# Formation and Failure Mechanism of the Xinfangzi Landslide in Chongqing City (China)
## 6 Plastic Deformation Analysis of Multiple Sections  
Since the boreholes are laid along with the typical profiles, the 2D numerical models have the most detailed geological data, involving the position and the mechanical parameters of the sliding zone. Compared to the numerical models in 3D, the 2D simulation results can be identified as the closest analysis to the actual condition and easier to reveal the failure mechanism of landslides with complex slip paths. Therefore, the numerical simulations in 2D were carried out on multiple typical sections of the landslide using the strength reduction method [33; 34], and the plastic deformation trends of the landslide under different conditions were compared. Specifically, the factor of safety (FOS) can be obtained through the c-\(\varphi\) reduction algorithm by the FEM [35; 36; 37; 38]. In addition, the numerical non-convergence occurs simultaneously when the system reaches instability. The FOS and plastic strain distribution are then obtained [39; 40]. The typical sections involved three types: a cross section, a longitudinal section, and a compound section (Figures 4 and 7). The multiple sections of the landslide were simplified into geo-mechanical models covering the sliding mass, slip zone, and slip bed. In addition, the distribution of the groundwater inside the landslide was considered. According to on-site investigations, after heavy rainfall, the surface soil of the landslide is almost fully saturated, and water flows out on the slope's surface. Therefore, the mechanical state of the landslide was simplified under rainfall conditions, and the sliding mass and the slip zone above the groundwater level were converted into saturation parameters. The impact of heavy rainfall on the landslide will be further analyzed in the discussion part of this paper. Notably, the unsaturated mechanical states were ignored. In this way, the simplification under the rainfall conditions can be better transferred, and the failure mechanism of the landslide with the complicated slip path can be highlighted. The MIDAS GTS numerical calculation software was used in this study. The Mohr-Coulomb model was selected to characterize the mechanical properties of the landslide. The unit weights and shear strength parameters were obtained from laboratory tests, and the elastic modulus and Poisson's ratio were adapted from the relevant literature [41; 42]. All relevant calculation parameters are shown in Table 1. The other parameters, such as the dilatancy angle of the soil, are not considered here. \begin{table}
\begin{tabular}{c c c c c c c} \hline \hline \multirow{2}{*}{\begin{tabular}{c} **Mechanical** \\ **State** \\ \end{tabular} } & \multirow{2}{*}{**Lithology**} & \begin{tabular}{c} **Unit** \\ **Weight** \\ **(kN/m3)** \\ \end{tabular} & \begin{tabular}{c} **Elastic** \\ **Modulus** \\ **(kPa)** \\ \end{tabular} & \begin{tabular}{c} **Poisson’s** \\ **Ratio** \\ \end{tabular} & \begin{tabular}{c} **Cohesion** \\ **(kPa)** \\ \end{tabular} & \begin{tabular}{c} **Friction** \\ **Angle (\({}^{\circ}\))** \\ \end{tabular} \\ \hline \multirow{4}{*}{\
\begin{tabular}{c} Natural \\ conditions \\ \end{tabular} } & Sliding & \multirow{2}{*}{20.10} & \multirow{2}{*}{46,400} & \multirow{2}{*}{0.36} & \multirow{2}{*}{18.46} & \multirow{2}{*}{13.4} \\  & mass & & & & & \\ \cline{1-1} \cline{2-6}  & Slip zone & 20.10 & 3800 & 0.30 & 19.30 & 11.5 \\ \cline{1-1} \cline{2-6}  & Slip bed & 25.70 & 11,500,000 & 0.24 & 1678.20 & 44.29 \\ \hline \multirow{4}{*}{
\begin{tabular}{c} Rainfall \\ conditions \\ \end{tabular} } & Sliding & \multirow{2}{*}{20.30} & \multirow{2}{*}{40,500} & \multirow{2}{*}{0.32} & \multirow{2}{*}{14.60} & \multirow{2}{*}{11.38} \\  & mass & & & & & \\ \cline{1-1} \cline{2-6}  & Slip zone & 20.30 & 3000 & 0.28 & 15.00 & 9.60 \\ \cline{1-1} \cline{2-6}  & Slip bed & 25.80 & 10,500,000 & 0.20 & 1568.30 & 42.30 \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Numerical calculation parameters.