Abstract: How to mitigate and eliminate the impact of debris flow on bridge pier is crucial to the safety of bridges. Taking the three-pile unit with trefoil-shaped distribution as the protective structures, a design principle for the size of this structures is proposed considering factors such as the characteristics of debris flow movement and the width of bridge piers. Firstly, an optimal arrangement of piles is obtained by simulating the impacts of block stone on different types of pile group structures. Then the impact of the debris flow with block stone on the bridge pier under pile structure was analyzed by using SPH-FEM method. The results show that the impact force of large stones and debris flow slurry on bridge piers is 3 843 kN, which is much greater than the impact force of debris flow slurry on bridge piers without protection, which is 1 840 kN, and the impact force of debris flow slurry on bridge piers with protection, which is 1 452 kN. The pile layout with a column spacing of 3.0 m and a row spacing of 1.0 m is the most stable. And after the protective structure was set up, the peak impact force of debris flow slurry, peak stress and peak displacement at the pier bottom whent=8.0 s were reduced by about 21.1%, 79.0% and 29.3%, respectively. Therefore the pile group structure has a significant protective effect on bridge piers impacted from debris flow.

Key words: debris flow, trefoil-shaped, pile structures, bridge pier, SPH-FEM simulation