Numerical modelling of the gravity-induced destabilization of a slope: The example of the La Clapière landslide, southern France
Introduction
Gravitational instability of topography results from the interplay of different processes. The most important seems to be the weathering and alteration caused by climatic factors and fluid circulation within the massif and dependent on the physicochemical and mechanical properties of the rock (Hill and Rosenbaum, 1998, Hall and André, 2001, Pellegrino and Prestininzi, 2007). Both weathering and alteration cause a progressive time-softening (strength reduction) of the superficial horizons. Although the kinematics of these processes and their variation with depth are poorly studied, the softening is generally maximal at the surface and diminishes with depth (Chigira, 2001, Maréchal et al., 2003) where it is concentrated along the fractures and faults (Migon and Lidmar-Bergstroem, 2002, Wyns, 2002).
Ignoring the influence of other factors (e.g., large spatial and temporal scale tectonic processes) on the slope evolution, we assume the following conceptual model: the initially homogeneous and stable mountain is subject to a progressive reduction of effective strength because of rock weathering and alteration. Ultimately, this mountain should undergo inelastic, gravity-driven deformation (damage) and macrofracturing increasing with time. The aim of the present work is to model this deformation numerically and compare the results with field data. Lack of similarity would mean that the conceptual model is too simplistic and that in reality other factors (e.g., structural heterogeneities, tectonic stresses, and more complex mechanical properties) have a dominant role in controlling the gravity-induced deformation.
The well-studied La Clapière landslide (Follacci, 1987, Follacci, 1999, Ivaldi et al., 1991, Guglielmi et al., 2002, Casson et al., 2005, Lebourg et al., 2005, Jomard, 2006) located in the southern French Alps (Argentera–Mercantour massif) was chosen as a natural example (Fig. 1). The numerical models well reproduce this superficial landslide (its depth, size, and along-slope position) and also reveal slower inelastic deformation (normal faulting) at a larger scale involving the whole mountain and resulting in crest sagging.
Section snippets
Geological framework
The La Clapière slope (Fig. 1, Fig. 2A) is situated in the Tinée valley, which represents the north-western edge of the Argentera–Mercantour metamorphic unit (southern French Alps). The region underwent polyphased tectonic deformations during Variscan and Alpine orogenesis (Follacci, 1999). The eastern side of the valley is mainly made of weathered metamorphic units characterized by a N. 150–60° E. foliation (average trend) (Bogdanoff, 1986, Gunzburger and Laumonier, 2002) with the dip varying
Numerical modelling
Accurate numerical simulation of gravitational instability (as of any other physical instability) is a delicate exercise. It requires application of a “time-marching” explicit solution scheme. Such a scheme is implemented in the dynamic, finite-difference calculation code FLAC3D. This code also uses mixed-discretization zoning technique that is believed to ensure accurate modelling of plastic collapse loads and plastic flow (Marti and Cundall, 1982). Therefore FLAC3D has been chosen for the
Discussion and conclusions
The simple and robust constitutive model without strain softening or hardening and with homogeneous reduction in cohesion with time has been chosen to define the first-order deformation pattern of the gravitationally destabilizing slope. As expected, this process is affected by both the slope material properties and the topography. For the topography of the La Clapière slope, the most realistic results correspond to an internal friction angle ϕ of about 30°. For this value, the maximal depth of
Acknowledgments
The authors thank G. Sanchez and Y. Gunzburger for providing the photos and the landslide and to the anonymous reviewers for the useful comments.
References (37)
- et al.
Large sackung along major tectonic features in Central Italian Alps
Eng. Geol.
(2006) Micro-sheeting of granite and its relationship with landsliding especially after the heavy rainstorm in June 1999, Hiroshima prefecture
Jpn. Eng. Geol.
(2001)- et al.
Coupling between hydrogeology and deformation of mountainous rock slopes: insights from La Clapière area (southern Alps, France)
C. R. Geosciences
(2005) - et al.
Origine tectonique du pli supportant le glissement de terrain de la Clapière (NordOuest du massif de l'Argentera–Mercantour, Alpes du Sud, France) d'après l'analyse de la fracturation
C. R. Geosciences
(2002) - et al.
Influence of daily surface temperature fluctuations on rock slope stability: case study of the Rochers de Valabres slope (France)
Int. J. Rock Mech. Min. Sci.
(2005) - et al.
Modelling brittle failure of rock
Int. J. Rock Mech. Min. Sci.
(2002) - et al.
New insights into rock weathering from high-frequency rock temperature data: an Antarctic study of weathering by thermal stress
Geomorphology
(2001) - et al.
Examples of multiple rock-slope collapses from Köfels (Ötz valley, Austria) and western Norway
Eng. Geol.
(2006) - et al.
Practical estimates of rock mass strength
Int. J. Rock Mech. Min. Sci.
(1997) - et al.
Anisotropie verticale de la perméabilité de l'horizon fissuré des aquifères de socle: concordance avec la structure géologique des profils d'altération
C. R. Geosciences
(2003)