TO PUMP OR NOT TO PUMP…THE “OTHER” BYPASS ALTERNATIVE

As the knowledge and use of trenchless technologies advance, the applications have become more diverse. With this diversity has come the introduction of supplementary considerations, including complex bypass pumping schemes, site restoration issues, and site ingress and egress. Many times, what appears to be a simple trenchless project turns into a complex exercise involving issues other than the actual rehabilitation itself.This paper provides a review of an emergency project that included the cured-in-place lining of approximately 1,200 linear feet of 30- and 36-inch interceptor sewer, and the unique implementation of a 30-MGD bypass pumping system. While the lining component involved relatively standard design and construction procedures, the bypasspumping component presented several complex issues, making this project less than ordinary. Upon construction, a bypass-pumping alternative, that did not include any pumps, was selected.The emergency project was initiated when a collapse occurred in a 30-inch gravity reinforced concrete interceptor, immediately downstream from a force main terminal structure. The interceptor is approximately 36-years old and is located in a mature, established residential area. An additional collapse had occurred down-stream from this location ten years prior. Based on the condition and location of the interceptor, the County needed to implement a quick, non-disruptive, and long-term solution. While the cured-in-place lining option provided resolution of each of these concerns, the 30 MGD bypass system required significant ingenuity to achieve these goals. Issues that needed to be addressed for the bypass system included: staging area; size, number, and location of the bypass lines and pumps; noise control; protection of existing force main and interceptor; method of installation for suction and discharge operations; coordination with pump station operations; protection of up- and down-stream structures; power requirements; and system maintenance. The original emergency design included the installation of approximately ten 12-inch pumpsets and two to three 18-inch bypass lines. However, upon receipt of the bids and further inspection of the collection system operations, it was determined that the bypass could be achieved by using the County pump stations/force main to pump through the bypass piping system; thus eliminating the need for temporary pumpsets.