Lake Oswego Interceptor Sewer HDPE Submerged Gravity Flow Pipeline

Session 1A
10:50 AM

Jon Holland, Project Manager, Brown and Caldwell

The existing Lake Interceptor Sanitary Sewer system was constructed from concrete and cast iron pipe. Over 90 percent of the pipe lay within Oswego Lake and its associated bays and canals. The existing interceptor is undersized, which can result in overflows during heavy rains. Additionally, more than 9,000 feet of the line in the main portion of the lake is supported on steel piles, designed and constructed at a time when seismic design standards were much less stringent. With the corrosion of the steel pile system, engineering studies predict that seismic ground movement sufficient to cause widespread failure of the pile support system has a 15% chance of occurring within 25 years. Severe breaks in the pipeline could cause millions of gallons of raw sewage to flow into Oswego Lake, potentially overwhelming the Tryon Creek Wastewater Treatment Plant and spilling to the Willamette River. Replacement of the interceptor is critical to ensuring the environmental protection of Oswego Lake and maintaining sewer service for residents.

After receiving the recommendation from the city’s engineers, Lake Oswego chose to replace the existing pipeline rather than try to increase the capacity of and maintain the current system. The new sewer interceptor system uses HDPE pipes to resist corrosion, withstand earthquake impacts, and endure drought and flood conditions for more than 100 years. The new pipeline design uses both pile-supported pipes along with a submerged, buoyant, gravity-flow pipeline. The new interceptor is believed to be the first of its kind to use a submerged HDPE pipe held under the lake’s surface by ground anchors for conveyance of wastewater. Custom-measured and fabricated stainless steel tethers connect the ground anchors to tether brackets that hold the main pipe. Additional buoyancy is in place at specified grades to allow wastewater to flow by gravity to the wastewater treatment plant. The project consisted of 10,800 feet of 42-inch DR 13.5 PE4710 HDPE for the main sewer line. An additional 8,000 feet of 30-inch DR 13.5 was used for the buoyancy float pipes, and 2,000 feet of 24-inch and 22-inch pipe for laterals. HDPE pipe was selected for the project based on its flexibility, resistance to corrosion and leak‐free reliability.

Compared to other options considered by the city, the price tag for the in‐lake system using HDPE was approximately $20 million less than an around‐the‐lake pumped system. Additionally, the overall environmental impact of this project is much lower than many projects of a similar scale. Using the gravity-flow pipeline consumed fewer natural resources for construction and will take less energy to operate, which in turn creates a smaller carbon footprint for the system.

Joel Komarek, Project Director, Lake Oswego Interceptor Sewer (LOIS)
Jon Holland, Project Manager, Brown and Caldwell
http://www.lakeinterceptor.com