Transient hydrological response of weathered clay shales and its implication for slope instability

Earthflows are very common in northern Apennines of Italy. They develop along slopes made of clayshales and are subject to periodic reactivations consisting of retrogressive failures in the alimentation zone which cause partial or complete mobilization of the earthflow deposit. A monitoring system aimed to measure pore pressures at various depth across the headscarp of a large earthflow was installed in summer ’04. Data collected so far give a consistent picture of the subsurface flow. During the wet season the soil is saturated at shallow depths (1-2 m), and rainfalls cause rapid pore water pressure growth: pore pressures increase sharply few hours after the onset of the rainfall then decrease smoothly in the following hours or days. Single rainfall episodes produce appreciable responses only in the weathered clayshales horizon. At greater depth, within intact clayshales, the gradients are oriented downward year around and it is difficult to relate measured pore pressures to climatic forcing. The back-analysis of a first-time failure illustrate two alternative failure mechanisms and their sensitivity to strength and pore pressure parameter variations. The adoption of the measured pore water pressure distribution implies a mainly translational failure developing within weathered clay shales and mobilizing fully softened strength parameters.