Shimmick Construction Company along with its Joint Venture Partner, Obayashi Corporation Inc., successfully completed this very large and complicated underground box sewer in the City of San Francisco. This project was one of five projects that together made up the Islais Creek Facilities. The Islais Creek Facility is for the storage of combined (rain water and sewage) sewage during wet weather in the City of San Francisco. San Francisco has an unique problem in that its storm water and its sewage utilize the same underground pipelines. Because of this, whenever it rains the San Francisco sewage treatment plants get overwhelmed with the amount of sewage that they must process. So much so, in fact, the City releases untreated sewage to the bay and the ocean during some storms because it is just too much flow for the system to treat. The Islais Creek Project provided a large underground storage facility in order to “buffer” the large amounts of rainwater so that the number of overflows of raw sewage would be minimized and the stored sewage/rainfall could be treated at the capacities of the existing treatment plants.

Contract “C” is a 6,000 feet long underground cast-in-place-concrete box structure. The box varied from 40 feet deep and 27 feet wide to 25 feet deep and 16 feet wide. The structure was built inside a shored excavation. The shoring for the excavation varied depending on the geotechnical characteristics of the ground. Because the project site was so long, the soil characteristics varied a great deal. In some areas, very large and long interlocking sheet piles were used as the shoring walls. The sheet piles were supported by a waler and strut system made from wide flange and H-pile sections. In another portion of the structure, the excavation shoring consisted of rock bolts and shotcrete. Furthermore, another portion of the structure had soldier pile and lagging as the means of excavation support with the same type of waler and strut system as the sheetpile area.

The structure was constructed from the bottom up with a series of concrete form travelers. The box was constructed on two separate headings, the 27 feet wide heading and the 16 feet wide heading. First the invert was poured in 50 feet long sections, then the walls were poured all the way to the soffit, lastly the soffit was poured. Once the structure was complete, it was backfilled with soil approximately 4 to 12 feet deep. Surface improvements were than constructed to rebuild the area.

Many storm drain tie-ins to the existing sewage and storm drain systems were constructed to link the new storage box. Also many new storm drain lines and water lines were relocated to provide the right-of-way for this storage box to be constructed. To further complicate this project, most of the construction took place in the middle of the busy streets in San Francisco’s Bay View / Hunters Point Area.