Environmental Engineering Reference
In-Depth Information
and the settlements as of 1957 occurring from consolidation of the clay strata were as pre-
dicted, or about 10 cm/year.
Long Beach, California (Oil Extraction: Subsidence and Flooding)
Geologic Conditions
The area is underlain by 2000 ft of “unconsolidated” sediments of late Pliocene,
Pleistocene, and Holocene age, beneath which are 4600 ft of oil-producing Pliocene and
Miocene formations including sandstone, siltstone, and shale.
Ground subsidence began to attract attention in Long Beach during 1938-1939 when the
extraction of oil began from the Wilmington field located primarily in the city (see site
location map,
Figure 10.3)
.
A peak subsidence rate of over 20 in/year was reached in
1951-1952
(Section 10.2.1),
By 1973, subsidence in the center of a large bowl-shaped area
had reached 30 ft vertically, with horizontal movements as great as 13 ft. Flood protection
for the city, which is now below sea level in many areas, is provided by extensive diking
and concrete retaining walls. Deep-well recharging by salt-water injection has halted sub-
sidence and some rebound of the land surface has occurred.
Baldwin Hills Reservoir Failure (Oil Extraction: Faulting)
Event
On December 14, 1963, the Baldwin Hills Reservoir, a pumped storage reservoir located in
Los Angeles (Figure 10.3), failed and released a disastrous flood onto communities down-
stream (Jansen et al., 1967).
Background
The dam was located close to the Inglewood oil field and the Inglewood fault passed
within 500 ft of the west rim of the reservoir. The Inglewood fault is part of the major
Newport-Inglewood fault system (Figure 10.3). During construction excavation in 1948,
two minor faults were found to pass through the reservoir area, but a board of consultants
judged that further movement along the faults was unlikely. The dam was well instru-
mented with two strong-motion seismographs, tiltmeters, settlement measurement
devices, and observation wells.
Surface Movements
Between 1925 and 1962, at a point about 0.6 mi west of the dam, about 10 ft of subsidence
and about 6 in. of horizontal movement occurred. In 1957, cracks began to appear in the
area around the dam. Six years later, failure occurred suddenly and the narrow breach in
the dam was found to be directly over a small fault (
Figure 10.4).
It was judged that 6 in.
of movement had occurred along the fault, which ruptured the lining of the reservoir and
permitted the sudden release of water. It appears likely that subsidence from oil extraction
caused the fault displacement, since there had been no significant seismic activity in the
area for at least the prior month.
10.2.5
Subsidence Prevention and Control
Groundwater Extraction
General
Subsidence from groundwater extraction cannot be avoided if withdrawal exceeds
recharge, resulting in significant lowering of the water table or a reduction in piezometric
levels at depth, and if the subsurface strata are compressible.
