# Analysis of a Landslide on a Railway Track Using Laser Scanning and FEM Numerical Modelling
## 4 Results of Numerical Modelling  
The parameters of each layer were obtained from geotechnical testing [27] (Table 1).  
Due to the assumed failure mechanism of the railway embankment, only characteristic parameters of the soil obtained from laboratory tests were used for the stability analysis. Because geotechnical studies were conducted after the landslide, it was not known precisely what the limits of the geotechnical layers were before the landslide (Figure 10). Cross-section II\(-\)II was the closest to the landslide main scarp. It was unknown whether the sandy layer was a continuous or a lens layer, as it would seem from geotechnical testing. The numerical model was based on the data obtained with some margin of uncertainty which was difficult to estimate. Figure 11 shows the geometry of numerical model of the area with the geotechnical layers. The load of the railway traffic was omitted in this numerical calculation, but it can be assumed to be a significant factor in activating the landslide process. The issue is important in the context of the impact of ground vibrations on the environment and anthropological constructions [28].  
\begin{table}
\begin{tabular}{c c c c c c} \hline \multirow{2}{*}{**Kind of Layer**} & **Unit Weight** & \multirow{2}{*}{**Cohesion c\({}_{u}\) [kPa]**} & \multirow{2}{*}{**Internal Friction Angle**} & \multirow{2}{*}{**Liquidity Index**} & \multirow{2}{*}{**Filtration**} \\  & \(\boldsymbol{\gamma}\) [kN/m\({}^{3}\)] & & & \(\boldsymbol{\varphi_{u}}\) [\({}^{\boldsymbol{o}}\)] & (I\({}_{\text{L}}\)) [-] & **Coefficient k\({}_{\text{10}}\) [m/s]** \\ \hline Crushed rock & 16.0 & 1 & 40 & - & \(10^{-3}\) \\ \hline Colluvium & 18.5 & 5 & 29 & - & \(10^{-4}\) \\ \hline Silty layer IIa & 20.5 & 28 & 16 & 0.30 & \(10^{-5}\) \\ \hline Silty layer IIb & 21.0 & 32 & 19 & 0.18 & \(10^{-7}\) \\ \hline Sandy silt layer III & 21.5 & 32 & 20 & 0.10 & \(10^{-7}\) \\ \hline Sandy layer IVa & 19.5 & 1 & 32 & - & \(10^{-3}\) \\ \hline \end{tabular}
\end{table}
Table 1: Parameters obtained from geotechnical testing [27].  
Figure 10: The top view of the critical area shows the drill hole locations and the set of geological cross-sections imposed on the numerical model (based on [27]). Descriptions: I-I—cross-section, OWDPLA—borehole number, x—geotechnical sounding point.  
Two numerical models were analysed-dry and wet. The dry model was built on the assumed natural moisture of the layers, while the wet model represented fully saturated layers.  
The results for calculating the stability for the dry model are shown in Figure 12. As can be seen, the obtained stability factor FoS = 1.17. Thus, it is close to the stability limit but was not exceeded. This explains why the slope did not lose stability until water influence changed the soil parameters.  
Considering wet model assumptions, the IIA silty layer was plasticised. Liquidity index \(\rm{I_{L}}\) = 0.75 was adopted. The strength parameter values were changed to \(\rm{\varphi_{u}}=11^{\circ}\) and \(\rm{c_{u}}=6\) kPa.  
Figure 11: The geometry of the numerical model of the area with the geotechnical layers and calculation mesh.  
Figure 12: The dry model with indicated shear strain isolines. Maximal shear strain denoted by red colour corresponds to FoS = 1.17.  
The results for calculating the stability for the wet model are shown in Figure 13. The obtained stability factor FoS = 0.9. This means the railway embankment lost its stability.