## 6 Discussion  
### Unique Geological Situation  
The occurrence of the Ludoialm landslide increases our concern on the stability of relatively flat slopes. Deep-seated landslides in Alpine mountainous regions are usually observed on slopes with a dip angle of the slope greater than 20\({}^{\circ}\), e.g., the Gradenbach landslide, the Reissenschuh landslide, or the Sechilienne landslide. The sliding mechanisms of these deep-seated landslides [9; 10; 11; 12; 13] differ significantly from the case investigated in this contribution. A case that also occurred in the Alps, the Gschliefgraben mudslide [87; 88], is somehow comparable to the Ludoialm landslide. The Gschliefgraben mudslide also occurred on a gentle slope with a dip angle less than 10\({}^{\circ}\), with alternating layers of less permeable and more permeable soils. This event was presumably triggered by a rockslide in the catchment area together with humid weather [88] and differs considerably from the case in this contribution, which was most likely caused by the pore pressure rise generated by snow melting.  
The layer of glacial sediments that covers the Ludoialm landslide area has a considerable thickness, which indicates that the soil is very weak from the slope surface down to a relatively large depth. A large thickness of glacial sediments that represent a weak strength is rarely observed for the geological compositions in most alpine deep-seated landslide areas [9; 10; 11], which could also be one contributing factor for the reactivation of the Ludoialm landslide. In this case, the unique features of this landslide can be summarized in the following two aspects.  
Firstly, the difference of permeabilities between the upper glacial deposits layer and the lower marl layer has a great influence on the stability of the flat slope, whose hydromechanical effect can be seen in Figures 15 and 17.  
Secondly, the shear strength of glacial deposits in the uppermost layer of the Ludoialm landslide is relatively low. The friction angle of about 19\({}^{\circ}\) is smaller than that of the glacial till in most cases, which is around 30\({}^{\circ}\), or at least larger than 20\({}^{\circ}\)[89; 90; 91; 92; 93]. This weak material fosters the instability of the slope, which was reactivated twice in the past decades even  
Figure 16: The pore pressure distributions obtained with different permeability coefficient settings for upper and lower layers at the location of the basal shear zone by the LEA.  
Figure 17: Schematic diagram of pore pressure distributions at the location of the basal shear zone (red dotted line) for cases with the same and different permeabilities for both layers.  
though the slope is quite gentle with the dip angle smaller than the soil friction angle of the sliding mass.  
The overall shear strength at the slope scale can also be estimated by a backward analysis of a failure. This could be more reliable than using a limited number of laboratory tests in some cases. However, in this case, we were focusing on the mechanical cause of the failure, and since the shear strength was only one of the parameters affecting the failure, a backward analysis would probably hide the cause of the failure and its mechanism.