GAINING UNDERSTANDING OF LANDSLIDES BY MONITORING (Invited Presentation)

The emergence of non-contact monitoring techniques such as SAR, TLS, or digital cameras has facilitated moving from point-based to spatially distributed observation. The analysis of the landslide kinematics, particularly of the rate of displacement and the deformation pattern, has benefited from these new tools.

The interpretation of the monitoring results however is not always straightforward. Understanding of the landslide behavior requires working with reliable geological models of the slope. In this contribution we present examples of instrumented landslides in the Pyrenean range. They illustrate the contribution of monitoring to the definition of the geological model and to the calibration of the simulations performed. They also evidence how detailed geological observations are fundamental for the correct interpretation of the instrumental data and for understanding landslide mechanisms. Vallcebre is a translational landslide composed of claystones and shales, whose fast response to rainfall may be modelled by integrating the observed geomorphic features into the hydrogeological model. The relationship between the groundwater table changes and the displacements also revealed a rate-dependent behavior of the landslide that otherwise would have remained unnoticed.

The onset of instability at the right abutment during the heightening of the Yesa dam demanded the initial geological model and the interpretation of the monitoring results to be revisited. Surface deformations monitored with an extensive array of geodetic targets combined with a network of inclinometers, provided evidences of the development of several superimposed failure surfaces. Their geometry was deciphered by the interpretation of the architecture of the sedimentary formation which composes the slope, the analysis of newly formed and inherited shear surfaces and the quality of the borehole cores. Based on the results and the examination of the 3D displacement field, the failure mechanism was redefined. Finally, in the Canillo landslide, radar techniques (GB-SAR and DInSAR) combined with conventional instrumentation yield consistent measurements of the landslide displacements. The greater spatial coverage of the radar measurements allowed a more representative interpretation of the behavior of the slope.