Regolith structure analysis — A contribution to understanding the local occurrence of shallow landslides (Austrian Tyrol)

This study focuses on shallow landslides on Alpine slopes, usually triggered by extreme rainfall events or rapid snowmelt. Shallow landslides cause soil loss of agricultural land and can even induce larger mass movements. To estimate shallow landslide susceptibility, it is crucial to understand the interactions of the conditioning factors. The structure of the shallow subsurface (regolith) represents the least well known variable. Hydrological characteristics of the slope and its resulting stability depend very much on the structure and thickness of the regolith. However, conditions of the shallow subsurface have hitherto not been investigated in detail, due to difficulties in area-wide data collection. By adapting a common method from engineering sciences, our research provides new insights into the small-scale structure of a regolith body on an Alpine slope in the inner Schmirn Valley (Tyrol, Austria). We carried out field measurements by applying densely spaced lightweight dynamic cone penetrometer tests (DCPT) on a test site of 30 m by 30 m. To cope with the three-dimensional (3D) characteristics of the regolith body, we suggest the application of a 3D volumetric lattice (voxel) model. For the analysis of patterns and variations in the penetration resistance, we propose a spatial interpolation by the regularized spline with tension (RST) method. We approximated three different geotechnical parameters: (i) the thickness of the local regolith, (ii) the penetration resistance within the regolith and (iii) the surface of the solid bedrock. An exploratory analysis allowed us to describe surface and bedrock morphology and to detect discontinuities in the regolith in the immediate vicinity of the landslide. In addition, we were able to establish a distinct spatial relation of the derived geotechnical parameters and slope failure. The proposed method was developed, tested and verified in an individual case study, but the results indicate a great potential for future applications in shallow landslide susceptibility analysis. The findings of this study improve knowledge on the relation between shallow subsurface structure and the occurrence of shallow landslides. They also allow predictions about the potential evolvement of existing shallow landslides through secondary gliding processes.