Constructed in the early 1960s for the city of Pontiac, Mich., the Auburn Sewage Treatment Plant is located on the east side of the Pontiac River and serves a significant portion of the community with wastewater treatment services. A 66-in. diameter steel sewer line is the primary influent line for the Clinton River Water Resource Recovery Facility (WRRF), crossing above the Clinton River.
The river crossing pipeline consists of a pile-supported fabricated steel pipeline with a total span of 140 ft, which uses three pipe-pile bents to span the river between underground sections of the interceptor. The effects of time, weather and the internal wastewater atmosphere gradually have deteriorated the steel pipeline.
During a video inspection of the sewer, Oakland County Water Resources Commissioner’s Office staff observed cracks in the sewer at two locations in the middle span over the river. The river crossing pipeline was originally constructed as a discrete segment in the overall influent sewer system but did not include any redundancy or adjacent facilities for a flow bypass procedure necessary to temporarily isolate the river crossing and allow worker access and repairs to take place.
For this project, the overall concept envisioned a bypass piping system with two parallel gravity lines supported by the existing pile bents, capacity-designed to handle both normal dry weather and maximum foreseeable wet weather flows. In addition, the chambers were configured with stop plate guides to allow flows to be maintained through the existing pipeline and when needed, flow could be diverted through one or both of the parallel bypass lines.
The chamber roof deck covers versatile, heavy-duty structural steel fabrications adequate for temporary hydraulic loading as well as potential future vehicular traffic loads. Upon completion of the entire bypass arrangement, those steel plates became the easily installed chamber cover plates readily available for instant reuse in the event of future needs.
The bypass pipelines were quickly demobilized and removed from the project and the diversion chambers were converted to their permanent configuration with the combination stop plate/cover plates secured in place.
The removal and replacement pipeline operation took six days to complete, which was within the maximum 10 day window. A temporary supported walkway platform spanning the three pile bents allowed full unrestricted safe access for the dismantling and erection crew.
“As always, the safety of the workforce completing this challenging task was a critical objective followed closely behind by the need to avoid any disturbance to wastewater flows that would affect the surrounding community served by the pipeline and wastewater treatment plant,” said NTH Principal Engineer Larry Gilbert.