# Analysis of a Landslide on a Railway Track Using Laser Scanning and FEM Numerical Modelling
## 5 Discussion  
LIDAR laser scanning allowed us to acquire data to model the area's morphology before the landslide occurred. Analysing the morphology (Figure 5), a recess of the terrain is visible in the form of a trench (marked by 1). Rainwater from this trench flowed out through the trough marked by the number 2. Based on meteorological data, it can be assumed that precipitation reached about 100 mm per day within the few days before the occurrence of the landslide, and that the soil moisture due to the previous rainfall was several dozen centimetres. These data indicate-in the absence of water drainage on the right side of the embankment (Figure 5)-the possibility of a large waterway stagnation with a length of several dozen metres and a width of a dozen or so metres in the time immediately before the occurrence of the landslide. Such a volume of water would undoubtedly exert a large hydrostatic pressure on the ground -which had an increased pore pressure- and filtration for the aquatic layers of sand below. Analysing the system of geotechnical layers, silty layer IIa was the most likely to develop the slip surface (Figure 14). In addition, a sandy layer, IIb, located below layer IIa, constitutes a natural water flow path. The accumulation of excess water in the layer caused it to exert additional pressure on layer IIa. There was probably a hydrostatic piercing of waters from the aquatic layer IVa to the silty layer IIa. This caused the values of the strength parameters of silty layer IIa to deteriorate significantly due to the influence of the water. This follows from the fact that there is a small difference between the plasticity and liquidity limits in this type of soil in comparison with those of other cohesive soils [29]. With the increase in soil moisture, the attraction force between soil particles decreases and, therefore, the ground exhibits less coherence. In cohesive soils which have moisture close to the liquidity limit, the cohesion force disappears almost completely. As a consequence, the low cohesion of deeper layers may contribute to movements of larger masses of soils [30].  
Figure 13: The wet model with indicated shear strain isolines. Maximal shear strain denoted by red colour corresponds to FoS = 0.9.  
The formation of a slip surface was shown in a 3D numerical simulation. The dry model indicated the railway embankment was stable in soil parameters before the landslide occurred (FoS = 1.15). For the wet model, the embankment became unstable (FoS = 0.9). Additionally, stability coefficients were small prior to the landslide occurrence. According to Eurocode 7, the FoS coefficient should be at least 1.25 [31]. The calculated FoS did not comply with the safety condition stated in the standards; however, there was no failure because the FoS was greater than 1. The numerical calculation indicated that weakened silty layer IIa, an unfavourable morphology and the geological structure of the area were the main causes of the landslide. Based on direct observation of the area, the deterioration of the material parameters was caused by prolonged rainfall--results confirmed by the wet model calculation.