The Portillo Rock Avalanche is a prehistoric rockslide (Upper Pleistocene) located on the eastern border of an Eocene-Oligocene volcano-tectonic basin, on the Chilean side of the Andean Cordillera. The primary source area of the rockslide is along the Caracoles range, and involves a series of volcano-clastic beds dipping approximately 55ºW. Current estimates of the thickness of the slide mass range from 10 to 50 m, with a total volume of 65 million m3. Five different geological models were constructed and tested for their stability response to varying rock mass and discontinuity strength parameters. Elasto-plastic modeling results showed that the most probable failure mechanism was one involving sliding and shearing along the bedding planes together with brittle fracturing and shearing through the toe. The distribution of plasticity indicators provided key insights into the kinematics and possibly staged nature of the Portillo Rock Avalanche that will be used in later stages of the study to assess the hazard potential of a reoccurring rockslide.
Analysis of massive rock slope failure and subsequent motion of rapid rock avalanches is a challenging task given the difficulty in reproducing the processes and circumstances under which failure had occurred. This task is even more complex when dealing with prehistoric rock avalanches due to the need of landscape reconstruction and knowledge of pre-failure conditions. In most cases, only approximations of reality can be provided, based on judgment and experience, due to geologic uncertainty and the inherently variable nature of rock. In the rugged slopes of the Andean Cordillera of Central Chile a number of large lobate-shaped diamicton deposits have been misidentified as glacial deposits. The misidentification occurred because of similarities between the partly preserved geomorphic surfaces of landslide deposits and glacial moraines, and the detrital characteristics of both types of material.