Impact of the wetting process on the hydro-mechanical behavior of unsaturated residual soil from flysch rock mass: preliminary results

Abstract

Slope instabilities in the form of shallow and deep-seated landslides in flysch formations throughout Europe typically occur after prolonged periods of heavy rainfall. The Rjecina River Valley, Croatia, is characterized by the presence of flysch material in the lower part of the Valley, where numerous historical and recent landslides have occurred. The weathering process and climate conditions result in a complex engineering geological profile of flysch slopes in the Valley, with unsaturated residual soil covering the slopes. To investigate the behavior of residual soil existing on the flysch slope under increasing water content due to the rainfall infiltration process, undisturbed soil samples collected at natural water content were tested in the modified direct shear apparatus. Under imposed stress conditions, samples of low hydraulic conductivity were subjected to a prolonged wetting process simulating the rainfall infiltration process in the field. The obtained results suggest that a gradual decrease of matric suction and an increase of water content resulted in an increase of displacement rates under constant shear stress, which was interpreted as a failure of samples in partially saturated conditions. A unique shear strength envelope expressed in terms of Bishop’s effective stress equation was found to be able to predict stress conditions at the slip surface at the time of failure, while the relationship between measured matric suction and water content closely matched with the main wetting curve. Although the testing results did not point out any special characteristics of residual soil from flysch rock mass behavior, the data about hydro-mechanical behavior of unsaturated residual soil from flysch rock mass, as well as similar fine-grained soils, are very rare and presented results would be valuable for further research. The presented testing procedure and obtained results are useful for studies of rainfall-induced landslides triggered in fine-grained soil materials in zones above the phreatic line, such as shallow landslides occurring in natural flysch slopes or in physical landslide models built in laboratories.