Environmental Engineering Reference
In-Depth Information
5.4 Land Subsidence
Land subsidence is the lowering of the land surface from changes that occur underground.
One of the most common human causes of land subsidence is compaction of fine-grained
sediments induced by extraction of water from the subsurface. It also can occur in certain
areas, notably in Los Angeles, California, and Houston, Texas, where oil extraction causes
similar compaction of underground strata. 35 Where significant amounts of these sensi-
tive subsurface strata are present, substantial ground-surface lowering can occur as water
levels decline. In some cases, land subsidence can continue for decades after a cessation or
reduction in pumping that stabilizes groundwater levels. 35 In Arizona, surface-elevation
subsidence of as much as 18 ft has been recorded west of Phoenix. 36 In Tucson, as much
as 4 in. of subsidence was associated with a water-level decline of 45 ft between 1989 and
2005. 37 Subsidence fissures (Figure 5.6), which typically occur near the margins of alluvial
basins between zones of differential subsidence, can rupture roads, canals, buildings, and
other infrastructure. 38
Extraction of groundwater is the major cause of subsidence in the deserts of the south-
western United States, and as pumping increases, land subsidence also increases. In allu-
vial aquifers, groundwater is pumped from the pore space between grains of sand and
gravel. If an aquifer has beds of clay or silt within or next to it, the lowered water pressure
in the sand and gravel causes slow drainage from the clay and silt beds. The reduced water
pressure is a loss of support for the clay and silt strata; because these beds are compressible,
FIGURE 5.6
Subsidence crack in Benson, Arizona. (Courtesy of S.R. Anderson.)
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