Conceptual and numerical models for the mechanics of fissuring caused by groundwater withdrawal

In this study the mechanisms of fissuring associated with groundwater withdrawal are analyzed systematically using interdisciplinary methods. It integrates a general hydraulic driving force into conceptual and numerical models (Helm, 1994a). The effects on fissuring of weakness planes, geometrical abnormalities of soil masses and aquifer movement are emphasized.; Five conceptual models are established. These conceptual models are designed around the following somewhat overlapping physical phenomena: aquifer movement and the resulting forces as well as the resulting displacements; tensile failure and the opening of cracks; initiation of a crack at depth and its migration towards land surface; effects of geological structures; effects of geometrical abnormalities; effects of a sudden increase in local pore water pressure. Two simple fracture mechanical models are established to estimate the likelihood of crack propagation for conceptual model 5.; Results are obtained using the boundary element analysis of some conceptual models. The results indicate that horizontal aquifer movement plays an important role in generating a crack at depth. Faults control the location of potential failure zones. The fissuring zone tends to focus on the hanging wall side of a fault. Pumping from two wells may cause a potential failure zone between the wells to deepen and widen.; Sensitivity analyses show that the confining stress plays an important role in earth fissuring. If a fault is involved, its friction angle and dip angle are controlling factors for tensile failure. In general, a shallow aquifer is more likely to be susceptible to tensile failure than a deep one.; Earth fissuring is a multistep process driven by multiple forces and influenced by multiple factors. An earth fissure is a result of multiple mechanisms. A tensile failure zone may occur in the material just above the top of a buried basement rock ridge when aquifer material on both sides of the ridge moves away from the ridge. Such failure may extend outward and initiate a crack in overlying materials with the ongoing progress of aquifer movement. This mechanism is distinct from the conventional "bending beam" model. It gives an explanation for a fissure occurring in areas where horizontal strain caused by vertical differential subsidence is insufficient to form tensile failure at land surface.; Finally, the mechanisms of earth fissuring in Las Vegas Valley, Nevada and Picacho, Arizona are analyzed. The effects of pumping from two pumping centers, roles of in-situ stresses and the effects of pre-existing mapped faults are emphasized.