February 2014

Sidewalk Sinks in Mission Bay

Keith Burbank

An entire block of sidewalk fronting 1401 Fourth Street—the Strata at Mission Bay building—has sunk three to four inches below the structure’s base. The sidewalk was laid just two years ago; it may have begun sinking soon after it was poured. Buildings in Mission Bay are built on Bay mud; the ground underneath the sidewalk may have settled when moisture in the clay material was displaced. Fortunately, the sidewalk’s movement hasn’t affecting the edifice’s integrity.

“We have been advised that the settlement of the sidewalk has not damaged the structure,” said Rachel Gordon, spokesperson, San Francisco Department of Public Works. According to DPW, the Strata sits on piles, and isn’t moving. The sidewalk, however, lies on compressible soils, which are sinking.

At one time Mission Bay was a tidal cove connected to San Francisco Bay. But beginning in 1859, the cove was filled-in with sand from other parts of the City, according to Vanished Waters: A History of San Francisco’s Mission Bay by Nancy Olmsted. Today the former cove is increasingly populated by housing developments, and the University of California, San Francisco-Mission Bay campus, with a new hospital set to open early next year. All that’s left of the tidal cove is Mission Creek, which stretches from AT&T Park to Seventh Street.

According to Justin Whitsitt, Strata at Mission Bay’s property manager, the sidewalk’s subsidence never posed a risk to the apartment building’s residents. Though plans to repair the sidewalk are underway, nothing has been approved by the City yet, Whitsitt said.

“A minor encroachment permit application has been submitted to Public Works’ Bureau of Street Use and Mapping to provide a temporary wood ramp for one of the doorways,” Gordon said by email. “We have been informed that drawings are under way to provide an interim solution at all building entrances by use of pavers and concrete.” One wood ramp was already in place last month. “We expect that after construction of the interim work, drawings for a long-range solution will be prepared to replace the sidewalk,” Gordon added.

Michael Hamman, a local general contractor, said most likely the developer failed to adequately supervise the sidewalk work. “That’s not normal,” he said of the subsidence. Hamman said the ground underneath the sidewalk is supposed to be compacted and tested before concrete is laid. If not, or if there are voids in the underlying ground, the sidewalk can collapse.

According to Darrin Ketter, regional manager, Sares Regis Group, which manages Strata, neither the company nor the building’s residents have concerns about the structure’s integrity or the safety of residents. He said that brick pavers will serve as either a temporary or long-term solution to the sunken sidewalk, with permits for pavers filed late last year. Work is likely to begin this month.

   

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