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In the San Francisco Bay area, much farther from the rupture zone, minor liquefaction as
evidenced by small sand boils also occurred beneath several elevated sections of the I-80/Grand
Avenue Highway “distribution structure” immediately inland of the Oakland-San Francisco Bay
Bridge approach fill. Figure 2.24 shows a linear zone of boils adjacent to a pile-supported
abutment.
Figure 2.24: Sand Boils at the Oakland-San Francisco Bay Bridge Approach Fill
The minor liquefaction and associated ground settlement did not result in damage to this
distribution structure. It should be noted that the area is flat and therefore, lateral movements
were relatively small. Also, piles are end-bearing beneath the liquefiable layers and any frictional
resistance lost during liquefaction of the subsoils was apparently adequately carried by the pile
tip. The post-liquefaction settlement of the approach fill and foundation soils did result in limited
access at the transition from the pavement supported on grade to the pile-supported approach.
2.3.4 1991 Costa Rica Earthquake
In 1991, a M w 7.5 earthquake centered in the Limon Province of Costa Rica resulted in
considerable damage to transportation lifelines. Strong shaking lasted for about 25 seconds with
most peak accelerations in the range of 0.06 g to 0.20 g. The maximum peak acceleration, 0.27 g,
was measured at a free field station close to the epicenter in an alluvial valley. Eight major
highway and railway bridges collapsed during the earthquake and several other bridges were
severely damaged. All of these were at river crossings and in nearly all instances, liquefaction-
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