Control of landslide retrogression by discontinuities: evidences by the integration of airborne- and ground-based geophysical information

Abstract : The objective of this work is to present a multi-technique approach to define the geometry, the kinematics and the failure mechanism of a retrogressive large landslide (upper part of the La Valette landslide, South French Alps) by the combination of airborne (ALS) and terrestrial (TLS) laser scanning data and ground-based seismic tomography data. The advantage of combining different methods is to constrain the geometrical and failure mechanism models by integrating different source of information. Because of an important point density at the ground surface (4. 1 pt.m-224 ), a small laser footprint (0.09 m) and an accurate 3D positioning (0.07 m), ALS data are adapted source of information to analyze morphological structures at the surface. velocities) may highlight the presence of low seismic-velocity presence of dense fracture networks at the sub-surface. The surface displacements measured from TLS data over a period of two years (May 2008-May 2010) allow one to quantify the landslide activity at the direct vicinity of the identified discontinuities. An important subsidence of the crown area with an average subsidence rate of 3.07 m.year-1 is determined. The displacement directions indicate that the retrogression is controlled structurally by the pre-existing discontinuities. A conceptual structural model is proposed to explain the failure mechanism and the retrogressive evolution of the main scarp. Uphill, the crown area is affected by planar sliding included in a deeper wedge failure system constrained by two pre-existing fractures. Downhill, the landslide body acts as a buttress for the upper part. Consequently, the progression of the landslide body downhill allows the development of dip-slope failures and coherent blocks start sliding along planar discontinuities. The volume of the failed mass in the crown area is estimated at 500,000 m3 with the Sloping Local Base Level method.
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Julien Travelletti, Jean-Philippe Malet, Kevin Samyn, Gilles Grandjean, Michel Jaboyedoff. Control of landslide retrogression by discontinuities: evidences by the integration of airborne- and ground-based geophysical information. Landslides, Springer Verlag, 2013, 10 (1), pp.37-54. 〈10.1007/s10346-011-0310-8〉. 〈hal-00649682〉

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