Transbay Tube Earthquake retrofit keeps farmers market in place


Transbay Tube Earthquake retrofit keeps farmers market in place

Engineers will get the job done with little disruption

BART officials unveiled a new Transbay Tube earthquake retrofit plan that will mean the San Francisco Ferry Building's world-renowned Farmers Market can remain in place while engineers work to strengthen the Tube to withstand a massive temblor.

As a result of the new plan, the impact on the Farmers Market vendors and the 40,000 people who visit them each weekend, will be far less than BART first predicted. BART worked extensively with both Farmers Market and Ferry Building vendors, the Port of San Francisco and other stakeholders to find a solution.

"As we've proven with the Farmer's Market, BART always tries to go the extra mile to accommodate the community," BART Board Vice President Lynette Sweet said today. The Transbay Tube retrofit project is one of the most essential components of BART's $1.3 billion Earthquake Safety Program.

"The Earthquake Safety Program is critically important to BART and to the entire Bay Area," BART Director James Fang said. "We are overjoyed that we can continue with this urgent project and yet not disrupt one of the city's true gems, the Farmers Market."

Everyone got a big break when BART engineers determined that some of the soil surrounding the Transbay Tube was stronger than initial studies predicted.

Each weekend, the Farmers Market sets up on the Ferry Building's pier. The Tube runs under that pier. BART's preliminary studies suggested that the soil surrounding the Tube under the pier needed strengthening in order to prevent the Tube from moving in a major earthquake. In order to thicken the soil, BART engineers thought they would have to move the Farmer's Market and bore through the pier to get to the soil.

But after months of intensely detailed engineering, BART was able to determine the soil under the pier was stronger than originally thought and the only section of soil that needed thickening was the soil surrounding the base of the World Trace Center located at the far eastern end of the pier. That meant the Farmers Market, which sets up on the western end of the pier, can remain in place when the Tube retrofit work begins in early 2008.

Protecting the Transbay Tube is just one part of BART's 10-year Earthquake Safety Program. It is, however, one of the most crucial portions. The 3.6-mile Transbay Tube carries roughly one-half of the 330,000 riders who take BART each weekday.

Many people believe the Tube is earthquake safe because it withstood the October 17, 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, and possibly cause the soil under the Tube to liquefy.

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.

"We're in a race against time to get all this work done," said Vice President Sweet.

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:

This method involves using a device that will vibrate the soil around the Tube to compact 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.

This method is virtually the same as the vibro-compaction except that stone columns are added to the hole created by the vibro-compaction. This method would prevent the soil from liquefying in a quake.

This method pumps a liquid cement-like material into the soil, which then hardens and densifies the soil.

Work to strengthen the Transbay Tube on the Oakland end began mid-summer. Condon Johnson & Associates, Inc. - the construction firm doing the work - is using both the grouting and vibro-replacement methods to strengthen the soil around the Tube.

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 Earthquake Safety Program will retrofit the Tube as well as BART stations and more than 1,900 columns that hold up elevated tracks. The 10-year, $1.3 billion program is funded through a $980 million property tax measure that voters passed in November 2004, bridge toll funds, the Caltrans Seismic Safety Retrofit Program and BART general revenues.