# Analysis of a Landslide on a Railway Track Using Laser Scanning and FEM Numerical Modelling
## 2 Landslide Area and Methods  
The landslide occurred across a railway track near the town of Kwidzyn, which is located in the geologically defined area of the Polish Lowlands. This area is not considered prone to the formation of land movements (Figure 1). Much more dangerous areas from this point of view are in the south of Poland. In this case, specific conditions occurred resulting in the activation of mass movements of soil. The research area is under the influence of all glaciation in Poland. There were three glaciations in Poland during the Pleistocene. The last of these was the Vistulian glaciation (Leszno phase) [22]. The thickness of the Quaternary deposits is, therefore, quite large in this area (Figure 2). Figure 1: The location of the research area concerning landslide areas in Poland (red spots) (www. pgi.gov.pl, accessed on 30 March 2022). The Quaternary deposits comprise the glaciation sediments in Poland's south, central, and central-north areas. They are separated by the Emski and Mazowiecki interglacial sediment settlements and the Holocene interglacial. The thickness of Quaternary deposits varies from 70 m to 250 m. These are mainly sand, gravel sands, individual clay layers, loam, silt and peat. In June 2020, damage to the railway track occurred due to a landslide. The event resulted in the complete failure of the railway embankment (Figure 3). Figure 3: A general view of the landslide that occurred on 30 June 2020 on the railway track near Kwidzyn (phot. [PERSON]). Figure 2: The thickness of Quaternary deposits in Poland according to [23]. It should be mentioned that in June 2020, the monthly rainfall exceeded 200 mm. The rainfall for June 2020 according to the Ringers system is presented in Figure 4. Several meteorological IMGW-PIB stations indicated that the period under consideration was the wettest since 1951. In the Kwidzyn region, rainfall was high at above 100 mm per month (Figure 4). The soil moisture was also high\(-\)above 60%. This abnormal amount of rainfall and the high soil moisture caused this hazardous landslide to occur. Two types of laser scanning measurements were employed: aeroplane LIDAR (Light Detection and Ranging) scanning and terrestrial laser scanning. The first was used to elaborate the morphology model before the landslide occurred. The second was used to analyse the landslide morphology, especially in comparison against the results of the numerical calculations. LIDAR data from the Airborne Laser Scanning (ALS) represent the terrain in the form of a cloud of measurement points with specific XYZ coordinates. In Poland, the entire country is covered by data from ALS measurements with different point densities, from 4 points/m\({}^{2}\) to as high as 20 points/m\({}^{2}\) (in the case of cities). The vertical resolution of the measurements is about 15 mm. Terrestrial laser scanning was performed in four locations using a RIEGL VZ-400. The measurement points were located at distances no greater than 400 m apart. The vertical resolution of the measurements was about 5 mm. Each measurement produced clouds of points in 3D with an accuracy of approx. 2-5 mm. GPS marked out each measuring point. The point cloud was developed in a RiSCAN PRO [24] because four scans were submitted. Figure 4: Map of the rainfall in June 2020 in Poland, based on data from IMGW\(-\)PIB stations (www.dobrapogoda24.pl, accessed on 5 April 2022). The point cloud is oriented in the local coordinate system. In the RiSCAN PRO program, the vegetation was removed within the analysed area. A 3D numerical model of the landslide was created based on geotechnical studies using the MIDAS FEM method [25]. The Coulomb-Mohr hypothesis was adopted with a linear plastic flow law, as is recommended for the stability analyses of slopes, e.g., [26]. The vertical stress in the model increased linearly with depth. The Factor of Safety (FoS) was determined using the shear reduction method (SRM).