Field evidence of pore pressure diffusion in clave soils prone to landsliding

The hydrologic behavior of shallow weathered soils commonly involved in slope failure is discussed. Pore water pressure response of a clay slope to rainfall is described by using experimental field data collected by an automated monitoring system. At depths of tens of centimeters to a few meters below the ground surface, moisture and pressure sensors have recorded relatively fast responses to precipitation. Peaks in pressure head are readily associated with single rainfall events and display a highly transient nature. Pulses advance downward within the saturated domain much faster than advective fronts driven by gravity, and attenuate with depth. Although pore pressure fields associated with actual slope failure have never been measured, three years of measurements indicate that shallow fine-grained soils are subject to substantial growth of pore pressure following infiltration of rainwater from the ground surface. The observed behavior is well reproduced by a 1-D linear diffusion model whose predictions have been tested against 129 pressure head responses. When initial moisture conditions match model assumptions, the model captures the essential physics of the phenomenon and is able to simulate observations, although good results can be attained only if diffusivity is treated as a calibration parameter. For areal slope stability assessment, the application of the linear diffusion model remains difficult owing to inherent uncertainties related to the definition of the initial “steady state” hydrologic conditions and to considerable natural variability inherent in hydraulic parameters. Both have a dramatic influence on the predictive capability of the model