FROM:
Sean Condry, P.E., CBO - Public Works and Building Director
SUBJECT:
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Emergency Sinkhole Response at Center Boulevard Bridge
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RECOMMENDATION
recommendation
That the Council acknowledge the emergency repairs required by the undermining of Center Boulevard Bridge at Sycamore Avenue.
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BACKGROUND AND DISCUSSION
On Monday, April 11, 2022, a business owner in San Anselmo notified Central Marin Police Authority (CMPA) about a sinkhole 3'-4' wide on the southwest corner of the Center Boulevard Bridge. Ross Valley Fire Department responded quickly by shutting down the Center Boulevard Bridge with the assistance of CMPA. Utilities were notified along with Public Works at approximately 3:30 pm and Public Works staff and crew responded immediately.
Upon inspection by Town staff, it was determined that the sinkhole was significantly larger underground and had undermined about two-thirds of the Center Boulevard Bridge abutment foundation on the west side of the bridge. The undermining appeared to be caused by a failed corrugated metal storm drainpipe (CMP) from Sycamore Avenue along with erosion on the creek side of the abutment. Ross Valley Sanitary District assisted by videoing the failed storm drain for the Town.
The bridge, street, and adjacent private parking lot were immediately barricaded off by Public Works crew while staff proceeded to move forward with design and repair options. Center Boulevard and the bridge serve over 10,000 vehicles per day. Center Boulevard is a main arterial roadway in San Anselmo and its repair was a high priority for the Town. On the morning of Tuesday, April 12 Van Midde Concrete began repair work of the undermined abutment foundation by excavating earth and installing rebar for a new below-grade retaining wall that sistered and underpinned the existing abutment.
On Wednesday, April 13 the sistered abutment and underpinning concrete were completed while the parking lot sinkhole and undermined street ...
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