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 were excavated. Marin Sanitary took the soil off-haul free of charge. On Thursday, April 14 Miksis Services repaired the end of the failed storm drain and the sinkholes were backfilled.
On Thursday, April 14, Nerviana Paving began grinding the road to prepare for paving. Paving plants were anticipated to be closed on Friday, due to pending rain but with the help of Nerviana Paving and Dutra Materials, paving was supplied and installed. Paving was completed midday on Friday and Fine Line Striping completed striping of the street, which allowed the bridge to reopen by approximately 3:30 pm on Friday, April 15.
FISCAL IMPACT
The cost for the emergency repairs is estimated to be about $175,000 to $200,000 in total. Funding for the emergency bridge and road repairs will come from the Emergency Fund.
CEQA
The Town finds this project to be statutorily exempt from the requirement for the preparation of environmental documents under the California Environmental Quality Act (CEQA) pursuant to CEQA Guidelines Section 15269 (Emergency Projects). Specifically, the emergency repair was undertaken by the Town to repair and restore an existing road damaged by land subsidence. (14 CCR §15269(d)). Additionally, the repair would be categorically exempt under the CEQA Class 1 exemption, because it involves maintenance of existing streets, will not create additional automobile lanes, and involves negligible or no expansion of use. (14 Cal. Code Regs. § 15301(c)).
CONCLUSION
Center Boulevard Bridge serves over 10,000 vehicles per day and is a main arterial road through San Anselmo and the Ross Valley. Caltrans inspects the bridges in Town on an annual basis. The bridge is approved for replacement by Caltrans and is structurally deficient. However, funding is delayed so it may be some time before a design is complete and the bridge is replaced. The Town inspects and has repaired other bridges; however, considering the issues associated with the undermining of this bridge by erosion and a failed storm pipe, the Public Works Department will do additional annual inspections of all bridges immediately following winter rains or large storms.
Staff believes that the undermining of this bridge may have occurred with the big storms in October during the flood event that occurred and by subsequent rains this past winter. Based on this, it is imperative that additional inspections occur beyond the routine annual inspection by Caltrans. In addition, staff has wanted to create a Storm Drain Master Plan (SDMP) which would look at all storm drains throughout Town but focused on CMP which are known to fail due to age and type of material. To create an SDMP the Town would need to allocate funding from existing funds or look at creating a drainage impact fee or parcel tax like other jurisdictions.
Town staff would like to thank Van Midde Concrete, Nerviana Paving, Miksis Services, Fine Line Striping, Marin Sanitary Services, Dutra Materials, Maggiora & Ghilotti, and Miller Pacific Engineering for their immediate response and assistance to the Town. Without this community response by local contractors, consultants, and Town staff and crew, the repair of the road in such an expeditious fashion would not have been possible.