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

Abstract

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.

Tree-root control of shallow landslides

“…For example, triggering could be due to the rapid increase in macropore water pressure and the saturation of the soil from top to bottom with little time for changes in matrix pore pressure to occur. In our example, preferential flow paths lead to local increases of pore-water pressure that, in combination with a loss of suction stress in the soil matrix, result in a critical drop of soil shear strength typical of forested soils on compacted bedrock (Lehmann et al, 2013). Yet in another situation, high pore-water pressure can originate from ephemeral springs or water exfiltration from fractured bedrock (Montgomery and Dietrich, 1994).…”

“…The model embodies the rapid increase in positive pore pressure in a preferential flow domain (representing macropores) and the slow decrease in suction in the soil matrix caused by slow water transfer from the macropores to the matrix. This decrease in suction is the equivalent of the increasing connections of water-saturated regions represented by the decrease in the standard deviation of water saturation observed by Lehmann et al (2013) that eventually caused slope failure in the Rüdlingen experiment. We assume that water flow in soils during a rainfall event is a combination of slow matrix flow (also called immobile water with capillary number lower than 1) and fast preferential flow (mobile water, capillary number higher than 1) (Sidle and Ochiai, 2006;Beven and Germann, 2013).…”

“…Our objective here is not to reproduce the detailed physical mechanisms by which changes in subsurface hydrology trigger a landslide but to develop a simple empirical model that realistically mimics observed changes in porewater pressure and water content during rainfall infiltration. Although diverse hydrologic triggers have been observed and described (e.g., Reid et al, 1997;Iverson, 2000), here we use, as a representative example for the hydrological conditions triggering a shallow landslide in our model, pore-pressure measurements during the artificial triggering of the Rüdlin-gen shallow landslide experiment in Switzerland (Askarinejad et al, 2012;Lehmann et al, 2013). Data from Lehmann et al (2013) indicate that high pore-water pressures were attained relatively quickly and remained steady across the slope long before failure occurred, and that the decrease in the standard deviation of the water saturation prior to failure indicated an increase in the connectivity of water-saturated regions that reduced soil shear strength across the full length of the slip surface leading to failure.…”

“…Although diverse hydrologic triggers have been observed and described (e.g., Reid et al, 1997;Iverson, 2000), here we use, as a representative example for the hydrological conditions triggering a shallow landslide in our model, pore-pressure measurements during the artificial triggering of the Rüdlin-gen shallow landslide experiment in Switzerland (Askarinejad et al, 2012;Lehmann et al, 2013). Data from Lehmann et al (2013) indicate that high pore-water pressures were attained relatively quickly and remained steady across the slope long before failure occurred, and that the decrease in the standard deviation of the water saturation prior to failure indicated an increase in the connectivity of water-saturated regions that reduced soil shear strength across the full length of the slip surface leading to failure. Other data in different localities (e.g., Matsushi et al, 2006;Bordoni et al, 2015) have also shown high, steady pore-water pressure prior to failure.…”

“…Figure 2 illustrates the model behavior for parameters shown in Table 1. The standard deviations are chosen so that macropore water pressure reaches its maximum before matrix water content, to mimic, but not reproduce, the behavior observed by Lehmann et al (2013).…”