MODELING VADOSE-ZONE PROCESSES TO FORECAST SHALLOW LANDSLIDE OCCURRENCE

Operational forecasting of conditions conducive to shallow landslides are desired by organizations responsible for managing hazard and issuing warnings. However, hydrologic and mechanical processes in the vadose zone complicate such predictions. Infiltrating rainfall must typically pass through an unsaturated layer before reaching the irregular and usually discontinuous shallow water table. This process is dynamic and a function of precipitation intensity and duration, the initial moisture conditions and hydrologic properties of the hillside materials, and the geometry, stratigraphy, and vegetation of the hillslope. As a result, pore-water pressures, moisture content, effective stress, and propensity for landsliding vary over seasonal and shorter timescales. We describe a general framework for assessing the stability of slopes under variably saturated conditions. The framework includes profiles or fields of soil suction and moisture content combined with a general effective stress for slope stability analysis. The general effective stress, or so-called "suction stress," provides a means for rigorous quantification of stress changes due to rainfall and infiltration, and thus the analysis of slope stability over the range of moisture contents and soil suctions relevant to shallow landslide initiation. We apply the framework using a one-dimensional analytical solution for flow in the vadose zone combined with an infinite-slope stability analysis to examine the effect of soil hydrologic properties, antecedent soil moisture conditions, and rainfall intensity and duration on the timing of slope instability. For the limiting case of hydrostatic initial conditions and in which rainfall intensity is equivalent to the saturated hydraulic conductivity of the soil, results for hypothetical hillslopes composed of sandy and silty soils show that the timing of potential instability ranges from less than 36 hours (Fig. 1) to more than 20 days (Fig. 2).

Figure 1. Profiles of (a) pressure head, (b) effective saturation, (c) suction stress, and (d) factor of safety for a hypothetical steep hillslope of sandy soil.

Figure 2. Profiles of (a) pressure head, (b) effective saturation, (c) suction stress, and (d) factor of safety for a hypothetical steep hillslope of silty soil.