Earth Fissure Formation from the Mechanics of Groundwater Pumping

This paper presents the formulation of the mechanics governing the changes in stress state from groundwater pumping. A two-layer alluvium consisting of a cemented alluvium layer on top of an unconfined aquifer alluvium layer is considered. It is shown that the stress state in the aquifer alluvium consists of compression, simple shear and couple stresses. The shear stresses on the vertical planes and horizontal planes are not equal. Consequently, a Cosserat rather than the conventionally used Cauchy continuum better simulates the stresses in the aquifer alluvium. Land subsidence from groundwater level decline consists of vertical compression (consolidation), shear displacement and macrorotation. The latter occurs when conditions are favorable for the microrotation imposed by asymmetric stresses to become macrorotation. Earth fissures are formed in aquifer alluvium from groundwater pumping by simple shear on vertical planes and rotation and not by tension as commonly suggested. Earth fissures are formed in the upper cemented alluvium by simple shear strains and rotations. Highly cemented alluviums are more prone to earth fissure formation than weakly cemented alluviums. It is shown that there is a critical characteristic length of √2 times the aquifer alluvium thickness below which earth fissures would, theoretically, not develop. The distances from the well face at which earth fissures will form depend not on the total drop in groundwater level but on the gradient of the groundwater profile. The slope of the subsidence bowl, which consists of simple shear strain and rotation, is established as an indicator for the initiation of earth fissures.