FORECASTING SOIL CRACKING ZONES USING THE VOLUMETRIC LOSS THEORY

In some cities north of Mexico, human water extraction surpass the natural recharge of the aquifer generating an important groundwater table decline. This phenomenon reduces the soil pore pressure on the layers located below the original water table, increasing the effective stresses on the soil mass and leading to the development of the consolidation phenomena. Additionally, when the rock basement underlying the compressible soil layers is irregular, differential settlements and tension stresses may appear, resulting in soil cracking and fissuring. In some cases, these cracks evolve into faults, when an offset between the two sides of the fissure appears and grows depending on the rate of water table decline.

In 1987 Sagaseta developed a method to determine the displacements in an infinite medium produced by a finite volumetric loss. In order to consider the free surface, he applied the image method, consisting in placing an image of the original ground loss symmetric to the free surface. This image annihilates the normal stresses on the surface while the tangential stresses can be nullified by considering superficial shear stresses of the same magnitude but acting in opposite direction. This procedure keeps the free surface unconstrained.

The same method can be applied to the problem of subsidence due to water table decline. In such case the volumetric loss depends on the compression index of the soil. An incompressible basement can be considered in this procedure by adding a positive image that nullifies the displacements on this surface. When both a free surface and an incompressible basement are considered, the procedure evolves into an infinite series of images that can be approximated with sufficient precision into a finite number with the aid of a computational program. This procedure may consider different soil strata and different rates of water table decline. It allows to determine the displacements and strains of a soil mass and therefore determine the tension zones where new cracks may appear as long as the rate of water table decline and the topography of the soil basement are known.

The objective of this study is to present a method that can be used to predict the potential zones of soil cracking due to water table decline.