The Bay Area Rapid Transit system (BART) is one of the San Francisco Bay Area's most vital transportation systems with more than 100 miles of track, 43 stations, and an average of 300,000 individual trips every day. It also represents a huge public investment, currently valued at nearly $15 billion.
BART - running quietly under the streets of San Francisco
So what if BART were hit by an earthquake? How would the system fare?
Its biggest test so far came in 1989. The BART system was successful in maintaining continuous service directly after the 1989 Loma Prieta quake, the epicenter of which was 50 miles south of San Francisco . But the next quake could be bigger and more direct.
When the BART system was built more than 30 years ago, it incorporated state-of-the-art seismic technology, but since then many engineering advances have been made. It was time to take another look.
The first step of the $l.3 billion program was to complete a systemwide Seismic Vulnerability Study an assessment of how system components would perform during a major earthquake. ( http://www.bart.gov/docs/eqs/BART_Seismic_Study.pdf ) Working with BART on this study were several Northern California structural engineers.
The study, completed in 2002, took one and a half years of engineering and statistical analysis to complete. More than 15,000 components were evaluated. It concluded that, if the Bay Area were to get hit by another major earthquake, it would take two and a half years or more to repair critical portions of the system. Moreover, if BART were made inoperable, riders would become drivers and commuting delays could increase by as much as 80 minutes.
BART System Map
The highest priority for upgrade became the Transbay Tube, which connects Oakland to San Francisco and where soil backfill is prone to liquefaction. Though the consequences of liquefaction on the TransbayTube are uncertain, a worst-case scenario could cause excessive movement of the seismic joints and structural stress that could result in significant damage.
The original BART system included more than 25 miles of elevated trackways, which the study showed are vulnerable to collapse. To fix this, nearly two thousand structures supporting the elevated trackways need strengthening. Two techniques are under consideration: enlarging their foundations, and wrapping their columns with steel jackets.
It's estimated that design and construction of the entire program will require approximately 10 years.