Pore water pressures and strembank stability: results from a monitoring site on the Sieve River, Italy

To investigate the role of pore water pressure in the stability of a streambank, a series of tensiometers and piezometers was installed in a bank of the Sieve river, Tuscany, Italy. Fluctuations in matric suction measured at the tensiometers reflected the overall response of pore water pressures to rainfall, evapotranspiration, rising and drawdown of the river stage and variations in water table. During low-flow periods, the shear strength term due to the matric suction allows the bank to remain stable at a steep angle.However, during rainfall and flow events, reduction of matric suction and increase in unit weight of the material form vertical and lateral infiltration may be sufficient to trigger a mass failure, without development of significant positive pore water pressures.