Between a Neighborhood and a Salt Marsh - Trunk Sewer Replacement Along a Tidal River Weymouth, MA
After ten years of major collection system overhaul, the Town of Weymouth approached the final significant piece with apprehension: replacement of the 30-inch lower central interceptor along the shore of the Fore River, between a dense residential neighborhood and a sensitive estuarine salt marsh ecosystem.
The majority of the Weymouth sewer system was constructed between 1947 and 1980. The system contains approximately 200 miles of sewer lines that are operated and maintained by the Town of Weymouth Sewer Department. The Weymouth sewer system contains 11 pump stations, 17 ejector stations, 3 submersible stations, and more than 5,000 manholes. The Town’s system is divided into 6 sub-basins which converge sewer flows into 5 main trunk lines (interceptors) that discharge directly into the MWRA system at 11 individual locations. Once entering the MWRA system, flow is directed to the Deer Island Wastewater Treatment Plant through either the Intermediate Pump Station or the Braintree-Weymouth Replacement Pump Station.
As part of a DEP Administrative Consent Order, the Town embarked on a CIP program which consisted of 5 major sewer system improvements designed and constructed over the course of 5 years at an approximate cost of $22 million. Since the CIP improvements were completed in 2007, there remained only one trouble spot associated with the Weymouth sewer system where periodic overflows occurred: the lower section of the Lower Central Interceptor (LCI) along the Fore River by the Montcalm siphon.
Environmental Partners evaluated several alternatives to alleviate this problem area using a hydraulic model of the Weymouth sewer system. The solution identified to resolve these periodic sewer system overflows includes the replacement of approximately 2,100 feet of existing 30-inch diameter reinforced concrete gravity sewer constructed in 1949 with 42-inch diameter PVC gravity sewer. The design and construction sequencing included bypassing the existing sewer pipe and installing the new 42-inch sewer pipe in the same trench as the existing 30-inch pipe. By far the biggest design, permitting, and construction challenge was completing this project along the shoreline of a tidal river.