Pore water pressures and hydraulic conductivity and in a clayey earthflow: experimental data in the landslide body, in the slip band and in the stable soil

The Costa della Gaveta earthflow occurs in a structurally complex clayey formation of the Italian southern Apennines. The slip band detected by inclinometers reaches 40 m depth. Sliding localized on a thin slip zone, under conditions of constant soil discharge, is the current prevailing mechanism of movement. Over the last 15 years, the displacements have occurred at almost constant average yearly rate, with seasonal variations ascribable to the hydrologic conditions. The total stress being constant, rain can influence displacements by an influence on pore water pressures. Pore water pressures were monitored by Casagrande piezometers and vibrating wire cells in three different zones of the subsoil: stable formation, landslide body and slip band. The experimental data show that along the shear band the response of pore water pressures to the site hydrological conditions is far faster than in the landslide body and in the stable formation. Furthermore, the deep displacement rates of all the inclinometers crossing the slip surface seem to be strictly correlated to pore water pressures in the slip band. To understand why and how this occurs, the hydraulic conductivities of the three different parts of the subsoil were evaluated by the analysis of two types of field tests: falling head tests in the Casagrande piezometers, and localized seepage measurements in test wells equipped to this aim. The experimental data show that in the slip band, the hydraulic conductivity is much higher than that in the landslide body which, in turn, is higher than that of the stable formation. Piezometric measurements support the hypothesis that such permeable zone is connected to the atmosphere.