Preliminary results from pore pressure monitoring on an unstable clay slope.

We monitored positive pore water pressure in a clay slope in the Apennines to investigate how rainfall influences the pore pressure distribution along the soil profile and, consequently, affects slope stability conditions. Data refers to the first season of monitoring activities (August 2001–June 2002). The study site is located at the head of a complex landslide, where incipient shallow failures are visible along the slope. The preliminary analysis of pressure data allowed to point out consistent trends, which manifested throughout 11 months. During the cold and wet season, saturation always extends close to the ground surface ( < 1 m) and pore water pressure measurements indicate a strong downward component of the flow, which is largely dominant over the downslope component, in any condition. Response to rainfalls at shallow depth (1 to 3 m) generally consists in a pressure pulse whose delay is relatively short if compared to low permeability of the soil and whose nature appears clearly transient. Although no failure occurred along the slope during the observation period, it is very likely such transient response would mimic the pore water pressure pattern capable to cause sliding. The presence of prefertial flow path in the sub-surface can only explain anomalous response of some sensors or substantial differences between buried at similar depths.