HR: 1340h
AN: H43D-0530    [Abstracts]
TI: Coupling Between Hydrogeology And Progressive Failure Of Mountainous Rock Slopes: Field And Modelling Results From La Clapiere Valley (Southern Alps, France)
AU: Guglielmi, Y
EM: guglielmi@geoazur.unice.fr
AF: Geosciences Azur, CNRS-UNSA-IRD-UPMC, Sophia-Antipolis, 06 560 France
AU: * Cappa, F
H43D-0530 AF: Geosciences Azur, CNRS-UNSA-IRD-UPMC, Sophia-Antipolis, 06 560 France
AB: Hydromechanical effects of water flow within fractures are predominant effects that induce strength decrease of rock slopes and progressive failure propagation. A multi-parametric approach was conducted on the 70 km long La Clapière valley (Southern French Alps) consisting in mapping geology, hydrogeology and gravitational features, dating gravitational scarps and in monitoring slope springs yields, water chemistry and slope deformation for more than 10 years. First a hydromechanical model of rock slope behaviour was established and compared to bibliography. Second, taking this model as a reference, relationships between slope failure and hydrogeology were parametrically investigated using the two-dimensional distinct element method program UDEC. Rock slopes general structure consists in a superficial weathered zone (a few hundreds of meters thick) overlapping a deep intact zone. Penetrative discontinuities cut both zones. In the weathered zone tensile cracks scatter from the middle to the top of the slope. Large landslides are located at the slope foot. A perched saturated-with-water zone nested within the cracks is drained towards the slope foot through the landslides. Annual precipitation infiltrations induce hydromechanical effects that participate to the rock strength decrease through tilting and diffuse shear plane development. Such progressive failure propagation lasts over thousands of years (10 000 years in the studied area). Numerical study shows that, at the early stage of slope alteration, hydrostatic pressures are concentrated in tensile features of the upper part of the slope for moderate infiltration yield (mean inter-annual value). Pressure increase induces fracture shear dilation and traction opening that in reverse modifies flow paths and pressure. Consequence is tilting of rock columns with progressive diffuse failure at the columns' foot. A thick high porosity and high permeability weathered layer (up to hundreds meters thick) is generated between the theoretical bottom of the perched aquifer and the slope surface. When the strength of the weathered layer is low enough, state of principal stresses is very low and the entire layer is in tensile stress. Because rock porosity and permeability became very high, interstitial pressure variations are low. To the contrary, the saturated volume of rock is very high and seasonal precipitation recharge of the perched aquifer induces large density variations. At this late evolution stage, rock remains un-failed only in sparse volumes that are located at intersections between major discontinuous surfaces and even moderate recharge can induce failure of those residual volumes and large landslides triggering.
DE: 1822 Geomechanics
DE: 1828 Groundwater hydraulics
DE: 1847 Modeling
DE: 1857 Reservoirs (surface)
DE: 1895 Instruments and techniques: monitoring
SC: Hydrology [H]
MN: Fall Meeting 2005