Hydromechanical modeling of a large moving rock slope inferred from slope levelling coupled to spring long-term hydrochemical monitoring: Example of the La Clapie're landslide (Southern Alps, France)

Taking the example of the La Clapiere landslide, the influence of water infiltration on large moving rock mass stability is investigated. Based on the analysis of geological, hydrogeological, hydrogeochemstry and landslide velocity measurements, a hydromechanical conceptual model is proposed. Then, a two-dimensional numerical modeling with the Universal Distinct Element Code (UDEC) was carried out to determine the influence of the location and the amount of water infiltration on the hydromechanical behaviour of La Clapiere slope. Geological and hydrogeological analyses indicate a perched water-saturated zone connected by large conducting-flow fractures to a basal aquifer. The comparisons of spring water chemistry data and meteorological data from the slope area show a large variability of groundwater transits in the slope through time (transit durations of 1-21 days) and space. Water infiltration transient signals correspond to accelerations of the slope downward motion. Infiltration rates are comprised between 0.4 and 0.81 s(-1). The most pronounced hydromechanical response of the slope instability is due to snowmelting in the stable area located between elevations 1800 and 2500 m above the unstable slope. The hydromechanical modeling performed with the UDEC code concerns firstly a model of a slope without any unstable zone, and, secondly, a model including a failure surface in order to simulate the current instability. Numerical computations are done in order to localize the area through which water infiltration is the most destabilizing. The most destabilizing area is the one that has the largest influence on the spatial distribution of strain fields. It corresponds to water infiltration located in the middle part of the slope and characterized by weak flow rates of 0.751 s(-1). This approach can easily be applied to the monitoring of other unstable rocky slopes. As it gives relevant information about the spatial and temporal effects of meteoric infiltration, it can be applied to improve remedial protocols. (C) 2004 Elsevier B.V. All rights reserved.