Pore water pressures and hydraulic conductivity in the slip zone of a clayey earthflow: Experimentation and modelling.

Rainfall is among the most relevant triggering factors of landslides. With the aim of defining a model of the influence of rainfall on deep landslides, this paper presents data of a long-term monitoring of pore water pressures in a clayey earthflow, and the results of many field permeability tests carried out with several procedures. Pore water pressures are being monitored in the Costa della Gaveta slope since 2005. In the last years, new instruments have been installed and permeability tests have been carried out in the stable formation, in the landslide body and in the slip zone. The hydraulic conductivities of the three different zones of the subsoil have been evaluated by two types of field tests: falling head tests (by Casagrande piezometers and test wells), and localized constant head tests (by a permeameter ad hoc designed). The experimental data show that: a) pore water pressures along the slip zone respond to rain more than in the landslide body and in the stable formation, even at about 25 m depth; b) the hydraulic conductivity k sz of the slip zone is much higher than that of the landslide body, k l , in turn higher than the hydraulic conductivity of the stable formation, k f. The numerical 3D modelling of steady-state reference conditions and of transient processes caused by the historical rainfall series shows that the occurrence of k sz higher than k l and k f determines an overall drainage effect which makes the global limit equilibrium safety factor SF greater than that evaluable for other possible conditions. On the other side, even the lowest measured hydraulic conductivity k sz provides seasonal rain effects on water pressures along the slip surface such to determine significant SF variations in phase with the observed seasonal displacement rates. • Pore water pressures along the slip zone of a clayey earthflow respond to rain more than in the landslide body and in the stable formation. • The hydraulic conductivity k sz of the slip zone is much higher than those of the landslide body and of the stable formation. • High k sz causes an overall drainage effect and seasonal changes in pore water pressures along the slip zone influencing SF and displacement rates. [ABSTRACT FROM AUTHOR]