Rainfall and its consequences are usually considered as the major factor triggering slope instabilities within clay deposits. The link between rainfall, water infiltration and landslide activity may be complex, and comprehensive sets of data are still necessary to understand how water infiltrates in cohesive material. On unstable slopes made of fine-grained sediments, gravitational deformation generates superficial shear ruptures and tension fissures whose continuous opening could be amplified by shrinkage during dry periods. This fissure network at the surface can be very dense and strongly controls the water infiltration process. However, it remains difficult to assess the fissure evolution at depth and its eventual relationship with shallow slip surfaces. This work presents the results of a geophysical study which aimed to characterize the fissures that are observed along the unstable clay slopes of the Trièves area (French Alps). The site was first investigated combining geomorphological analysis, drilling, borehole logging, geotechnical tests and geophysical experiments. This investigation evidenced the presence of at least three rupture surfaces (at 5, 10–15 and 42 m) with a water flow at the second one. In a second step, permanent instruments were installed (piezometers and soil water content probes). Results indicate high water infiltration velocities, down to the water table located at 2·5 m depth, that are likely to be linked with sub-vertical preferential paths, such as fissures. Geophysical parameters (electrical resistivity, S-wave velocities and Rayleigh waves attenuation) were monitored. Geophysical results suggest that fissures are permanently open, that they serve as preferential infiltration paths, and that they can reach, at least, a depth of 2 m. The methods employed in this research put forward that the system of imbricated fissures drains water from the surface down to the shallow slip surfaces at 5 and 10–15 m. This work highlights the role of fissures network in water infiltration in the numerous clay landslides of the Trièves area. Copyright © 2011 John Wiley & Sons, Ltd.