Bedford Street Force Main Rehabilitation
Weston & Sampson designed the rehabilitation of the Town of Bedford’s 20-inch ductile iron force main, the town’s most critical sewer. The Bedford Street force main, which runs along a major commuter artery between Route 128 and numerous businesses in Bedford, carries 100% of the town’s wastewater flow through Lexington to the Massachusetts Water Resource Authority's (MWRA’s) Mill Brook Valley relief sewer. Significant pipe failures occurred in multiple locations along the force main due to internal and external corrosion.
To repair about 2,700 feet of the pipe, Weston & Sampson designed and bid three different rehabilitation techniques: cured-in-place lining, sliplining, and excavation and replacement. This approach allowed Bedford to weigh the pros and cons of each method against a competitively bid price to make a fully informed decision on a complicated project. The town selected sliplining an 18-inch HDPE pipe through the existing 20-inch ductile iron pipe based on lowest bid cost and least traffic disruption. The project included: hydraulic modeling of the pumping and force main system, 2,700 feet of 18-inch sliplining, bypass pumping and a piping plan, an automatic air release valve, and a force main drain structure.
Design of the rehabilitation was particularly challenging due to heavy traffic, sliplining beneath an existing stream culvert, bypass flow from two pump stations, adjacent wetlands areas, and coordination with Hanscom Air Force Base. Planning of traffic control and design of a bypass pumping system were significant components of the project. Also, a 12-inch force main from Hanscom Air Force Base wyes into the 20-inch force main within the project area. This connection increased the complexity of the hydraulic modeling and added significant logistical issues such as bypass pumping, facility access, and security concerns, to construction sequencing.
Weston & Sampson also designed Phase II of the project, which included: complex hydraulic modeling of the Bedford and Airforce pumping stations and their separate and shared force main system, 8,200 feet of 18-inch sliplining, a 150-foot river crossing, a 630-foot highway crossing, bypass pumping and piping plan, an automatic air release valve, force main drain structure, complex shut-down and flow trucking plan to transfer onto temporary bypass, major traffic management plan including Route 128 and State Highway 4/225, and project management and coordination with the Town of Bedford, the Town of Lexington and United States Airforce at Hanscom.
Weston & Sampson provided construction engineering services during the rehabilitation work of both phases.