Observation and analysis of pore pressure fluctuations in a thin colluvium landslide complex near Cincinnati, Ohio

Field observations in a thin colluvium landslide complex along the Ohio River near Cincinnati, show that measurable, late winter and spring pore water pressure increases occur when several centimetres of rain fall over a period of one or more days. Average pre-storm fillable porosity of 9% suggests that the colluvium was near saturation during the period of record. Dimensional analysis of flow through a sloping homogeneous layer of uniform thickness shows that short-term pore pressure fluctuations in very nearly saturated soils can be approximated by a one-dimensional linear diffusion equation. The concept of slope-parallel and slope-normal characteristic times is also introduced to determine the conditions under which slope-parallel and slope-normal flow will predominate. Analytical solutions to the diffusion equation are then used to investigate the effects of storm frequency and magnitude on pore pressure fluctuations. Comparison of solutions for layers of finite and infinite thickness shows that an impermeable substrate nearly doubles pore pressures along the substrate for storms of short duration; however, the effect of an impervious boundary vanishes with increasing duration. Finally, measured rainfall values are incorporated into a finite-difference simulation of the observed pore pressures, with good results.