Hydromechanical constraints on piping failure of landslide dams: an experimental investigation

Background: Understanding the internal structure and material properties of landslide dams is essential for evaluating their potential failure mechanisms, especially by seepage and piping. Recent research has shown that the behaviour of landslide dams depends on the internal composition of the impoundment. We here present an experimental investigation of the hydromechanical constraints of landslide dam failure by piping. Experiments were conducted in a 2 m × 0.45 × 0.45 m flume, with a flume bed slope of 5°. Uniform dams of height 0.25 m were built with either mixed or homogeneous silica sands. Uniform-sized pebbles encased in a plastic mesh were used to initiate internal erosion. Two laser displacement sensors were used to monitor the behaviour of the dams during the internal erosion process while a linear displacement transducer and a water-level probe were deployed to monitor the onset of internal erosion and the hydrological trend of the upstream lake. Results: Five major phases of the breach evolution process were observed: pipe evolution, pipe enlargement, crest settlement, hydraulic fracturing and progressive sloughing. Two major failure modes were observed: seepage and piping-induced collapse. Majority of the dams composed of homogeneous material failed by seepage and downstream slope saturation, whereas dams built with mixed material failed by piping. Conclusions: We found that an increase in soil density and homogeneity of the dam materials reduced the potential to form a continuous piping hole through the dams. Furthermore, the potential for piping and progression of the piping hole through the dams increased with an increase in the percentage of fines and a decrease in hydraulic conductivity. The rate of pipe enlargement is related to the erodibility of the soil, which itself is inversely proportional to the soil density. This study provides new insights into the governing conditions and breach evolution mechanisms of landslide dams, as triggered by seepage and piping.