BART’s plan to repair tunnels involves bentonite and a ballot measure

Written by Mischa Wanek-Libman, editor
image description
A broken rail in one of BART’s tunnels.
BART

"Save the rainforest" is not a mantra embraced by Bay Area Rapid Transit (BART) maintenance crews. Their "rainforest" is a nickname bestowed on the miles of underground track in downtown San Francisco that have developed problematic water intrusion issues following more than 40 years of use.

 

BART explains that water has made its way through caulking material injected through steel plate holes to serve as insulation and a liquid seal when the tunnels were built in the 1960s. To fight these leaks, BART repair crews have been repacking the joints over the years with a waterproofing compound. However, the compound degrades relatively quickly and must be checked often.

The transit district explains that the BART system already has pumps installed in the lowest tunnels and highest-volume underground streams, but a larger drainage system isn’t feasible because draining that much water could cause the sinking or collapse of Market Street. Additionally, micro fractures can form by condensation being forced into the rail steel under the weight of fully-loaded trains.

“In constantly wet conditions, these micro fractures can grow and grow – the same way a pothole forms as cars run over the same weak spot in the road over and over. Eventually, these microscopic imperfections can actually cause the rail to break,” said BART.

Engineers are exploring options for a more permanent fix to keep BART dry and they believe the current answer lies with bentonite, a naturally occurring clay compound known for waterproofing tunnels.

BART points out that the project scope for such a project is “huge,” requiring miles of tunnels to be repaired with complicated engineering challenges.

Cost is also an issue. BART estimates repairing the tunnels and structures would cost $570 million and the transit district hopes a measure on the November ballot will be approved to pay for the project. Measure RR would raise $3.5 billion to help cover the cost of big-ticket repair projects, with the money statutorily dedicated to these capital improvements and protected by an independent audit committee.

“The bones of BART—including assets beyond leaking tunnels such as eroded trackway and deteriorating electrical cabling—are decaying,” the transit district said. “Waterproofing our tunnels is definitely one of those big-ticket repair items, because keeping water out keeps [riders] safe on the way to your destination. Plus, having dry tunnels lowers the cost of maintaining BART to taxpayers in the future.”

Measure RR funds would pay for BART’s planned system overhaul to repair and replace the agency’s 90 miles of deteriorating rail, corroding tunnel walls and aging speed control equipment.

BART said the funds spent to repair damage to key structures would support continued passenger safety and reliable BART operations. In addition to repairing water intrusion in the Market Street tunnels, the funds would fix water intrusion at 16 stations and repair the Berkeley Hills Tunnel, which shifted alignment due to the continuous movement of the Hayward Fault near the western edge of the tunnel.

More information on BART’s renewal plan is available at http://www.bart.gov/better-bart.

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