granular flow in a numerical model. Unique to this study is an original relationship between soil
moisture and the inertial number for soil particles. Our numerical model can be applied to
arbitrary soil slab profile configurations and to the analysis of natural disasters, such as
mudslides, glacier creeping, avalanches, landslips and other pyroclastic flows. Here we focus
on mudslides.
We examine the effects of bed slope and soil slab thickness, soil layered profile
configuration, soil moisture content, basal sliding and the growth of vegetation, and show that
increased soil moisture enhances instability primarily by decreasing soil strength, together with
increasing loading. Moreover, clay soils generally require a smaller relative saturation than sandy
soils for sliding to commence. For a stable configuration, such as a small slope and/or dry soil,
the basal sliding is absorbed if the perturbation magnitude is small. However, large perturbations
can trigger significant scale mudslides by liquefying the soil slab.
The role of vegetation depends on the wet soil thickness and the spacing between vegetation
roots. The thinner the saturated soil layer, the slower the flow, giving the vegetation additional
time to extract soil moisture and slow down the flow. By analyzing the effect of the root system
on the stress distribution, we show that closer tree spacing increases the drag effects on the
velocity field, provided that the root system is deeper than the shearing zone.
Finally, we investigated a two-layer soil profile, namely sand above clay. A significant stress
jump occurs at the interface of the two media.
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