Observatoire Gravitaire Géoazur-en Observatoire Gravitaire Géoazur-fr
[Observatoire Gravitaire Géoazur] Modeling

- Numerical modelling of the gravity-induced destabilization of a slope: The example of the La Clapière landslide, southern France
A.I. Chemenda, T. Bois, S. Bouissou and E. Tric

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.

Keywords: Rock mass deformation; Sagging; La Clapière landslide; Weathering; Numerical modelling
Geomorphology, Volume 109, Issues 3-4, 15 August 2009, Pages 86-93, doi:10.1016/j.geomorph.2009.02.025

- Analysis of massif fracturing during Deep-Seated Gravitational Slope Deformation by physical and numerical modeling
D. Bachmann, S. Bouissou and A.I. Chemenda

Both 3-D physical modeling and 2-D numerical modeling were used to analyze the development of Deep-Seated Gravitational Slope Deformations. We focused especially on the link between fracturing and morphological features such as Sagging/Sackung. Physical modeling was performed by using properly scaled analogue materials as well as an original vertical accelerator device. This device enables cyclic loading of the model, and to advance step by step in the deformation process. This technique is thus particularly well suited to analyze rupture initiation and evolution. Surface deformation is registered with a high resolution digital camera. Internal model deformation was studied by making cross-sections at different experiment stages. The failure of one or both mountain sides was observed, with a deep-seated failure zone that follows an almost circular trajectory. It intersects the surface perpendicularly in the upper part. Other fractures are generated in the moving mass in response to its motion. Fracture network becomes wider and morphological features such as counterscarps are generated. Numerical models were performed with a 2-D finite element code named Adeli. We tried to reproduce physical modeling results by using the same mechanical behaviour and parameters of the analogue material. The results reproduced failure initiation well, but poorly described moving mass deformation. Furthermore rupture initiation in the 2-D numerical models occurs more easily (e.g. for a lower acceleration value) than in the 3-D physical models, confirming the importance of the three dimensional analysis.

Keywords: Deep-Seated Gravitational Slope Deformation; Sagging; Sackung; Fracture; Physical modeling
Geomorphology, Volume 103, Issue 1, 1 January 2009, Pages 130-135, doi:10.1016/j.geomorph.2007.09.018

- Influence of major inherited faults zones on gravitational slope deformation: A two-dimensional physical modelling of the La Clapière area (Southern French Alps)
T. Bois, S. Bouissou and Y. Guglielmi

Inherited faults are known to influence rock slope stability and gravitational deformation. In spite of that, in many studies few faults are identified in field and properly used in models of gravitational slope deformation. The aim of this work is to study the influence of inherited faults zone density and geometry on gravitational failure processes at the massif scale using a physical modelling technique which satisfies the similarity criteria. Models are scaled to the well-documented natural example of La Clapière in the Southern French Alps. Experiments were conducted using mechanically homogeneous material with variable fault geometry. In each of tested configurations, the mobilized volume was almost the same. Results confirmed the hypothesis that the La Clapière landslide is a shallow section of a deep-seated gravitational slope deformation. Furthermore, among the various configurations tested, only one is enable to reproduce the observed superficial deformation on the La Clapière hillside. This result demonstrated that the geometry of the faults at depth plays a major role on the style of gravitational deformation patterns. Regarding the particular case of La Clapière, our results give new insights on the shape of the faults affecting the massif at depth. In particular, normal listric faults seem to have shallow inflexions compared to a deep-seated thrust fault that was either gravitationally formed or pre-existing but inactive (i.e. sealed) before slope destabilization and then gravitationally reactivated.

Keywords: Deep-Seated Gravitational Deformation; Sackungen; Sackung; Deep-seated failure; Landslide; La Clapière; Structural heterogeneities; Listric faults; Thrust fault; Physical modelling
Earth and Planetary Science Letters, Volume 272, Issues 3-4, 15 August 2008, Pages 709-719, doi:10.1016/j.epsl.2008.06.006

- Hydromechanical modeling of a large moving rock slope inferred from slope levelling coupled to spring long-term hydrochemical monitoring: example of the La Clapière landslide (Southern Alps, France)
F. Cappa, Y. Guglielmi, V. M. Soukatchoff, J. Mudry, C. Bertrand and A. Charmoille

Taking the example of the La Clapière landslide, the influence of water infiltration on large moving rock mass stability is investigated. Based on the analysis of geological, hydrogeological, hydrogeochemistry 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 Clapière 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 0.8 l 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 0.75 l 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.

Keywords: Large moving rock mass; Groundwater chemistry; Coupled hydromechanical modeling; Universal Distinct Element Code
Journal of Hydrology, Volume 291, Issues 1-2, 31 May 2004, Pages 67-90, doi:10.1016/j.jhydrol.2003.12.013