Environmental Engineering Reference
In-Depth Information
Large submarine slides may also occur, such as the one depicted on the high-resolution
seismic profile given in Figure 9.57, presumably caused by an earthquake affecting the
Kayak Gulf of Alaska.
Earth Dams and Embankments
Occurrence
If earth embankments , such as those for roadway support, fail during earthquakes, it is usu-
ally by lateral spreading due to foundation failure such as that occurred in San Francisco
and the coastal cities of Alaska as described in the previous chapter.
Earth dams , when well built, can withstand moderate shaking, of the order of 0.2 g or more,
with no detrimental effects. Dams constructed of clay soils on clay or rock foundations have
withstood extremely strong shaking ranging from 0.35 to 0.8 g (from an M
8.5 event) with
no apparent damage. The greatest risk of damage or failure lies with dams constructed of
saturated cohesionless materials that may be subjected to strong shaking. A review of the
performance of a large number of earth dams during a number of earthquakes is presented
by Seed et al. (1978). They list six dams in Alaska, California, Mexico, and Nevada that are
known to have failed, three dams in California and Nevada known to have suffered heavy
damage, and numerous dams in Japan that suffered embankment slides.
Foundation failure appears to have caused the collapse and total failure of the Sheffield
Dam, near Santa Barbara, California, during the 1925 quake (Seed et al., 1969).
Case Study: The Upper and Lower San Fernando Dams (Seed et al., 1975)
Event: During the 1972 San Fernando earthquake ( M
6.6), the two dams on the lower Van
Norman Reservoir complex, located about 9 mi from the epicenter, suffered partial fail-
ures. If either dam had failed completely, a major disaster would have occurred, since
some 80,000 people were living downstream.
Description : The Upper San Fernando Dam was 80 ft high at its maximum section. During
the earthquake, the crest moved downstream about 5 ft and settled about 3 ft. Severe lon-
gitudinal cracking occurred on the upstream slope, but there was no overtopping or
breaching. The Lower San Fernando Dam was 142 ft high at its maximum section, and it
suffered a major slide in the upstream slope and part of the downstream slope, leaving
about 5 ft of freeboard in a very precarious position as shown in the photo (Figure 11.37).
Both dams were constructed by a combination of compacted fill and semihydraulic fill
placed during times when little was known about engineered compacted fill. The Lower
Dam was completed in 1915, and raised twice, in 1924 and 1930. The Upper Dam was com-
pleted in 1922. The dams had withstood the not-so-distant Kern County event of 1952
( M
7.7). The locations of the events of 1971 and 1952 are shown on the isoseismal maps,
(Figure 11.15).
Instrumentation had been installed in recent years, including piezometers in the Upper
Dam and two seismoscopes on the Lower Dam, one on the crest and one on the rock of the
east abutment.
Pseudostatic analysis performed before the earthquake had evaluated the stabilities
against strong ground motion and found the dams to be safe.
Dynamic analysis of the response of the dams to earthquake loadings appeared to pro-
vide a satisfactory basis for assessing the stability and deformations of the embankments
(Seed et al., 1975).
Lower dam : Dynamic analysis indicated the development of a zone of liquefaction
along the base of the upstream shell, which led to failure. Evidence of liquefaction
was provided by the seismoscopes, which indicated that the slide had developed
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