Land Subsidence in California
Land subsidence, the loss of surface elevation due to removal of subsurface
support, occurs in nearly every state in the United States. Subsidence
is one of the most diverse forms of ground failure, ranging from small
or local collapses to broad regional lowering of the earth's surface.
The causes (mostly due to human activities) of subsidence are as diverse
as the forms of failure, and include dewatering of peat or organic soils,
dissolution in limestone aquifers, first-time wetting of moisture-deficient
low-density soils (hydrocompaction), natural compaction, liquefaction,
crustal deformation, subterranean mining, and withdrawal of fluids (ground
water, petroleum, geothermal).
Although land subsidence has caused many negative impacts on human civil works for centuries, especially in the highly developed urban or industrialized areas of Europe, the relation between subsidence and ground-water pumpage was not understood or recognized for a long time. Recognition began in 1928 when pioneer researcher 0.E. Meinzer of the U.S. Geological Survey realized that aquifers were compressible. At about that same time, Karl Terzaghi, working at Harvard University, developed the one-dimensional consolidation theory. The theory generally states that compression of soils results from the slow release of pore water from stressed clay materials and the gradual transfer of stress from the pore water to the granular structure of the clay.
Joseph Poland noted large elevation discrepancies between U.S. Coast and
Geodetic Survey first-order re-levelling runs for the Santa Clara Valley
and the San Joaquin Valley, California. Beginning in 1952, a major subsidence
research effort by the U.S. Geological Survey, under Poland, paralleled
unprecedented growth in the use of ground water, especially in California,
Texas, and Arizona. The results of field studies done by members of Poland's
research team not only verified the validity of the application of Terzaghi's
consolidation theory to compressible aquifers, but they also provided
definitions, methods of quantification, and confirmation of the interrelationship
among hydraulic-head declines, aquitard (clay) compaction, and land subsidence.
In other words, the theory of aquitard compaction had been tested and
essentially verified.
