The following is a continuation of information being provided by the city of Lake Oswego about the Lake Oswego Interceptor System. Today's format comes in a question-and-answer form.
Ask LOIS: Why are portions of the interceptor designed as a buoyant system and other portions pile-supported?
Some parts of the interceptor replacement pipeline are proposed to be supported on piles below the lake's surface and some are proposed to be buoyant and tethered to the bedrock beneath the lakebed. Since the buoyant portion is less expensive to build, some citizens ask why not just make the entire pipeline buoyant and tethered. This is sometimes followed with 'Since one reason the existing interceptor is being replaced is that the piles won't hold up in an earthquake, why would piles be used again?'
The pile-supported sections are at the west and east ends of the main lake and at four locations on the north and south shore where on-shore trunk lines enter the lake. These areas are shown on the project map at http://www.loisnews.com/ . At each of these locations, the pipe is either below or only slightly above the existing lake bottom. For the buoyant system to be effective, it needs at least 10 feet of water beneath the pipe to ensure the proper grade for gravity flow as the lake temperature varies from season to season. To make the buoyant system work in these areas the city would have to excavate and remove lake sediments. However, the city's permit from the U.S. Army Corps of Engineers requires restoring the lakebed to its existing contours. Therefore, the buoyant system is not possible in these locations.
Since the buoyant system won't work in these shallow areas, why not support the pipe directly on the lakebed without driving piles?
There are two reasons. First, in many locations, the lake sediments are too soft to support the pipe system, which would have to include heavy concrete anchor blocks to keep it from floating up out of the lakebed during summertime low flow, when the pipe is the lightest. Second, in an earthquake, much of the existing lakebed in these areas will 'liquefy' and would not be able to support the pipe system.
That leaves pile supports as the only feasible option. The new piles, however, will be substantially different than the existing piles. They will be designed to resist the 1,000-year earthquake and they will be protected against the effects of corrosion, two of the fatal flaws in the existing system.
An added benefit of pile supports is that future drawdowns of the lake will not affect operation of the sewer system. The Lake Oswego Corporation periodically draws the lake down 10 to 12 feet to allow property owners to make repairs to their docks and boat houses. This level of drawdown will expose much of the upstream reaches of the pipeline, which, if buoyant, would result in sags and require sewer cleaning. With the planned pile system, the grade of the trunks and interceptor will be maintained.
Questions? Call 503-699-7466 or go to http://www.loisnews.com/ .
Jane Heisler is Communications Director for the Lake Oswego Interceptor Sewer Project.