LAND SUBSIDENCE AND GENERATION OF EARTH FISSURES: A PROBLEM OF STRESSES

In basins that are filled with alluvial material where there are an intense mining or pumping of the ground water a phenomenon is observed more and more frequently: the sinking of the surface and the generation of earth fissures.

There is a relationship between the decrease of the water level of an aquifer, the sinking of the surface and the generation of earth fissures. This relationship is not so obvious because it is very difficult to obtain values of the variables that are interacting in this problem such as the soil strata sequence at any depth of the aquifer system, the mechanical properties of the soil to depths longer than common exploration depths in soil engineering (depths longer than 10 meters), the variation of the water level at any point of aquifer etc. Due to these difficulties the behaviour of an aquifer system that includes deformations is calculated by means of simplified models.

In terms of a stress problem the unknowns are the boundary conditions of our model (boundaries without capacity of consolidation or capacity of deformation that are forming the basin), mechanical properties of the body to deforming (stress-strain behaviour of the materials that are filling the basin) and the load of the system (conditions of applied stress).

Works carried out in Querétaro Valley show that the boundary conditions of the system are decisive in the generation of earth fissures.

In this work we presented the analysis of stresses of an hypothetical aquifer by means of Finite Elements, we varied the mechanical properties of the materials and the boundary conditions as well as load conditions of the system (a parameter at a time) in order to determine which conditions are critical for generating earth fissures. We also present the results of an analysis of stresses of four partial sections of the basin of Querétaro valley based on gravimetric data.