Rainfall induced instabilities: a field experiment on a silty sand slope in northern Switzerland

Two full scale field tests were planned and performed successfully on a steep forested slope located on the east facing banks of river Rhine in Ruedlingen, in canton Schaffhausen, northern Switzerland. The aim of the experiments was to study the triggering mechanisms of the landslides due to rainfall. Intensive field investigations were carried out, including in-situ geotechnical tests, characterisation of hydrological properties of the soil and reinforcing effects of vegetation, geological and hydrogeological mapping, and subsurface investigations by means of geophysical methods. Additionally, several series of saturated and unsaturated laboratory tests were conducted on undisturbed and disturbed samples taken from different depths from the vicinity of the selected slope. The test site was intensively instrumented and monitored over a period of 6 months in the course of artificial rainfall and natural precipitation. The instrumentation includes conventional and novel methods to measure pore water pressure, volumetric water content, piezometric height, soil pressure, acoustic emissions, surface and subsurface movements, soil temperature, and meteorological data. This paper introduces briefly the measurements and findings of this multi-disciplinary project. This paper focuses on numerical and analytical methods used to explain the behaviour of a marginally stable slope before, and during the failure induced by rainfall. Simple stability calculations are described that still offer realistic predictions of the status of a slope prone to failure due to increase of the pore water pressure. The basal and lateral reinforcing effects of vegetation and unsaturated shear strength of soil are introduced in these two and three dimensional simulations.