D45 Methodology for landslide hydrological coupling - CNRS(ULP), CNRS (UFC)

Leader : Yves GUGLIELMI

Co-leaders : Frédéric CAPPA, Catherine BERTRAND, Stéphane BINET

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Methodology for landslide hydrological coupling

Abstract:

CHEMICAL SPRING WATER MEASUREMENTS COUPLED WITH 2D U.D.E.C HYDROMECHANICAL MODELLING AS AN INVESTIGATION METHODOLOGY OF WATER INFILTRATION INFLUENCE ON LARGE MOVING ROCK MASS STABILITY : APPLICATION TO THE "LA CLAPIÈRE" LANDSLIDE (FRANCE, 06).

F.Cappa (1), Y.Guglielmi (1), V.M.Soukatchoff (2), J.Mudry (3), C.Bertrand (3), A.Charmoille (3).

(1) UMR 6526 Géosciences Azur – CNRS-UNSA, (2) LAEGO – Ecole des Mines de Nancy – INPL, (3) EA 2642 Géosciences - Université de Franche-Comté.

Frederic.Cappa@geoazur.unice.fr / Fax : (+33) 04.92.94.26.10

We present an investigation method of water infiltration influence on Large Moving Rock Mass (LMRM) stability. In the case of huge unstable mountainous slopes, it has been clearly shown that the main driving of instability is gravity and that the major triggering and increasing factor is water located in interstices and fractures of rocks (Noverraz & al., 1998). More particularly, groundwater originates from a localized hydro-mechanical deformation inside fractures that can induce a generalized destabilization of large rock masses (Guglielmi, 1999). However, the understanding of groundwater mechanical effects on landslides and their neighbouring environment is rendered more complex given the large anisotropy of the rock mass as well as the difficulties to apply classic hydrogeological investigation methods in a moving environment. For these reasons, we developped an indirect investigation method based on chemical groundwater measurements coupled with a two-dimensional hydro-mechanical modelling with the Universal Distinct Element Code (UDEC) numerical program, taking the example of the La Clapière landslide (Alpes-Maritimes, France).

The methodology we develop firstly establishes a hydro-mechanical conceptual scheme through the analysis of geological, hydrogeological, hydrogeochemistry and landslide velocity measurements. Then, a two-dimensional numerical modelling with UDEC was performed to test the influence of the locations and the intensities of water infiltrations on the hydro-mechanical behaviour of La Clapière’s slope.

A geological and hydrogeological analysis reveals a perched saturated zone connected by large conducting-flow fractures to a basal aquifer. The correlations of spring water chemistry data and meteorological events on the slope highlight a large variability of groundwater transits in the slope in time (transit durations of 1 to 21 days) and in space. Infiltration transients correlate with landslide accelerations. Infiltration yields range between 0.4 and 0.8 l.s-1 . The most intensive hydro-mechanical response of the landslide is linked to snowmelt in a stable area in the upper part of the slope located between 1800 and a 2500 m high.

On the one hand modeling hydro-mechanical effects with UDEC considers a model corresponding to a slope without any unstable zone, and on the other hand, a model including a failure surface in order to simulate the current instability. In the two numerical tests, calculations show that the most unstabilizing water infiltration corresponds to water infiltrations located in the middle part of the slope for weak flow rates of 0.75 l.s^-1. This is due to the water infiltration influence on the spatial distribution of strain fields. This result fits with field measurements.

This methodology can easily be applied to the monitoring of landslide movements. As it gives relevant information on the spatial and temporal effects of various meteoric infiltrations, it can be applied to improve remedial protocols. This work was partly funded by the French National Program on Natural Hazards (PNRN) and Retina European Program.

§ Guglielmi Y., 1999. Apport de la mesure des couplages hydromécaniques à la connaissance hydrogéologique des réservoirs fissurés. Habilitation à diriger des recherches, Université de Franche-Comté, E.A. 2642 Géosciences : Déformation, Écoulement, Transfert. 187 p.

§ Noverraz F., BonnardC., Dupraz H., and Huguenin L., 1998. Grands glissements de versants et climat. Rapport final PNR 31, vdf hochschulverlag AG an der ETH Zürich, 314 p.

Publications:

Cappa F., Guglielmi Y., Merrien-Soukatchoff V., Mudry J., Bertrand C., Charmoille A. (submitted) - Chemical spring water measurements coupled with 2D hydromechanical modelling as a preliminary investigation method of water infiltration influence on a large moving rock mass stability: example of La Clapière landslide (France, 06). (submitted to Journal of Hydrology – 2002, December)

Cappa F., Guglielmi Y., Merrien-Soukatchoff V., Mudry J., Bertrand C., Charmoille A., 2003 - Chemical spring water measurements coupled with 2D U.D.E.C hydromechanical modelling as an investigation methodology of water infiltration influence on a large moving rock mass stability: application to the "La Clapière" landslide (France, 06). EGS-AGU-EUG Joint Assembly, Nice, France, 06-11 April 2003.