Winding Underwater

High-density polyethylene pipe provides the answer to the challenge of replacing and upsizing a sewer interceptor beneath an Oregon lake

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The 24,000-foot-long concrete and cast-iron Lake Oswego (Ore.) interceptor sewer (LOIS) had insufficient capacity to handle development and peak wet-weather flows, resulting in surcharged manholes.

 

Part of the gravity-flow system traverses Oswego Lake. The 9,700-foot-long buoyant middle section floats 14 to 21 feet below the surface. On either end of it are 5,200 feet of submerged pile-supported pipe. A modest earthquake could cause the corroded steel pilings and hardware to fail, causing an ecological nightmare.

 

The City of Lake Oswego, under a state Department of Environmental Quality mandate to replace the interceptor by 2012, retained Brown and Caldwell Inc., an environmental engineering firm with a local office, to design a solution. Vice president Jon Holland recommended upsizing and replacing the interceptor with another in-lake gravity-flow system. With the emphasis on flexibility, buoyancy, and durability, high-density polyethylene pipe (HDPE) was the only material to meet the project’s goals.

 

Compound challenges

Constructed in the early 1960s, LOIS has 40 manholes and includes more than 5,000 feet of smaller tributary lines constructed through canals and bays. The system, designed to handle flows from 3,500 developed acres, serves more than half the city’s businesses and residences, or nearly 4,500 developed acres. The interceptor increases from 16 to 36 inches along a 7-foot slope to the City of Portland’s Tryon Creek Wastewater Treatment Plant.

 

Part of the estimated $110 million renovation is upsizing 18,868 feet of the interceptor to 42-inch pipe submerged 8 to 17 feet below the surface of the lake. Water temperatures varying up to 40 degrees F will lengthen and shorten the pipeline by 14 feet, so tethers anchored to the lake bottom restrain it from bowing up, the buoyant plastic restrains it from bowing down, and installation in an S-curve limits side-to-side movement.

 

“With up to 200 feet of soft sediment overlying bedrock, it was difficult and costly to install pilings in much of the lake, which is why buoyant HDPE was essential,” says Holland. “The pipe and fused joints are corrosion- and impact-resistant, enabling them to take hits from recreational boat anchors, while being flexible enough to withstand seismic activity.”

 

LOIS is designed to resist an earthquake with a 10 percent chance of happening in 100 years. The pipe walls are 1.75 inches thick to withstand 2.5 times the maximum water pressure. Closely spaced rock anchors, tested to 150 percent of extreme load, control any upward movement of the pipe to one-quarter inch. Stainless steel wire rope tethers provide 30 percent more strength than needed.

 

“To enhance maintenance, the new system has a maximum manhole spacing of 1,600 feet,” says Holland. “The existing system has a manhole every 2,700 feet and a joint every 32 feet. We specified a joint every 1,000 feet to reduce opportunities for failure.”

 

Assembly logistics

Advanced American Construction Inc. (AAC), the general contractor from Portland, built a large L-shaped trestle dock, then set up a custom-fabricated winch and roller conveyor on the long leg. A Manitowoc crane, staged at the junction of both legs, moved 50-foot sticks of Driscoplex DR 32.5 PE 3408 pipe into position and took receipt of shipments as they arrived.

 

Workers elevated a McElroy Manufacturing Inc. MegaMc 1648 fusion machine from Ferguson Industrial Plastics (FIP) in Washougal, Wash., to align with the winch, then used the winch to pull the fused pipe gently along the conveyor and into the water.

 

“The water’s buoyancy often pulled in the pipe without much assistance from us,” says David Marcum, FIP pipe fusion technician. “However, we had to build a tent over the fusion machine to protect it from the persistent rains, which would have contaminated and ruined the joints.”

 

The FIP team used a wireless McElroy DataLogger to record and verify that they followed proper fusion procedures. The 1,000-foot lengths were anchored to piles near the shore until a tugboat towed the segments to their installation point.

 

Bending the pipe lengths into S-curves required a 40- by 10-foot barge and a 300- by 1,200-foot space on the lake. Using winches, the barge pulled the pipe into the first curve. Internodal cables kept the shape intact. The barge then repeated the process on the opposite side of the pipe. Seven AAC commercial divers attached the length to tethers, four dives per attachment.

 

Pile-supported pipe lengths will be installed during the Lake Down phase beginning in September 2010. The project’s completion is scheduled for between the summer of 2011 and the end of the year.



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