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HDPE’s Flexibility and Durability Required for Oregon Gravity Sewer
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| Pipe extends out into Oswego Lake, where it will be pulled by boat to its final location within the lake. |
The thought of sewage running through pipes in a scenic lake might scare some people. In northwestern Oregon, the Lake Oswego Interceptor Sewer (LOIS) has been piping wastewater to the City of Portland’s Tryon Creek Wastewater Treatment Plant since the 1960s. However, the LOIS system that has performed admirably for five decades has two major problems – capacity problems due to an increase in population in the Oswego Lake area and worries about structural soundness should an earthquake occur in the vicinity.
Earthquakes have been the primary focus of worldwide news recently, with the unfortunate situation in Haiti. The Caribbean isn’t a primary focus of earthquake activity, but nonetheless is affected by several major fault lines in the area. The Cascadia region between Oregon and British Columbia is often overshadowed by the more active San Andreas Fault, but is still susceptible to major earthquakes as well. An October 2009 report in Time magazine, reports that the Pacific Northwest is one of five locations around the world where the next big earthquake could occur. In fact, the area suffered an earthquake in 1700 that was possibly as severe as a 9.2 on the Richter Scale.
The LOIS project communications team filmed a short informative clip regarding the fusion process to explain the benefits of pipe fusion to their constituents. View the YouTube clip by clicking the screen capture above. The movie will open in your web browser.
The old LOIS piping system has steel pile supports and hardware that are corroding and are under risk of collapse should seismic activity occur. In the case of a sizeable earthquake, millions of gallons of untreated wastewater would flow into the lake and lake water would overrun the Tryon Creek treatment plant.
Locals in the service area will soon be able to sleep a little easier about capacity and structural issues. The LOIS system is undergoing a $110 million renovation that will include 10,581 feet of 42-inch high-density polyethylene (HDPE) pipe to replace smaller 16- to 36- inch pipe. Smaller pipes were originally used when the project was designed for a smaller service area. The population growth has dictated that LOIS increase it's capacity as well.
The LOIS is a gravity sewer that uses changes of elevation from the west to the east side of the lake to keep material moving through the line without the use of pumps. At the end of the line, the contents are and will continue to be processed at the Tryon Creek Wastewater Treatment Plant.
The piping system will be attached to the lake bottom with anchors and tethers, which allow the buoyant HDPE to float in a controlled manner 8 to 17 feet below the surface. With such emphasis placed on flexibility and durability, HDPE was the only buoyant piping material that could meet the LOIS project’s goals.
The large temporary trestle extends out into Oswego Lake and is the base for the fusion operations. “We designed LOIS to resist an earthquake with 10 percent risk of occurrence in 100 years (a 1,000-year event),” said Jon Holland, vice president and engineer for Brown and Caldwell, the environmental engineering firm for the project. “With deep (up to 200 feet), soft sediments overlying bedrock, it was difficult and costly to make a pile support system work in much of the lake, although we did use piles with HDPE in some areas where water depth was insufficient for the buoyant system. The only alternative to a structure to hold the pipeline up is a buoyant system to hold the pipeline down. We went with the buoyant system in equal parts because of its cost-effectiveness relative to piles and because of the seismic-resistance benefits inherent in its flexibility. We certainly viewed HDPE as an essential element in the flexibility and feasibility (due to its low unit weight) of the buoyant system. Fused joints, toughness to resist impact from pleasure boat anchors and corrosion resistance all factored into the decision to use HDPE.”
In one of the most environmentally conscious parts of the United States, all happenings of the LOIS project are updated via website and social networking outlets for locals in the area and others keeping a watchful eye on the project. According to the project’s website (www.lakeinterceptor.com), HDPE was chosen over steel, ductile iron, PVC, carbon fiber reinforced resin, fiberglass and concrete for the following reasons:
- It’s tough: HDPE has a 50-year track record in severe marine environments – offering impact resistance to anchors from recreational boats on Oswego Lake.
- It’s strong: The walls of the new pipe will be more than three inches thick to resist 2.5 times the maximum water pressure for 100 years and ensure that nicks, cuts and scrapes are structurally insignificant.
- It’s flexible: HDPE’s flexibility makes it the ideal material for seismic events and handling the pipe during installation.
- It can’t corrode: Materials such as concrete, steel and ductile iron would likely corrode in this environment.
Fusion Process
With HDPE pipe the natural selection for the project, engineers and contractors needed a way to get the pipe into Oswego Lake. Advanced American Construction, lead contractor on the project, built a large L-shaped dock to be a multi-functional base for construction.
Approximately 150 feet out from shore, a McElroy MegaMc® 1648 machine provided by Ferguson Industrial Plastics was staged in an elevated position to bring it to a level of a custom-fabricated winch and pipe rolling system that ran the length of one arm of the dock.
In lieu of typical pipe stands, the contractor built a custom pipe-rolling system and elevated the McElroy 1648 machine to the level of the pipe tracks.
As each 50-foot stick of Driscoplex pipe was about to move into position, workers from Advanced American Construction would use the winch system to gently maneuver the length of pipe into the water. The buoyancy of the water often pulled the pipe into the water without much assistance from the winch.
The Pacific Northwest’s persistent rains required that a shelter be created on the dock to keep water from contaminating the fusion joints. One of the design features of the MegaMc 1648 is the hydraulic pivoting heater and facer. This feature allows the machine to be tented and for work to continue in most weather conditions.
The delicate nature of passing sewage through pipes gave McElroy’s Wireless DataLogger® an opportunity to shine. The DataLogger is used to record and document key parameters of the fusion process. The device can be used to verify that proper fusion procedures were followed prior to installation.
Timeline
The massive LOIS project began with pre-design studies in 2002 by Brown and Caldwell, Inc., an environmental engineering firm.
In August 2007, the Lake Oswego City Council recommended the replacement of the LOIS system. Work began at the lake in fall of 2008, and will continue with a full lake until September 2010, when the lake will be drawn down to make some installations easier. The project’s completion is scheduled for the later part of 2011.
Sincerely,
Tyler Henning
P.S. – Do you have an interesting job site that you would like to share? McElroy is always looking for fusion job sites where HDPE is being used and fused to solve an infrastructure problem. Contact Tyler Henning, public relations specialist at (918) 831-9286 or by email at thenning@mcelroy.com
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