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This study describes transient hydrogeological and geomechanical models realized jointly in 2006 by the Geolep office (Géologie de l’Ecole Polytechnique) of the Polytechnical School of Lausanne (EPFL) and GeoMod SA within the framework of the stabilization work of the La Frasse landslide (Vaud, Switzerland). These models evaluate the impact of a deep drainage gallery with subvertical drainage boreholes towards the surface in terms of reduction of the deformation velocities and increase of the landslide’s factor of safety. The paper presents the effect of different pipe spacing of the drainage system. Considering the local heterogeneities, the results show that a mean spacing between the drainage boreholes of the order of 10 m is sufficient to control the temporal head fluctuations between the wells within a range of a few metres. Moreover, this solution induces a strong diminution of the predicted displacements during a specific crisis, from 101 cm for the model without drainage to around 14 cm for the drained model, and a significant gain of stability (from a factor of safety (FoS) of 1.05 to 1.30).

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