UNDERWATER SEWER REPLACEMENT OPTIONS – NO EASY ANSWERS IN, UNDER, OR AROUND LAKE OSWEGO, OR
Abstract:Prior evaluations of the in-lake portion of the City of Lake Oswego's Lake Interceptor sewer system determined that nearly 19,000 feet of the existing 12-inch to 36-inch-diameter cast iron and concrete cylinder pipe have serious hydraulic capacity and structural support deficiencies. The capacity deficiencies have resulted in overflows of untreated sewage during severe wet weather conditions on a few isolated occasions. Continued growth will result in greater frequency of these events. The line at both the west and east ends of the lake is buried while much of the line in the main lake, more than 9,000 feet, is supported above the lake bed on a pile foundation that would not withstand a moderate seismic event.
The City undertook this predesign project to evaluate alternatives to replace the existing interceptor and support system so that capacity and structural deficiencies would be reliably addressed for the long-term. This paper reviews the alternatives initially considered and screened out for much of the system—continued O&M only, construct new tunnel under or around the lake, replace/rehabilitate existing system in-place—as well as the three alternatives carried forward for more detailed evaluation: the around-the-lake option and the two in-lake solutions: pile supported and buoyant.
The around-the-lake option requires construction of six pump stations, 23,000 feet of force mains, and 13,000 feet of new gravity sewers. Most of these facilities would require significant rock excavation. The new pump stations would be located along the lakeshore at or near the points where existing tributary sewers enter the interceptor. This alternative also requires significant work in the lake's west end. Major disruption to local residents is expected during construction.
A new pile-supported pipeline requires placing piles on a new alignment in the most geologically stable location, furthest from the steep slopes of the lake shore and the formations near the middle of the lake. The piles would typically extend to bedrock, which varies from approximately 35 to 150 feet below the lakebed. The pipe material selected for the new interceptor must be rigid enough to span the distance between the pile supports without excessive deflection (upward when empty and downward when flowing full). Corrosion of steel or ductile iron components is a significant concern.
A new buoyant interceptor was explored in detail. This alternative would utilize anchors at the bottom of the lake and flexible tethers to hold the pipe to the required elevation. HDPE, while an ideal pipe material for this application in certain respects would both shrink and grow more than 14 feet from a neutral temperature length due to the seasonal lake temperature range of nearly 35 degrees F. Various methods of accommodating this movement, as well as anchor and tether options are presented.
Manholes for a new in-water pipeline, whether buoyant or pile supported, would be submerged with their top elevations a safe distance below the lake water surface elevation. Access to the interceptor would be through a standpipe that would be clamped by divers to a mating surface on the top of each submerged manhole. Once the standpipe has been placed, the water would be pumped out and a maintenance worker would descend the standpipe to open the water-tight manhole cover and allow equipment installation.
Predesign cost estimates for all alternatives greatly exceed those made during feasibility studies in 2001. As such, the City finds itself having to make difficult choices between funding of competing improvements on its capital improvement program. In addition to affordability and technical feasibility issues, public involvement efforts, permitting strategies, longevity, and O&M issues are discussed.
Document Type: Research Article
Publication date: January 1, 2007
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