The objective of this work is to present a multitechnique 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 and
terrestrial 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
sources of information. Because of an important point density at
the ground surface (4. 1 points m?2), a small laser footprint (0.09
m) and an accurate three-dimensional positioning (0.07 m), airborne
laser scanning data are adapted as a source of information to analyze
morphological structures at the surface. Seismic tomography surveys
(P-wave and S-wave velocities) may highlight the presence of low-seismic-velocity
zones that characterize the presence of dense fracture networks at
the subsurface. The surface displacements measured from the terrestrial
laser scanning data over a period of 2 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 preexisting 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 preexisting 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.