# Slope Failure of Noncohesive Media Modelled with the Combined Finite-Discrete Element Method
## 4 Example II: Landslide Triggered by Chi-Chi Earthquake  
The Chiu-fen-erh-shan landslide induced by the Chi-Chi earthquake which occurred in 1999 is examined in this section. Various methods have been employed for previous simulations [7, 8]. Huge amount of rock mass high in the mountain travelled a long distance and deposited at the foot of the mountain. According to previous studies, the sliding rock masses are initially weak and disintegrated. Consequently, noncohesive rock blocks are used in the FDEM modelling. Once the rock masses are excited by the earthquake, they will fall along the mountain slope surface and mass flow can be formed. Configurations of landslide induced by the Chi-Chi earthquake are presented in Figure 6. The pre-and postslide configuration lines in Figure 6a are taken from reference [8] based on the site measurements where details can be found in Ref. [28]. In the FDEM modelling, 3072 elements are meshed in the mountain and 936 elements are meshed with rock blocks. The total computational time for this simulation is 119.1 h (up to t = 120 s). Fine mesh with the characteristic size of 10 m is employed for the slope surface of the mountain so that the shape of the slope surface is close to reality. On the other hand, coarse mesh with the characteristic size of 100 m is employed in the far field for the sake of computational efficiency. Rock blocks, with the characteristic size of 10 m, are modelled with individual noncohesive discrete elements. Horizontal ground motions recorded by the TCU072 station (see Figure 7) are used as the input earthquake excitation and applied to the mountain slope in the FDEM modelling. Material properties of rock blocks and the mountain are tabulated in Table 3. Time step for this problem is 5 \(\upmu\)s.  
Figure 5: Evolution of slope distance \(L\) versus time. Figure 6: Configurations of landslide induced by Chi-Chi earthquake: (**a**) topography after [8]; (**b**) FDEM modelling. Figure 8 shows the progression of mass flow resulting from the landslide induced by ground excitations. The horizontal travelling distance of the front of the sliding mass is marked. It can be observed from Figure 8 that the sliding mass reaches the equilibrium around t = 120 s. Triggered by ground motions, the initially disintegrated rock blocks gradually slide to the foot of the mountain under gravity along the slope surface. The mass flow piles up at the foot of the mountain slope, resulting in a new slope surface which is in good agreement with the dash line in Figure 6a [8]. To better demonstrate the effectiveness of the FDEM in modelling this landslide process, results simulated by DEM [7] and MPM [8] are given in Figure 9 for comparison. It should be noted that the friction angle \(\Phi=22^{\circ}\) in Figure 9a. This is because in reference [7], only data with \(\Phi=22^{\circ}\) is available when cohesion \(c=0\). Figure 8: Progression of landslide by FDEM. Figure 7: Time history of the horizontal velocity. \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Young’s Modulus (GPa)** & **Poisson’s Ratio** & **Density (kg/m\({}^{3}\))** & **Friction Angle (\({}^{\circ}\))** \\ \hline
7.57 & 0.19 & 2550 & 24 \\ \hline \hline \end{tabular}
\end{table} 
Table 3: Material properties of rock blocks and the mountain. The conclusion can be reached that the FDEM simulation agrees well with results from the DEM and MPM. For the travelling distance of front mass, results from the FDEM and DEM are in good agreement. For the slope process, the FDEM results are closer to the MPM results (particularly at t = 15 s and 30 s when the mass flow is dispersed a long distance along the slope surface). Fine-meshed rock blocks make the flowing mass more like granular material and thus results are closer to the MPM simulation. It is also worth mentioning that although the FDEM results agree well with those from the DEM and MPM in general, there are still differences (e.g., the travelling distance of front mass).