Mechanisms of earth fissuring caused by groundwater withdrawal

Earth fissures associated with groundwater withdrawal are complex products of both human activities and natural forces, and they occur in definable geological environments. In this paper, the authors first characterize the driving forces for earth fissures caused by groundwater withdrawal. Then, the effects of various factors, such as stresses and pre-existing geological structures,are examined using conceptual models. Numerical results show that the fissuring process is controlled not only by the induced movement at depth and pre-existing structures but also by the in situ stress field. In addition, the degree to which aquifer movement and pre-existing structures actually trigger fissuring depends greatly on the in situ stress field. The authors conclude that earth fissuring related to groundwater withdrawal is a multi-step process that is influenced by a multiplicity of factors, one being the aquifer movement. With groundwater, withdrawal, hydraulic and gravitational forces tend to drive aquifer material to deform both horizontally and vertically. Cumulative deformation or strain results in movement. In turn, this aquifer movement results in differential displacements at depth along planes of weakness, such as pre-existing faults and material interfaces. This differential movement (both horizontal and vertical) then generates tensile zones at depth. Once formed, such a tensile zone may migrate upward, form a crack (fail) where the vadose zone is brittle, and eventually express itself as an earth fissure at the land surface in and or semi-arid regions. In humid regions, the same tensile zone (lateral stretching) at depth will simply express itself as a transient sub-vertical plane of enhanced porosity within and crossing the vadose zone.