ENGINEERING GEOLOGY APPROACH TO THE EFFECTS OF LAND SUBSIDENCE IN MEXICO CITY

In this work and its associated field trip, we examine the regional land subsidence and its relationship to fracturing in selected sites within Mexico City; which is a Megacity exposed to natural and man induced hazards affecting many aspects of urban development. Land subsidence is a geo-hazard imposing important constraints to urban development due to the gradual decrease in elevation of the land surface. This is caused either naturally, or by the extraction of water, oil, minerals, or gas from the subsurface, or by the interaction between natural and anthropogenic forces. Due to the rapid growth of the city, groundwater has been over-exploited for human consumption during the last fifty years, leading to a marked decline of piezometric levels. Land subsidence is thus generally related to the consolidation of fine grained materials in response to effective stress increments caused by extensive extraction of groundwater. Nevertheless, the geological and geomechanical processes become more complex as the phenomena reach regional scales, either laterally or downwards. As the consolidation of the lacustrine deposits advances, their static and dynamic geomechanical properties change, and so does the fracture vulnerability of the whole soil-rock system. Changes occur in the totality of the former lake zone and given the present trends in the urban and demographical evolution of the city, regional subsidence will continue in the foreseeable future. Monitoring and direct and indirect surveillance should be implemented for a better assessment of its spatial and temporal effects. Moreover, the combined effects of differential deformation, regional subsidence and earthquakes associated risks pose serious threats to the sustainability of the urban infrastructure in the Metropolitan Zone of the Valley of Mexico.