SAN FRANCISCO CSO

Today, even a light rain can overload San Francisco's combined sewer overflow (CSO) system, sending untreated sewage and stormwater into the bay and ocean. This is especially a problem in the Islais Creek Basin area, where excess combined sewage discharges into Islais Creek about 46 times per year. To reduce these occurrences, the city and county governments of San Francisco are constructing an extensive 900 mi long CSO control system that will consist of a series of underground reinforced concrete box sewers (transport/storage structures), tunnels, pipes and pumping stations to store the combined sewage until it can be pumped to a treatment plant. The Islais Creek transport/storage portion of the CSO system is about 11,000 ft long, and will add 32 million gal. of storage capacity and reduce overflows to an average of fewer than 10 times a year. Islais contract E consists of two tunnels in Bay stratum mud that connect the transport storage structure. To stabilize the excavation for the two tunnels, designers chose jet grouting in what is reportedly, the first application of jet grouting, at this scale, for full-face tunnel excavation in the U.S.