Environmental Engineering Reference
In-Depth Information
Prevention
Only control of overextraction prevents subsidence. Approximate predictions as to when
the water table will drop to the danger level can be based on withdrawal, precipitation,
and recharge data. By this time, the municipality must have provisions for an alternate
water supply to avoid the consequences of overdraft.
Control
Where subsidence from withdrawal is already troublesome, the obvious solution is to stop
withdrawal. In the case of Mexico City, however, underlying soft clays continued to con-
solidate for many years, even after withdrawal ceased, although at a much reduced rate.
Artificial recharging will aid the water balance ratio.
Recharging by pumping into an aquifer requires temporary surface storage. The Santa
Clara Valley Water District (San Jose, California) has been storing storm water for recharge
pumping for many years (ENR, 1980). In west Texas and New Mexico, dams are to be built
to impound flood waters that are to be used as pumped-groundwater recharge (ENR, 1980).
Where the locale lacks terrain suitable for water storage, deep-well recharging is not a
viable scheme, and recharge is permitted to occur naturally. Venice considered a number
of recharge schemes, in addition to a program to cap the city wells begun in 1965.
Apparently, natural recharge is occurring and measurements indicate that the city appears
to be rising at the rate of about 1 mm every 5 years ( Civil Engineering , 1975).
Oil and Gas Extraction
Prediction of subsidence from oil and gas extraction is difficult with respect to both mag-
nitude and time. Therefore, it is prudent to monitor surface movements and to have con-
tingency plans for the time when subsidence approaches troublesome amounts.
Control by deep-well recharging appears to be the most practical solution for oil and gas
fields. At Long Beach, water injection into the oil reservoirs was begun in 1956; subsidence
has essentially halted and about 8 mi 2
of land area has rebounded, in some areas by as
much as 1 ft (Allen, 1973; Testa, 1991).
Construction Dewatering
Prediction and Control
Before the installation of a construction dewatering system in an area where adjacent
structures may be affected, a study should be made of the anticipated drop in water level
as a function of distance, and settlements to be anticipated should be computed consider-
ing building foundations and soil conditions. Peat and other organic soils are particularly
susceptible to compression. In many cases, condition surveys are made of structures and
all signs of existing distress recorded as a precaution against future damage claims. Before
dewatering, a monitoring system is installed to permit observations of water level and
building movements during construction operations (see Figure 4.37). The predicted set-
tlements may indicate that preventive measures are required.
Prevention
Prevention of subsidence and the subsequent settlement of a structure are best achieved
by placing an impervious barrier between the dewatering system and the structure, such
as a slurry wall (see Section 8.4.2). Groundwater recharge to maintain water levels in the
area of settlement-sensitive structures is considered to be less reliable.
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