Seismic Design of Tunnels - Parsons Brinckerhoff

• The collapse of the I-880 viaduct claimed more than 40 lives.
• The direct damage costs to the transportation facilities alone totalled nearly $2 billion
(Werner and Taylor, 1990).
• The indirect losses were several times greater as a result of major disruptions of
transportation, particularly on the San Francisco-Oakland Bay Bridge and several
major segments of the Bay area highway system.
The San Francisco Bay Area Rapid Transit (BART) subway system was found to be
one of the safest places during the event, and it became the only direct public
transportation link between Oakland and San Francisco after the earthquake. Had BART
been damaged and rendered inoperative, the consequences and impact on the Bay area
would have been unthinkable.
The 60-mile BART system was unscathed by the earthquake because PB engineers
had the foresight 30 years ago to incorporate state-of-the-art seismic design criteria in their
plans for the subway tunnels (SFBARTD, 1960; Kuesel, 1969; and Douglas and Warshaw,
1971). The Loma Prieta earthquake proved the worth of their pioneering efforts.
Seismic Design Before the ‘90s
Based on the performance record, it is undoubtedly fair to say that underground
structures are less vulnerable to earthquakes than surface structures (Dowding and
Rozen, 1978; Rowe, 1992). Interestingly, some tunnels and shafts built without special
earthquake provisions have survived relatively strong earthquakes in the past — for
example, the Mexico City subway during the 1985 Mexico City earthquake. On the other
hand, some underground structures have been damaged severely in other events (see
Section 1.5).
Limited progress has been made in seismic design methodology for underground
tunnels since the work for BART, possibly because of favorable performance data, and
limited research work has been done toward a practical solution. The lack of a rational
methodology for engineers and the nonexistence of applicable codes has led to widely
varied measures taken by different engineers. For example:
• Some ignore seismic effects and fail to check the resistance of the structures to
earthquakes, even in highly seismic areas.
• Others conduct their seismic design for underground structures using the same
methodology developed for aboveground structures, without recognizing that
underground structures are constrained by the surrounding medium.
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