Evolution of soil wetting patterns preceding a hydrologically induced landslide inferred from electrical resistivity survey and point measurements of volumetric water content and pore water pressure

The hydrological state of a hillslope prior to a sprinkling-induced shallow landslide was monitored using electrical resistivity tomography (ERT) along a 47 m long transect, supplemented by local time-domain reflectometry (TDR) and tensiometer measurements. The spatial and temporal evolution of wetting patterns in the soil material indicated attainment of a stationary fully saturated profile in a slope region underlain by shallow sandstone bedrock. The significant decrease in spatially averaged standard deviation of water saturation has not been observed during an earlier failed attempt to trigger a landslide by intense sprinkling. While for the stable experiment (no landslide was triggered) water saturation and soil moisture variability were still increasing with time, the unstable experiment reached a time-invariant state of high pore water pressures and saturations, until it finally failed. The results indicate that when large and interconnected regions of hillslope are saturated (as confirmed by high volumetric water content and low standard deviation of water saturation), additional water cannot be redistributed to empty drier regions and may eventually enhance local pore water pressure and seepage force, initiating large shear deformation and failure. Accordingly, a transition to such a critical steady state of high average water saturation, associated with low and constant spatial standard deviation, may serve as additional hydro-geophysical indicator for the imminence of a landslide release.