Canadian Consulting Engineer
Pipe Bursting for the Millstone Sanitary Trunk Sewer, NanaimoEngineering
Dayton & Knight, West VancouverCategory: Water Resources and TreatmentThe Millstone sanitary trunk sewer was constructed in 1976 and services about a quarter of the 72,000 residents of Nanaimo, B.C. F...
Dayton & Knight, West Vancouver
Category: Water Resources and Treatment
The Millstone sanitary trunk sewer was constructed in 1976 and services about a quarter of the 72,000 residents of Nanaimo, B.C. Flows in the sewer had increased to the extent there was a danger it would overflow into sensitive fish-bearing watercourses, wetlands and residential areas.
Several options for rehabilitating the pipe were considered. Building a parallel sewer through traditional open-cut construction was unacceptable to environmental agencies due to sensitive fish habitats in the adjacent Millstone River. Another option of diverting flows into other sewerage areas meant constructing five pump stations and would have resulted in long-term costs of $300,000 a year.
The adopted pipe bursting approach solved both these problems and is the largest and most extensive pipe bursting project undertaken in Canada, and one of the most complex in North America to date.
The Nanaimo sewer travels close to the shore of Brannen Lake and through 19 private properties, including a horse farm with horses valued in excess of $250,000 and a federal correctional facility.
Another complication was the scheduling of the work, 45 per cent of which had to be completed within a two-month window to protect the fisheries. The project team developed mitigation measures that reduced the amount of work undertaken during the two-month window, such as the use of a portable sedimentation tank to process the excavation’s de-watering liquid before it was released into Brannen Lake.
Despite the complexities, the project was completed in October 1999, one month ahead of schedule, and at a cost of $4.8 million — $600,000 under budget and $1.0 million less than the open cut alternative.
The pipe bursting method involves a bullet-type expansion head that is pulled through the existing pipe. As the head proceeds it breaks apart the old pipe, pushing the pipe fragments and surrounding soil outwards and at the same time pulling the replacement pipe into place. Excavation is not needed except at the entry and exit pits, which are located at manholes.
Pipe bursting is considered an unproven method and many municipalities are still reluctant to use it. In this case, the city of Nanaimo assumed part of the risk from the contractor by awarding a demonstration section first to test the feasibility of the approach before committing the contractor to the entire program. Research involving the monitoring of the pipe strain and ground movements was undertaken in conjunction with the University of Alberta Department of Civil Engineering.
The project involved upgrading approximately four kilometres of 350 mm diameter asbestos cement sanitary pipe to 650 mm diameter high-density polyethylene pipe (HDPE). The application was considered well beyond the limits of typical pipe bursting installations. Typically, pipe bursting is used to upsize pipes by 20% to 40%. Lengths are usually in the range of 100 to 120 metres. The Nanaimo project required upsizing of nearly 100% and installed lengths of 150-180 metres.
The greatest difficulty occurred when the bursting reached the first of seven deep sections and the team encountered a thicker pipe than they had expected. The process was stopped at this level and tests revealed that the load was about 10% greater than the machine’s capacity. Dayton & Knight kept detailed records of the pull loads on each burst segment, which were developed into graphical form and plotted to show that installation depths greater than three metres would exceed the machine’s capacity. With this information, the contractor modified the bursting head and used trenchless directional drilling to reduce the soil pressure above the pipe.
The B.C. Environment and Department of Fisheries and Oceans personnel were so impressed with the technique they are now supporting its use in other environmentally sensitive areas.
The project won the Lieutenant-Governor’s Award 2000 and Award of Excellence from the Consulting Engineers of British Columbia.CCE
Project name: Pipe Bursting of the Millstone Sanitary Trunk Sewer
Award winner: Dayton & Knight Ltd., West Vancouver (prime consultant)
Project team leaders: Brian L. Walker, P.Eng., Jack Lee, P.Eng., Richard Harper, P.Eng.
Client/owner: City of Nanaimo/Bill Sims
Other key players: Terraco Trenchless Technologies (principal contractor); Alberta University Dept. of Civil Engineering/ Dr. Sam Ariaratman (monitoring), Triton Environmental Consultants (environmental monitoring and liaison)