The Lake Oswego Interceptor Sewer is no electron supercollider in terms of complexity, but its design has received serious scrutiny by a team of engineers to ensure it provides reliable, long-term service. Last week's column (by Joel Komarek, project director for the city of Lake Oswego's LOIS project) addressed how the principles of gravity, thermal expansion and buoyancy are guiding the LOIS pipeline design.
This week, the focus is on interceptor component design and how the LOIS replacement will be stronger, more corrosion resistant, more flexible and more seismically resistant than the existing pipeline.
The sewer pipe: The design team evaluated a wide range of materials to find the most suitable for the environment (the lake) and for the contents it will carry (sewage). These materials included steel, ductile iron, polyvinyl chloride (PVC), carbon fiber reinforced resin, fiberglass, high density polyethylene (HDPE), and concrete.
The team determined that HDPE pipe is the ideal material:
It's tough. A 50-year track record in severe marine environments demonstrates its impact resistance. Unlike more brittle materials, HDPE will not be damaged by the anchors from recreational boats on Oswego Lake.
It's strong. Fifty-foot pipe lengths joined into 1,500-foot segments by heat fusion welding results in a continuous, jointless pipe, unlike the existing interceptor, which has a gasketed, push-on joint every 32 feet. The walls of the pipe are over 3-inches thick to resist more than 2.5 times the maximum water pressure for 100 years and to ensure that any nicks, cuts, or scrapes are structurally insignificant.
It's flexible, making it the ideal material for seismic events and handling during installation.
It can't corrode, unlike concrete, steel and ductile iron.
The buoyancy pipe: The smaller buoyancy pipe below the sewer maintains the proper grade for gravity flow in all conditions. This pipe is made of the same HDPE material as the sewer pipe and is designed to maintain buoyancy in typical operations and through extreme circumstances. For instance, the system will remain buoyant even if the sewer is full of sediment and the outside of the pipes are covered by an invasive species like zebra or quagga mussels.
Anchors: Anchors are needed to hold the pipe solidly into the bedrock under the lake. A wide range of anchor types were considered. Rock anchors were selected because they are assured to stay in place in the lake's unique geologic conditions. They have a long track record of performance in critical facilities such as dams, buildings, bridges and tunnels. Upon installation, every installed anchor will be tested to 150 percent of the extreme load condition to ensure proper installation. By keeping the anchors close together, any upward movement of the pipe is controlled to ¼ inch over 100 years.
Tethers and Brackets: Made of stainless steel wire rope, the tethers holding the pipe down will also be sized to handle extreme load conditions. For instance, if the sewer pipe were entirely empty and half the anchors were out of service, the tethers are still designed to provide 30 percent more strength than needed. If anything unanticipated should disrupt the final grade of the pipeline, it can be fine-tuned by adjustable turnbuckles on each tether. The tether brackets connect the tethers to the sewer and the sewer and buoyancy pipes to each other. All bracket parts will be HDPE and stainless steel to prevent corrosion.
Seismic: The entire system is designed to survive without any repairs an earthquake with a 200-year average recurrence interval and with only minor repairs an earthquake with a 1,000-year average recurrence. This high level of protection comes at no additional cost as the buoyant approach provides an inherently flexible, seismically resistant system.
For more information about the LOIS Project, go to ttp://www.loisnews.com . And watch this column in future weeks for answers to further questions about this critical pipeline.