Elsevier

Geomorphology

Volume 109, Issues 3–4, 15 August 2009, Pages 86-93
Geomorphology

Numerical modelling of the gravity-induced destabilization of a slope: The example of the La Clapière landslide, southern France

https://doi.org/10.1016/j.geomorph.2009.02.025Get rights and content

Abstract

A finite difference two-dimensional model with Hooke–Mohr–Coulomb properties and topography derived from the DEM are used to reproduce the La Clapière landslide. The principal factor defining the gravity-driven destabilization of the model is a gradual reduction in the cohesion. This reduction simulates a degradation of the material properties with time because of weathering/alteration processes. The inelastic deformation, fracturing, and faulting first occur at mountain scale and results in normal fault formation causing crest sagging. Later, the failure process is concentrated in the lower part of the slope and leads to the formation of a localized fault subparallel to the slope surface at a depth of ca. 100 m. This corresponds to the initiation of the La Clapière landslide and its propagation upslope. A slow crest sagging continues during the whole model evolution.

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.

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