Environmental Engineering Reference
In-Depth Information
Interior Basins
In the semiarid to arid regions of the western United States, the basins are often filled with
hundred of meters of sediments, which serve as natural underground reservoirs for the
periodic rainfall and runoff from surrounding mountains. Subsidence can reach signifi-
cant amounts: for example, as much as 30 ft in the San Joaquin Valley of California, 12 ft in
the Santa Clara Valley in California, 15 ft in Elroy, Arizona, 6 ft in Las Vegas, Nevada, and
9 ft in Houston (Leake, 2004). Around the Tucson Basin, where the water level has dropped
as much as 130 ft since 1947, it has been suggested that minor faulting is occurring and
may be the reason for distress in some home foundations (Davidson, 1970, Pierce, 1972).
Other effects will be increased flooding due to changes in stream gradients and the loss of
canal capacity due to general basin lowering.
In 1989, the U.S. Department of Housing and Urban Development began requiring spe-
cial subsidence hazard assessments for property located near subsidence features in Las
Vegas (Bell, et al., 1992). The requirement resulted from structural damage to a major sub-
division in North Las Vegas that required the repair or displacement of more than 240
damaged or threatened homes at a cost of $12-13 million. The assessments included
guidelines specifying detailed studies and specialized construction for all new develop-
ments within 150 m of a mapped fault.
The basin in which Mexico City is situated is filled with thick lacustrine sediments of
volcanic origin, and groundwater withdrawal has resulted in serious consequences.
10.2.4
Significant Examples
Houston, Texas (Water Extraction: Flooding and Faulting)
Between 1906 and 1964, 5 ft of subsidence occurred, and reports place the subsidence at 9 ft
at some locations (Civil Engineering, 1977). The cost of the subsidence, including flood
damage, between 1954 and 1977 has been estimated to be $110 million and in 1977 was
growing at the annual rate of $30 million (Spencer, 1977). The subsidence results in
“growth” faults that cause distress in structures, large deflections of roadways, and rup-
ture of utility lines; in flooding, resulting in homes being abandoned along Galveston Bay;
and the lowering of bridges over the Houston Ship Canal.
The problem of growth faults in the Houston metropolitan area is severe. Activity has
been recognized on more than 40 normal faults, which are prehistoric according to Van
Siclen (1967). Major surface faults and the cumulative subsidence between 1906 and 1964
are given in
Figure 10.1
,
and a profile of subsidence and groundwater decline for a dis-
tance of about 14 mi is given in
Figure 10.2.
The drop in water level of almost 300 ft causes
an increase in overburden pressure of about 9 tsf, which is believed to be causing down-
ward movement along the old faults as clay beds interbedded with sand aquifers consol-
idate (Castle and Youd, 1972).
Holdahl et al. (1991) report that the area has been divided into two zones, west and east
of downtown Houston. In 1987, west zone subsidence was ranging up to 72 mm/year (2.8
in./year), but after 1978 the east zone has experienced 60-90% decreases in subsidence
rates due to regulated groundwater withdrawal and the use of canals to carry water from
Lake Houston. The east zone is the industrial zone. Subsidence patterns in Houston are
being monitored with InSAR (
Section 10.2.2)
.
Mexico City (Water Extraction: Subsidence and Foundation Problems)
Geologic Conditions
The basin of the valley of Mexico City, 2240 m above sea level, has been filled with 60 to
80 m of Pleistocene soils including interbedded sands, sands and gravels, and lacustrine
