Some Issues in Simulating Land Subsidence and the Formation of Earth Fissures from Ground Water Withdrawal

Land subsidence is affecting many nations and the problems with it will increase with climatic changes.  Land subsidence causes land morphological changes that lead to increased susceptibility to flooding, distress of structures, infrastructures and lifeline systems, and the formation of ground fractures.  Although land subsidence has received considerable attention, prediction of its amount and rates, and the formation and growth of earth fissures are elusive.  The main problem is that a single model that can simulate the vagaries of geology, pumping rates and pump locations, and natural recharge is elusive. There are several types of ground fractures associated with land subsidence but earth fissures – linear to arcuate cracks caused by differential land subsidence from groundwater withdrawal – are particularly dangerous and can inflict significant damages to infrastructures and lifeline systems.
In this paper, some key issues in modeling land subsidence and in the mechanics of earth fissure formation are presented.  Terzaghi and Biot consolidation theories have been used with Navier-Stokes fluid flow equation to predict the initiation of earth fissures.  Consolidation of geomaterials does not lead to failure and thus these theories cannot be used to simulate earth fissure formation. Since land subsidence is intimately linked to earth fissures, its simulation must couple fluid flow-consolidation-stress-strain response-failure and fracture growth.  Even if a model is developed that incorporates fluid flow-consolidation-stress-strain response-failure and fracture growth it would be difficult, if not impossible, to obtain the desired input soil parameters. In this paper, some techniques are suggested for modeling land subsidence and earth fissure formation given the constraints imposed by the vagaries of geology and management practice in well location and pumping rates. Results from a numerical simulation of a site in Arizona are presented.