## 4 The Large-Scale Landslide Model Experiment and Bayesian Updating
### FEM Simulation  
In the Bayesian updating process, a numerical model of the experimental setup is needed to provide simulation results. The SEEP/W software, a part of Geo Studio is a numerical simulation software for seepage analysis. In this study, Geo Studio software (2007) was used. It can mathematically simulate the real physical process of water flowing through the soil medium based on the finite element model. The equation used in the process is a steady-state seepage equation. The work of this section aims to model the seepage analysis of the landslide model in Figure 3 through the FEM. For this reason, a geometric model of a large-scale landslide model was built in FEM software. The purpose of this research was to study the seepage characteristics of landslides through FEM to simulate the changes in water content of landslides when the water level rises. We used SEEP/W software to develop and calibrate computer models of landslide model, and compare observed data with simulated data.  
Figure 5: Pressure plate instrument: (**a**) pump; (**b**) pressure gauge.  
Figure 6: (**a**) Water content sensor (the rated voltage was 5 V); (**b**) signal conversion device (converting the RS485 signal to USB signal).  
In the FE model process, the main steps are to establish a landslide model, set material parameters, divide calculation units, and set boundary conditions [35]. Firstly, SEEP/W was used for geometric modeling of the landslide model according to the dimensions in Figure 3. The material models and properties of the soil were put into saturated-unsaturated seepage model, defined with a seepage constitutive relationship requiring two functions: the permeability coefficient function and water content function (SWCC) [35]. In this process, the saturated permeability coefficient was set to \(1.6\times 10^{6}\), and the SWCC parameters were set according to the prior distribution. Figure 8 shows the geometric model and the mesh used in this work. In this figure, the yellow area represents the landslide body. The unit accuracy of the finite element mesh was set to 0.1 m. The red part represents the boundary conditions. Two boundary conditions were set for the numerical model based on water level changes: water level boundary conditions and pressure boundary conditions. The boundary conditions of the model were a key component of the numerical analysis, and the ability to control the boundary conditions was also a direct reason for the powerful numerical analysis. In this process, the calculation method was the steady-state calculation, and the solver adopted the implicit solution method.  
The FEM can evaluate water content values, depending on the input parameters (a,n) of the SWCC model. This results were the simulation samples used in Bayesian updating. The main purpose of this was to obtain the complete results of the FE model evaluation by SEEP/W software calculation, including water pressure, moisture content, etc. The results calculated by the SEEP/W software are analyzed and compared with the monitoring data of the landslide model in Section 4.1 to check for errors. The calculation of the SEEP/W software in this section also provides a model for the establishment of the surrogate model in the next section.  
Figure 8: Mesh generation of finite element model (FEM).  
Figure 7: Monitoring data of water content.