significantly changed the seepage-erosion pattern of landslide dams before overtopping. The degree of slope erosion and crest deformation was quantified, and that of the dam with a sandwich structure was the largest. The breach erosion rate of heterogeneous dams was controlled by the erodibility of soils, the difference in soil properties of the contact zone and the seepage erosion. Compared with the homogeneous dam, the peak flood discharge of the dams with sandwich structure and reverse grain sequence were significantly higher, with the peak discharge increased by 11.1% and 6.9%, respectively. The breaching duration of the dams with sandwich and reverse grain sequence was significantly shortened, and the emerging time of the flood peak was advanced, whereas the situation was opposite for the dam with positive grain sequence. Interlayer seepage shear stress dominated contact erosion initiation and overall failure evolution of heterogeneous dams. The typical longitudinal evolution patterns of heterogeneous dams were stepped profile, multiple scarps, and inflection point in the erosion surface compared with a homogeneous dam.