New Sunnydale sewer to avert floods
New Sunnydale sewer to avert floods Feb 2010
R. John Caulfield and Renée Fippin, Jacobs Associates
During heavy storms, flooding can impact hundreds of residents in the Sunnydale neighborhood of southern San Francisco. This flooding - a mixture of storm and sewer water - often overwhelms Sunnydale's existing 1910-era sewage system, which only has the limited capacity to convey the city's design storm flows. The San Francisco Public Utilities Commission (SFPUC) plans to resolve the problem with a new sewer tunnel named the Sunnydale Auxiliary Sewer, ranging from 8ft to 11ft (2.4m - 3.3m) in diameter.

Challenging project alignment

The SFPUC awarded Jacobs Associates the prime design contract for the Sunnydale tunnel in July 2007. Key members of the design team included Arup and AEW Consultants. Although the proposed tunnel alignment runs a relatively short 4,000ft, (1,219m) it is a challenging project, due to the highly complex geologic conditions and congested infrastructure.
On the eastern end of the alignment, the ground consists of Young Bay Mud.The central portion of the alignment transitions to Franciscan Formation bedrock, expected to consist primarily of chert with some sandstone and siltstone. The character of this rock can vary dramatically over short distances. Along the western portion of the alignment, it is anticipated that the tunnel will primarily traverse Colma sands below the water table.
Another twist in the geological mix is the presence of a contaminated site along the alignment, caused by pollutants released by adjacent inactive industrial plants. The effects of contamination on the soils and groundwater, site remediation, and handling and disposal of spoils and groundwater, are important construction issues.

Surface structures

The congested infrastructure in the project area requires special attention to protect nearby facilities from surface settlement. The project, located in an industrial area, will cross under Highway 101, as well as active CalTrain tracks and structures at the Norcal Waste Facility. There are also numerous utilities along the corridor that require special consideration.
The Jacobs Associates design team has completed the design efforts and the project has been advertised by the SFPUC. The bids are due in late March 2010 and the Notice-to-Proceed is expected in August 2010.
Procurement - TunnelTalk, Feb 2010

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