## 7 Conclusions  
In this paper, for a Yangtze River Bridge in the Three Gorges reservoir area, the FEM-SPH conversion coupling algorithm is used to establish a three-dimensional finite element model of the bank slope and bridge, simulating the whole process of bank slope deformation, slip, and instability evolution under the reservoir water level rise and fall cycle, revealing the interaction mechanism between bank slope slip and bridge pile foundation, studying the bridge pier deflection law and failure mode, and obtaining the following conclusions:  
1. The FEM-SPH conversion coupling algorithm can simulate the whole slope process, from the initial deformation slip to the deterioration of geotechnical properties leading to the final instability. It is more intuitive and accurate than the traditional criterion by completely converting the finite element mesh of the sliding area into SPH particles as the slope instability criterion;
2. The first instability damage will occur during the 16 th water level lift cycle. In the 17 th to 19 th water level lift cycles, the slope is in a new steady state, but the unbalanced soil thrust on the pile foundation gradually increases, and the bridge pile foundation plays an anti-slip role in the slope deformation instability during the 20 th water level lift cycle. The second type of instability damage will occur;
3. The overall deformation of the pile pier is characterized as follows: the displacement of the pile body in the bedrock embedded section is almost zero; the pile body is displaced horizontally from the soil-rock interface to the pile-soil interface; and the displacement of the pile body is linearly increasing from the pile-soil interface to the top of the pier. With the deformation, slippage, and instability of the slope, the deflection of the top of the pier shows a trend of \"slow increase-surge\", and the increment of the deflection changes from slow increase to surge during the first destabilization of the bank slope;
4. Pile damage and cracking occur first at the soil-rock interface of the pile foundation and then gradually develop upward; with the deformation and destabilization of the bank slope under the action of the reservoir water level cycle, the pile foundation develops from transverse cracks to mixed cracks until the second destabilization of the bank slope occurs in the process of damage, the pile foundation tensile area fails on a large scale, shear damage occurs at the upper part of the soil-rock interface, and the angle between the damage surface and the horizontal plane is about 60\({}^{\circ}\). Therefore, during the operation of the bridge, the structural performance monitoring at the soil-rock interface of the pile foundation should be strengthened, and the later remediation should focus on the reinforcement treatment at this location;
5. By relying on the bridge rescue project for the Three Gorges reservoir area landslide, the comprehensive reinforcement treatment method of the bridge structure was studied. The deflection of piers and main girders was corrected by jacking the main girders, piers were treated by increasing the cross-section; foundations were reinforced by adding pile foundations; and bank slips were reinforced by adding anti-slip piles. After adopting the comprehensive reinforcement treatment described in this paper, the bridge pier of a Yangtze River Bridge in the reservoir area has a good restoration effect, among which pier six has the most obvious treatment effect. The top upstream and downstream sides of the pier are restored to 33.5 cm and 33.0 cm, respectively, and the bottom upstream and downstream sides of the pier near the ground are restored to 5.1 cm and 4.2 cm, respectively. After the soil removal and anti-slip pile reinforcement treatment, the bank slope is in a stable state. Through reinforcement measures such as adding a pile foundation, enlarging the bearing platform, and expanding the pier section, the bearing capacity and stability of the substructure were greatly improved.