Numerical investigation of the failure of the colluvium slope with retaining piles induced by rockslides

Retaining piles is the common mitigation measure for stabilizing colluvium slopes due to ease of design and cost-efficiency. Although there are many successful applications of retaining piles in Taiwan, several cases failed due to lack of consideration in design stage. We focused on a specific case in the downstream of Chiaohu River, central Taiwan. Remote-sensing analysis shows that this site has collapsed at least five times since 1948 and the rockslides from source area of colluvium dominate the stability of colluvium. The latest colluvium failure that occurred in 2019 had damaged the retaining piles. The design report of the retaining piles reveals that the stability analysis only considered the colluvium as the acting force. In the scenario, limit equilibrium method can be used to evaluate the factor of safety of the colluvium slope with piles. However, the limit equilibrium method is not applicable when the colluvium slope failed because the rockslide at the upper slope. This study aims to provide insight into the complex behavior of rockslide-induced colluvium slope failure and the stability of retaining pile in context of this design factor, facilitating the development of a numerical model. We first conducted geological investigations and remote sensing analysis to understand the evolution of landslide history. Next, three-dimensional full-scale numerical model was built based on coupled FDM-DEM technique. DEM was used to simulate the rockslide source and colluvium material, and FDM was used to model the piles in the colluvium. The back-analysis shows that simplified consideration of only colluvium material on the stability of retaining piles underestimate the effects of sliding force of the rockslide on behavior. In addition, the rockslides not only increase total landslide volume, but also are the source of the colluvium deposit in landslide history. This study demonstrates that the assessment of the stability of the colluvium slope with retaining piles induced by rockslides can be aided by using coupled FDM-DEM simulation.