ROCK FALL HAZARDS EVALUATION AT CERRO CABEZA DE AGUILA, MEXICO CITY

Cerro Cabeza de Aguila is part of the Sierra de Guadalupe, a volcanic range in northern Mexico City. At the foot skirts of this range a large amount of people has settled during the last decades. This has resulted in a strong exposure to geological hazards, mainly associated to gravitational removal of rock blocks of several sizes. On September 4, 1999 a rock block slide from the eastern cliff of Cerro Cabeza de Aguila endangering alumni of three schools: kindergarten, primary, and secondary schools. Civil Defense and other concerned authorities decided to temporarily evacuate and close the schools, and asked for an evaluation of the likelihood for future occurrence of similar events and possible damages to infrastructure, property, and more importantly to life. Evaluation was done first, in terms of geological reconnaissance and recognition of past events in the area; second, identification of unstable blocks and possible trajectories; third, simulation for new rock fall events by using computing software; and fourth, construction of hazard and risk maps. Geologic work showed that rocks of a size of 16 tons and up to 1000 tons in weight fell from the cliff and were still present in between houses and schools. In fact, several blocks were recognized to have fallen or slide along the slope permitting identification of the different failure mechanisms of the blocks. A catalogue of the blocks together with information of distribution, dimensions, distance to the source, etc. resulted in a series of calibrated parameters to perform simulations. Trajectory simulations allowed to identify possible exposure to destruction of a water reservoir, representing an additional hazard to the community. Simulations also permitted to portray different scenarios based in probability ranges for different block sizes falling from the cliff. Probability isolines were decided to feature the hazards map. The resulting mapping depicts in a very accessible way, the most endangered areas. Trajectory analysis together with geotechnical simulations allowed the identification of range and likely energy the blocks might yield during a rock fall event and thus, permitted to locate and design of structures to protect the people, schools, and other important infrastructure.