Many large cities such as Tokyo, Bangkok, Río de Janeiro, Recife, Bogota and, of course, Mexico City, to name only a few, were built and are still being developed on soft soils. In many cases, these cities also experience regional subsidence induced by pumping of groundwater from deep local aquifers. Among the sources of uncertainty prevailing in the geotechnical characterization of these sites, soil properties spatial variability is one of the most difficult to deal with since the associated uncertainty cannot be eliminated only by improving laboratory and field-testing techniques. For an accurate evaluation of the subsoil conditions, spatial variations of the soil profile and mechanical properties together with the groundwater conditions must be assessed by performing a sufficient number of soil explorations, processing a generally large amount of data and developing either deterministic or probabilistic models of these variations. The techniques available to develop such models and some difficulties encountered to implement them are examined in this lecture. Some geotechnical analysis and design methods that take into account soft soils spatial variations are also reviewed together with constructions techniques aimed at mitigating consequences of soil variability.

The above considerations are illustrated with reference to Mexico City’s highly compressible volcanic lacustrine clays. Models of the spatial variability of these materials developed over the years for different projects using traditional and geostatistical techniques are presented. Some of the geotechnical analysis and construction methods used by geotechnical engineers to deal with soil spatial variability in this megacity once called by Terzaghi “the paradise of soil mechanics”, are also discussed.