As an increasing number of large diameter pipe bursting jobs are attempted, more is being learned about the capabilities and potential of pneumatic pipe bursting. The large diameter pipe bursting project in South San Francisco is one of the most challenging and innovative bursting projects ever attempted in North America. It is being heralded as a landmark pipe bursting job.
In 1999, the City of South San Francisco was confronted with a Cease and Desist Order from the Regional Water Quality Control Board mandating the elimination of severe wet weather overflows within ten years. The City completed a comprehensive evaluation of their wastewater collection, conveyance, treatment, and disposal system. Locally deficient sewers, undersized pump stations, and hydraulic restrictions in the effluent outfall system were identified as the major contributors to the wet weather overflow problem. A five-phase program was developed to address these conditions. Resolving the wet weather overflow problem was such a monumental task that many different elements were required. This massive “tool box” of solutions means that every presentation attendee should be able to take away valuable insights to benefit his or her own agency.
Construction of the first phase of improvements has recently been completed. Recognizing the importance of making significant strides early in the program, the first phase was the largest project in the group. Phase 1 Wet Weather Improvements included:
- Replacement of an existing pump station with a new 45-MGD pump station.
- Construction of a new 49-MGD pump station located alongside an existing pump station.
- Construction of 4,100 linear feet of 36-inch diameter force main (1,880 lf installed by pipe bursting from 27-inch to 36-inch); 6,860 linear feet of 42-inch diameter force main (525 lf installed by microtunneling); and 3,340 linear feet of 8-inch to 16-inch gravity sewer replacement (300 lf installed by horizontal directional drilling at Slope=0.0030).
- Expansion of the effluent pump station to the fullest extent possible without constructing a new facility – 60 MGD.
- Construction of a 7.5 million gallon effluent storage pond.
- Addition of 30 MGD influent pumping capacity.
This paper focuses on the design and construction of the force main installed by pipe bursting.
Design and construction challenges included construction sequencing, constrained sites, coordination with other projects, heavy rain and flooding during construction, soft, corrosive, and contaminated soils, high groundwater, congested utility corridors, heavy traffic, freeway crossings, and property issues. Initially, a route was chosen for the new force main to be constructed by traditional open-cut construction. Soils borings performed for the geotechnical investigation, however, identified a high potential for significant differential settlement and a need for extensive shoring that would include, at a minimum, interlocking sheet piles. The geotechnical engineer warned against constructing the force main by open-cut construction methods.
The design team then considered other alternatives for the force main. One part of the project involved constructing a new gravity sewer to replace a 27-inch diameter VCP sewer. An approach that involved utilizing the abandoned sewer as the host pipe for the new force main was developed. This approach would replace the 27-inch sewer with a 36-inch force main, installed using the pipe bursting process.