Underwater interceptor

January 2011 » PROJECT CASE STUDY
Submerged, buoyant HDPE pipeline provides gravity-flow for wastewater collection system.
Steve Cooper
Engineers designed the HDPE pipeline to allow for 14-foot expansions and contractions during temperature changes that could vary 35 to 40 degrees Fahrenheit, depending on seasonal shifts. To allow for these shifts, the pipeline was formed in S-Curves to limit horizontal movement.

The new wastewater interceptor line being built in Lake Oswego, Ore., is the first of its kind to use a submerged pipeline in a gravity-flow design to convey wastewater. Located in northwest Oregon, 8 miles south of Portland, the area is primarily a residential community of 35,000 residents and a 405-acre lake. High-density polyethylene (HDPE) pipe, which will create a corrosion-resistant pipeline with an estimated life cycle in excess of 100 years that can withstand earthquake activity, was selected for the project. The Lake Oswego Interceptor Sewer (LOIS) project started in October 2009 and is expected to be completed in mid to late 2011.

Project
Lake Oswego Interceptor Sewer, Lake Oswego, Ore.

Civil engineer

Brown and Caldwell

Product application

Submerged gravity-flow wastewater pipeline uses Performance Pipe’s 42-inch-diameter HDPE pipe.

After nearly a half century of serving the Lake Oswego community, the in-lake, concrete and cast iron interceptor system installed in the 1960s needed to be upgraded. The system was corroded, undersized, and at risk of failing in an earthquake. If a collapse were to occur, millions of gallons of untreated wastewater would enter the lake and millions of gallons of lake water would drain downstream to the receiving treatment plant, overwhelming its hydraulic capacity.

Between 2000 and 2006, engineers at Walnut Creek, Calif.-based Brown and Caldwell evaluated options for addressing the weaknesses in the existing interceptor and concluded that replacing the existing system was the best way for the city to proceed. In 2007, after many public hearings and community briefings on replacement alternatives, the city council accepted the recommendation to replace the system with a combination of pile-supported pipe and a submerged, buoyant gravity-flow pipeline.

“The [LOIS] project is using HDPE pipe in a new application,” said Tony Radoszewski, executive director of the Texas-based Plastics Pipe Institute Inc. (PPI), a technical, engineering, and industry knowledge resource and non-profit trade association that publishes data for use in development and design of plastic pipe systems. “Not only does the project further the application of HDPE pipe and impact future wastewater projects, but it also demonstrates the unique benefits and flexibility of HDPE.”

According to Radoszewski, engineers evaluated alternative plans for going around Lake Oswego or using different materials for the in-water interceptor, but decided to use HDPE to go through the lake because it was the best material to withstand corrosion, would result in the fewest number of joints, and could function as a submerged, buoyant gravity-based system to move wastewater.

Joel Komarek, project director of the city of Lake Oswego, stated in an online LOIS project video, “We chose [HDPE pipe] because it’s impact resistant and corrosion resistant and better than all the other pipe materials we looked at.”

The new sewer interceptor is the first to use a submerged HDPE pipe held under the lake’s surface to convey wastewater.

The main sewer line will use 10,800 feet of 42-inch-diameter, DR 13.5 HDPE pipe. The lake will contain 8,000 feet of 30-inch-diameter, DR 13.5 HDPE pipe that will be used to construct the buoyancy float system. For the laterals to connect the main line, 2,000 feet of 24-inch-diameter and 22-inch-diameter pipe will be used.

Performance Pipe, a division of Chevron Phillips Chemical Co., LP, manufactures the pipe used and is a PPI member. Other PPI members participating in the LOIS project include Ferguson Waterworks and Ferguson Industrial Plastics.

“HDPE pipe will enable creation of a pipeline with the lowest projected 100-year life-cycle cost because it won’t rust and can withstand seismic activity and still perform under drought and flood conditions that would lower and raise the level of water in the lake,” said PPI’s Radoszewski. “Another favorable attribute of this design is that using HDPE pipe to construct a shorter, less-disruptive gravity-flow pipeline is a ‘low-impact-development’ approach. It will consume fewer natural resources during construction, limit digging up land, and it will take less energy to operate, all of which produces a smaller carbon footprint.”

Designing and constructing
The engineering team designed a strong pipe system utilizing HDPE. Fifty-foot pipe lengths were fused together into 1,200 to 1,500-foot segments. With walls 3 inches thick, the pipe can resist more than 2-1/2 times the maximum water pressure.

Engineers designed a pipe system that allowed for 14-foot expansions and contractions of the pipe during temperature changes that could vary 35 to 40 degrees Fahrenheit, depending on seasonal shifts. To allow for these shifts, the pipeline was formed in waves or “S-Curves” to limit horizontal movement. Engineers also had to control the upward and downward movement by holding the pipe under the lake surface using ground anchors connected to pipe tether brackets, which held additional buoyancy pipes to ensure proper tension of the wire-rope tethers under all design conditions — from empty through full flows with 8 inches of debris in the pipe. These and other design considerations ensure that the pipe will continue to maintain the proper 7-foot slope along the 2-mile-long pipe run to deliver wastewater through a gravity system on its way to the treatment plant.

To float the pipeline, 30-inch-diameter buoyancy pipes filled with air create additional upward buoyant forces to maintain the required elevation for gravity flow and control tension on the tethers. HDPE was selected because of the pipe’s inherent buoyancy in water.

The new interceptor system will be held under the lake’s surface by ground anchors. Custom-fabricated stainless steel wire rope tethers connect the 428 ground anchors to tether brackets that hold the main pipe and additional buoyancy “baskets” in place.

The ground anchors were drilled through lake-bottom sediments, sometimes hundreds of feet thick, and grouted into bedrock. Every installed anchor was tested to withstand 150 percent more load than needed. Anchor spacing was limited to 25 feet on center and the anchors could not vary from specified coordinates by more than 6 inches in the XY position regardless of depth. At this spacing, the upward deflection or “sea-serpenting” of the HDPE pipe between anchors was limited to no more than one-quarter inch. These exacting tolerances will ensure proper slope and flow velocities in the pipe to minimize future cleaning.

Plans for future infrastructure
“Our city, like all the cities in our region and like tens of thousands of cities across our nation, has an aging infrastructure,” said Lake Oswego Mayor Jack Hoffman. “If we don’t responsibly invest and maintain it, we risk potential loss of property or life that could financially burden our citizens. Clean water, safe sewerage systems, and good roads are just part of our paramount responsibilities.”

The cost of the in-lake portion of the new system is estimated at $95 million, which is $25 million less than an around-the-lake pumped system. The city is financing the project through revenue bonds; residents will see a 30-percent rate increase for the next two years, followed by a 17-percent and a 14-percent increase to pay for the new system.

“In 2010, Lake Oswego celebrated its centennial,” Hoffman said. “For me, this was a time to contemplate [not only] how far we’ve come with our infrastructure, but [also] how much attention we need to continue to give to it in order to ensure that our investment lasts. The city of Lake Oswego is fortunate for the good decisions of the past 100 years that have created, protected, and enhanced our property values and the quality of life we continue to enjoy today.”

Video updates of the project are available at the LOIS website: www.lakeinterceptor.com

Steve Cooper is president of SCA Communications, based in Baldwin, N.Y. He can be contacted at steve@scacommunications.com.

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