John J. Carroll / Walnut Hills Water Treatment Plant

Location: Marlborough, Massachusetts
Contract Value: $212,825,969
Project Dates: Spring 1999 - April 2005

The John J. Carroll / Walnut Hill Water Treatment Plant in Marlborough performs several major treatment processes, including primary and secondary disinfection, pH level adjustment, and fluoridation.  Water enters the plant from the Hultman Aqueduct’s Cosgrove Tunnel, flows through the Ozonation Building, the Treatment and Post-Treatment Buildings, and into the storage tank, from where it ultimately makes its way through the MetroWest Water Supply Tunnel to Metropolitan Boston’s homes and businesses.

MWRA Contract No. 6488/CP-3 was the first of the two contracts for this project to be awarded.  The focal point of the project was construction of a 55-million-gallon reinforced concrete water storage tank consisting of two bays with interior reinforced concrete baffles.  The structure includes fifteen 120-inch diameter sluice gates with electric motor operators, two 30-inch diameter butterfly valves, a six-inch and 8-inch PVC underdrain system, access hatches and vents, and a precast concrete drainage pump station for the storage tank.

Transmission of water to and from the tank required constructing 3,600 feet of 120-inch precast concrete cylinder pipe (PCCP) crossing the Wachusett Aqueduct Forebay and connecting to the Aqueduct, 400 feet of 144-inch buried PCCP that also crosses the Forebay, a 30-inch buried ductile iron pressure pipe, and 1,000 feet each of 96-inch and 120-inch buried PCCP including a concrete overflow structure in the Wachusett Aqueduct Open Channel.

MWRA Contract No. 6489/CP-4 involved constructing the Chemical and Post-Treatment Chemical Buildings, the Ozonation Building and the Operations Building, as well as a 69kV Substation and Generator Building.

The five separate buildings total over 215,000 square feet. All buildings were constructed of structural steel frame with reinforced masonry walls and brick veneer, include fire protection and HVAC systems, and with the exception of the Generator Building, are all multi-story and contain mechanical and electrical rooms. The Ozonation and Post-Treatment Buildings both include overhead doors into the tank bays, and the Ozonation Building includes bulk storage rooms and a loading dock. The Chemical Building contains a freight elevator. The Operation Building is built on a spread footing concrete floor foundation, is entered through a public lobby, includes a passenger elevator, and houses the control room, administrative offices, conference rooms, training rooms, a lunchroom, a testing laboratory, and men and women’s locker rooms and washrooms.

The most critical portion of this project involved the construction of a 100-foot diameter watertight, reinforced concrete earth support wall within which the connection between the new 144-inch raw water line and the exiting Hultman Aqueduct was made.  This work interrupted the main transmission line supplying water to the Metropolitan Boston area, and was performed during the MWRA’s low demand period of November through March.

Between the two projects, over 35 acres of land was cleared, more than half a million cubic yards of material was excavated, 95,000 cubic yards of concrete was poured, 12,000 tons of reinforcing steel was placed, and just under 12,000 feet of pipe ranging in size from 60” to 144” was installed.