Civil Engineering Reference
In-Depth Information
FIGURE 4.26
Ground surface acceleration in the east-west direction at Yerba Buena Island and at Treasure
Island for the Loma Prieta earthquake in California on October 17, 1989. (
From Seed et al. 1990.
)
45 ft (13.7 m) of loose sandy soil over 55 ft (16.8 m) of San Francisco Bay mud (a normally
consolidated silty clay). Note the significantly different ground acceleration plots for these
two sites. The peak ground acceleration in the east-west direction at Yerba Buena Island
was only 0.06
g,
while at Treasure Island the peak ground acceleration in the east-west
direction was 0.16
g
(Kramer 1996). Thus the soft clay site had a peak ground acceleration
that was 2.7 times that of the hard rock site.
The amplification of the peak ground acceleration by soft clay also contributed to dam-
age of structures throughout the San Francisco Bay area. For example, the northern portion
of the Interstate 880 highway (Cypress Street Viaduct) that collapsed was underlain by the
San Francisco Bay mud (see Figs. 4.27 to 4.29). The southern portion of the Interstate 880
highway was not underlain by the bay mud, and it did not collapse.
As these two examples illustrate, local soft ground conditions can significantly increase
the peak ground acceleration
a
max
by a factor of 3 to 5 times. The soft ground can also
increase the period of ground surface shaking, leading to resonance of taller structures. The
geotechnical engineer and engineering geologist will need to evaluate the possibility of
increasing the peak ground acceleration
a
max
and increasing the period of ground shaking
for sites that contain thick deposits of soft clay. This is discussed further in Sec. 5.6.
