The Lower Central Interceptor (LCI) sewer collects more than half of the flow from the Town of Weymouth’s collection system prior to entering the 42” MWRA Mill Cove Siphon at Newell Park.
The LCI sewer flows through an elaborate system through north from Commercial Street, under the Mill Cove Stream, through the Cadman Conservation Area, under the MBTA Commuter Rail tracks, where the Montcalm Siphon then connects to it before the LCI follows an alignment along the Fore River shoreline in the Idlewell neighborhood prior to finally discharging into the Newell Park Head House. The goal of this project was to reduce nuisance Sanitary Sewer overflows (SSOs) along the LCI and Montcalm Siphon through the removal of hydraulic restrictions and increasing the capacity of a portion of the sewer.
Environmental Partners (EP) was retained to conduct a comprehensive evaluation of Weymouth’s sewer system, with the intent of identifying potential infrastructure upgrades and improvements needed to address existing capacity limitations and system overflows and to adequately plan for additional flows from future development projects. At the time of the sewer system study, one such redevelopment project was for the former Weymouth Naval Air Station (now Union Point), which was projected to contribute up to 1MGD of additional flow. The flow from the Union Point project would be conveyed to the MWRA system by the LCI.
The system evaluation confirmed that the Town’s Montcalm Siphon was subject to periodic overflows into the Fore River due to hydraulic restrictions in the 2,100 foot long terminal section of the LCI sewer, a 30 inch diameter reinforced concrete pipe. Subsequent to the sewer system study Environmental Partners conducted an alternatives analysis of the LCI sewer to determine the recommended approach to improve the hydraulic capacity of the sewer for existing system conditions as well as future increases in flows from the air station’s redevelopment.
After establishing the existing condition of the interceptor and its appurtenances through flow metering, topographic surveying, and CCTV inspection, Environmental Partners completed a comparative analysis of potential improvements to the LCI sewer including: alternative sewer alignments through the neighborhood, a parallel relief sewer, a sewer pump station, trenchless technology, and newer materials of construction for sewer pipe and manholes. The purpose of the analysis was to complete several model runs using Weymouth’s existing calibrated hydraulic model to investigate the capacity issues associated with the LCIS as it experienced SSO and surcharge conditions from Commercial Street to Nowell Parking during rain events. The design considered construction impacts to the residential neighborhood, impacts to sensitive environmental resource areas, construction under the MBTA right-of-way, maintaining operations of the interceptor during construction, and long-term operations and maintenance of the interceptor.
The recommended solution was to remove and replace the existing sewer with a larger diameter gravity sewer within the existing sewer easement. Environmental Partners proceeded with the design and permitting phase of the project following receiving Town approval. The project included extensive public outreach to neighborhood residents as the neighborhood had been recently subjected to the construction of a major MWRA sewer trunk line upgrade project and the construction of the MBTA’s Greenbush Line, which crossed through the neighborhood. Several residents also had unlicensed waterfront structures that required Chapter 91 licenses from the state to allow for them to be replaced following construction. Public outreach included hosting several public meetings for the neighborhood, providing regulatory support for Chapter 91 license applications, and incorporating restoration of a neighborhood park into the design.
The improvements minimized the occurrence of sanitary sewer back-ups and overflows which are a public health risk and harmful to the surrounding environment. The LCI improvements increased sewer capacity by replacing the existing 30” reinforced concrete pipe (6.4 MGD peak capacity) with a 42” polypropylene (PP) pipe (SaniTite® HD, 16.3 MGD peak capacity). The section of the LCI sewer replaced was approximately 2,038 feet long and followed a predominantly cross-country alignment through environmentally sensitive resource areas including: low lying marsh lands; bordering vegetated wetlands; coastal beach; and coastal bank along the Fore River shoreline. Coordination and permitting were completed with the Massachusetts Environmental Policy Act (MEPA), Weymouth Conservation Commission, US Army Corps of Engineers, and the Massachusetts Department of Environmental Protection.
The selected materials of construction for the PP interceptor, monolithic custom FRP manholes, and FRP composite frames and covers were selected to address concerns with internal corrosion from wastewater, external corrosion from salt water, and infiltration from tidally influenced groundwater and inflow from the Fore River during high tides and storm surges. The Town elected to undertake construction in the winter months to minimize disruption to the residential neighborhood, when outside activities were more limited, owner use of their waterfront amenities had been discontinued, and impacts on the salt marsh ecosystem could be reduced. The Town’s contractor utilized daily adjustments of the sewer bypass pumping system around the active work zone, which significantly reduced bypass operation costs, eliminated the threat of bypass failure during evenings and weekends, and minimized impacts to the residents. Construction was successfully completed in Summer of 2013, taking fewer than 12 months, in accordance with all regulatory requirements and with contract change orders less than 5% of the original contract price.