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Integration of geological properties in the study of the subsidence and fracturing phenomena in two urban areas of Mexico

We compare the mechanical behaviour of the near surface materials in two sedimentary sequences with contrasting grain size and water content. The methodology includes detailed laboratory measurements of physical properties such as specific gravity, grain size distribution, water content, Atterberg limits, electrical conductivity, and compressibility performed in samples from geotechnical excavations. Contrast in the physical properties can be identified in Ground Penetrating Radar (GPR) profiles to obtain the geometry of the deformational features. The basin of Mexico is characterized by a recent lacustrine sequence with high water content and clay bearing sediments intercalated with pyroclastic deposits. Deformational features in this basin reflect an interplay between the geological history (depositional conditions), load history, seismic activity, and faulting. Plastic mechanical behaviour predominates in these clayey sediments and differential deformation locally triggers brittle fracturing and/or subsidence of the surface. In this case, the fractures are planes of weakness that reduce the bulk strength of the clay and increase their hydraulic conductivity. In contrast, the near surface sequence of the Valley of Queretaro is characterized by highly heterogeneous unsaturated sand- and-silt bearing sediments with intercalated pyroclastic layers. The mechanical behaviour of these materials is characterized by dilatant fractures and associated shear bands with a normal displacement. Failure of the sequence is caused by a general compaction of the sedimentary refill but the presence of a pre-existing buried fault scarp localizes strain in a 5 km long narrow band with a N-S orientation. Differential deformation plays an important role only in the near surface sequence as a response to applied loads. Relating the mechanical behaviour of the studied sequences with physical and geological properties should be taken into account to estimate subsidence and risk of fracturing for urban development.