The first major construction to strengthen BART to withstand a massive earthquake is about to begin on the most critical part of the BART system - the Transbay Tube.
Each weekday, nearly half of BART's 325,000 daily riders go through the Tube, which is entrenched in the San Francisco Bay floor and connects the East Bay with downtown San Francisco.
Today the BART Board of Directors awarded a $9.7 million contract to the construction firm Condon Johnson & Associates, Inc. to strengthen the soil around the Tube on Port of Oakland property from the shoreline to the Tube's end. Work should begin this summer and take about seven months. Many believe the Tube is earthquake safe because it withstood the 1989 Loma Prieta earthquake. However, that quake was about 60 miles from the Bay Area. Geologists say the next big one will likely be much closer. In fact, U.S. Geological Survey geologists say there's a 62% chance of one or more 6.7 magnitude or greater earthquakes striking the Bay Area between now and the year 2032. Recently, a panel of world-renowned, independent earthquake engineers and geologists determined the next major earthquake in the Bay Area could potentially cause the Tube to fail, unless engineers compact the soil surrounding it to prevent liquefaction.
TUBE STRENGTHENING METHODS
Today's contract award is to strengthen the Oakland end of the Tube. However, the overall goal is to strengthen the entire length of the Tube. Engineers will compact the soil surrounding the Tube to prevent the soil from liquefying in an earthquake. BART engineers will use a combination of the following different strengthening methods to secure the Tube:
VIBRO-COMPACTION: This method involves using a device that will vibrate the soil around the Tube so that it compacts it. The process is roughly similar to what happens to cake mix when you turn on the cake mixer and the mix starts to compact along the edge of the bowl. This method would prevent the soil from liquefying in an earthquake.
VIBRO-REPLACEMENT: This method is virtually the same as the vibro-compaction except that stone columns are added to the hole created by the mixer. This method would prevent the soil from liquefying in a quake.
GROUTING: This method pumps a liquid cement-like material into the soil, which then hardens and densifies the soil.
The Condon Johnson & Associates, Inc. construction firm will use both the grouting and vibro-replacement methods along both sides of the Tube at the Oakland end when construction begins this summer. Construction to strengthen the entire Transbay Tube will take about four years to complete and cost approximately $330 million in 2004 dollars. Toll bridge money will cover $143 million of the costs. The rest will come from BART's $1.3 billion Earthquake Safety Program.
BART'S $1.3 BILLION, 10-YEAR EARTHQUAKE SAFETY PROGRAM
This Transbay Tube construction begins the first phase in BART's overall $1.3 billion Earthquake Safety Program, which will strengthen not just BART's Transbay Tube, but stations and elevated tracks as well. In November of 2004, more than two-thirds of voters in Alameda, Contra Costa and San Francisco counties approved Measure AA, a $980 million property tax bond, to help pay for the $1.3 billion. The rest of the funding will come from state, local and BART sources. BART expects to complete the entire Earthquake Safety Program in 10 years.
ONE OTHER TUBE STRENGTHENING CONTRACT AWARDED
The BART Board of Directors also awarded a $9.6 million, six month demonstration contract to Hayward Baker/Soletanche/Traylor, JV to:
- Test whether vibro probe equipment can penetrate the rock blanket over the Tube
- Determine the effectiveness of the vibro-compaction or vibro-replacement methods in the Bay
- Install a limited number of vibro-replacment stone columns at two locations along the Tube in the San Francisco Bay
- Provide additional information for use in the final design of the Tube's retrofit