Dynamics of large bedrock landslides, numerical modelling and Himalayan case studies.
Abstract:
Landslides are a common phenomenon on the Earth’s surface. They come in many forms as a wide range of
environmental conditions determine the characteristics of slope failure. They are a threat to human society
and play an important role in the denudation of hillslopes and thus in the evolution of the Earth’s surface.
Factors that precondition and prepare slopes to failure are diverse and include the characteristics of the
failure material as well as external factors such as climate and seismicity. A conceptually coherent
understanding of these factors is required to better assess landslides, especially their large representatives
which occur with low frequency and activity rates, but are however critical in terms of natural hazards and
development of reliefs.
This PhD-thesis is focused on bedrock landslides, which are slope failures that occur in rock masses. In the
first part of the thesis, two subtypes of bedrock landslides located in the Annapurna Massif of central Nepal
are investigated: Giant rock avalanches (> 0,1 km3 failure volume) and slow moving Deep-Seated
Gravitational Slope Deformations (DSGSDs). Absolute dating techniques, including cosmogenic nuclide
exposure measurements (10Be and 36Cl isotopes) and 14C carbon burial dating were used to determine the
age and volumes of 3 giant rock avalanches and reconstruct the paleo-activity of a DSGSD. Our results
indicate that the giant rock avalanches occurred predominantly at the end of Holocene periods with warmer
and wetter climatic conditions, i.e. during the Early Holocene Climatic Optimum (EHCO) and the Medieval
Warm Period (MWP). This highlights the role of climatic forcing on slope failure. We also identified a higher
activity of the DSGSD at the end of the EHCO, further emphasizing the role of climatic forcing on slope
destabilization. Besides their implications for natural hazards, our results offer new perspectives on
mountain-scale erosion fluxes and landscape denudation in the region.
In the second part of the thesis, a discrete element model is used to investigate how rock strength anisotropy
affects the failure of a 1 000 m high slope with constant slope angle. After setting up the transverse isotropic
material with the mechanical characteristics of a gneiss, the whole range of possible isotropy plane
orientations with respect to the slope face is systematically explored in two dimensions (0 – 180°). Our results
indicate that if the isotropy plane is slightly less inclined than the topographic slope (i.e. cataclinal overdip
configuration), slope stability requires a material strength one order of magnitude higher than in a
configuration where the isotropy plane is perpendicular to the slope (i.e. anaclinal configuration). Moreover,
as expected from field observations, slope failure modes are directly constrained by the isotropy plane
orientation: sliding is observed for cataclinal overdip slopes, buckling for cataclinal underdip slopes, toppling
for anaclinal slopes with steeply dipping isotropy planes, and crumbling for anaclinal slopes with less steeply
dipping isotropy planes. By analysing the south-facing slopes of the Annapurna Massif (Nepal), we were able
to evidence the role of material anisotropy in landscape shaping in the area. The relative orientation of the
anisotropy with respect to the topography is an important precondition of slope failure, controlling both the
stability and the failure mode. The systematic investigation performed in this thesis contributes to slope
stability analysis in general as well as to a better understanding of landscape shaping by slope failure.
Our work highlights a diversity of critical slopes and landslide processes that depend on both internal factors
(in this case, anisotropy) and external factors (tectono-climatic context).
Thesis supervisors:
Jérôme LAVÉ et Luc SCHOLTÈS (LMV, Clermont-Ferrand)
Members of the jury :
Federico AGLIARDI Università degli Studi di Milano-Bicocca, Milan
Pascal LACROIX Laboratoire ISTerre, Grenoble
Marin CLARK University of Michigan
David AMITRANO Laboratoire ISTerre, Grenoble
Monique FORT Professeure émérite, Université Paris Cité (Paris Diderot)