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A large-scale, stepped-plane failure analysis in the Central Andes uplands, Chile, using roughness profiles from terrestrial laser scanning.
Simposio; International Symposium of landslides.; 2012
Large volume (>106 m3) rock slides and rock avalanches are common in the highlands of the Andes of central Chile and Argentina. Most of them have failed with a massive failure mode. A stability analysis for such events need rock mass strength assessment or complex modelling of rock properties and sliding surface geometries. A distinct 0.7 km2 rock slide with a clear plane failure mode on two major steps is found in the headers of the Maipo river drainage basin at the latitude of Santiago (33ºS), : This is one of a larger cluster of landslides and rock avalanches deposits known as La Engorda landslide complex of Holocene age. The failure surfaces were scanned with a terrestrial Lidar, from which 35 high-resolution dip-slope topographic profiles were obtained. This information allowed computation of joint roughness of the shear surfaces, which were used along with Schmidt hammer measurements and topographic reconstruction of the slope palaeotopography to perform limit equilibrium back-analyses. The analyses were carried out using plane failure and slices methods. The shear strength of the sliding surface was modelled using the Barton´s failure criteria, after statistic analysis of JRC, JCS and residual friction angle data. Deterministic and probabilistic analyses confirm that the block would be safe under static conditions, suggesting a seismic trigger. Pseudo-static analyses show that horizontal seismic coefficients between 0.4 and 0.6 would be necessary to induce the failure, depending on the amount of vertical acceleration. These results are in agreement with the presence of faults and shallow seismicity in the area and a historic shallow crustal earthquake with peak acceleration estimates around 1 g. The results along with observations that subduction earthquakes do not produce such large accelerations in the area reinforce the hypothesis of crustal seismicity as a key factor for large landslide generation in the Andes highlands.