Land Subsidence and Cracking Due to Ground‐Water Depletion ^a

Abstract

ABSTRACT Subsidence of the land surface due to ground‐water overdraft is caused by an increase in the intergranular pressure in unconsolidated aquifers and other underground materials. For unconfined aquifers, this increase is the result of a loss of buoyancy of solid particles in the zone dewatered by the falling water table. For confined aquifers, increases in intergranular pressure are caused by decreases in the upward hydraulic pressure against the bottom of the upper confining layer, due to a drop in piezometric surface. Compression of layers in which the intergranular pressure is increased can be calculated with elastic or logarithmic theory. Sample calculations yield rates of subsidence that agree with those observed, i.e., about 5 to 50 cm (2 to 20 inches) per 10‐m (33‐ft) drop in ground‐water level. Ground‐water depletion can also produce surface cracks, particularly above discontinuities in bedrock depth along the periphery or in other parts of subsiding basins. Calculations based on the rotating‐slab theory show that the initial surface width of such cracks is about 1 cm (0.5 inch), which agrees with field observations.