Dynamic evolution from shallow landslide to fluidized debris flow in a field-scale experiment

Abstract Worldwide, debris-flow mobilization from landsliding is widespread, deadly, and the focus of various warning systems. However, synchronous deformation and pore-pressure data during debris-flow fluidization events are rare. We performed a well-instrumented field-scale experiment that triggered a shallow landslide and ensuing debris flow on a natural hillslope. Following hours of intense sprinkling, slow sliding commenced, continued for ~ 1 h, and then accelerated shortly before abrupt failure. Our data revealed that during the few seconds of rapid failure, pore pressures dropped (indicative of soil dilation) then oscillated greatly, enhancing liquefaction of the mass into a debris flow. Although loose soils can collapse upon shear and abruptly mobilize into debris flows, mechanisms controlling the transition from slow motion to rapid flow are less well understood. We explored our observed slide behavior using a 1D model (Iverson, 2005) that couples slide motion and pore pressure with evolving shear-zone dilatancy. We modified this approach by having dilatant effects gradually diminish with increasing shear strain in the shear zone. Modeling results illustrate that dilatancy can promote slow motion; when these effects are exhausted, rapid motion occurs. The dynamic evolution of dilatancy enables dense soils, common on many hillslopes, to initially slide slowly and then be driven by prolonged rainfall to abruptly transform into debris flows, as was observed during our experiment.