Scientific Investigations Report 2007–5251
U.S. GEOLOGICAL SURVEY
Scientific Investigations Report 2007–5251
Version 2.0, June 2013
Land subsidence is known to occur in valleys containing aquifer systems that are, at least in part, made up of fine-grained sediments and that have undergone extensive ground-water development. The pore structure of a sedimentary aquifer system is supported by the granular skeleton of the aquifer system and the pore-fluid pressure of the ground water that fills the intergranular pore space (Meinzer, 1928). When ground water is withdrawn in quantities that result in reduced pore-fluid pressures and water-level declines, the reduction of the pore-fluid-pressure support increases the intergranular stress, or effective stress, on the skeleton. A change in effective stress deforms the skeleton—an increase in effective stress compresses it and a decrease in effective stress causes it to expand. The vertical component of this compression sometimes results in nonrecoverable compaction of the aquifer system and land subsidence. An aquifer-system skeleton that primarily consists of fine-grained sediments, such as silt and clay, is much more compressible than one that primarily consists of coarse-grained sediments, such as sand and gravel. Inelastic (nonrecoverable) compaction of coarse-grained sediment is negligible.
Aquifer-system deformation is elastic (recoverable) if the effective stress imposed on the skeleton is smaller than any previous effective stress (Leake and Prudic, 1991). The largest historical effective stress imposed on an aquifer system—sometimes as a result of the lowest ground-water level—is the “preconsolidation stress.” If a stress imposed on the skeleton is greater than the preconsolidation stress, the pore structure of the granular matrix of the fine-grained sediments is rearranged; this new configuration results in a reduction of pore volume and, thus, inelastic (largely irreversible) compaction of the aquifer system. Furthermore, the compressibility of the fine-grained sediments, and any resulting compaction under stresses greater than the preconsolidation stress, are 20 to more than 100 times greater than they are under stresses less than the preconsolidation stress (Riley, 1998).
For an aquifer-system skeleton that contains an appreciable thickness of fine-grained sediments, a significant part of the total compaction may be residual compaction (delayed compaction that occurs in thick fine-grained interbeds and confining layers while heads equilibrate with heads in the adjacent aquifers [Terzaghi, 1925]). Depending on the thickness and the vertical hydraulic diffusivity of a confining layer, pressure equilibration—and thus compaction—lags behind pressure, or head, changes in the adjacent aquifers. For a more complete description of aquifer-system compaction, see Poland (1984), and for a review and selected case studies of land subsidence caused by aquifer-system compaction in the United States, see Galloway and others (1999).
U.S. Department of the Interior | U.S. Geological Survey
URL: https://pubs.usgs.gov/sir/2007/5251
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