Milestone Reached on MWRA’s Deep Rock Tunnel for South Shore Wastewater
MWRA’s effort to improve the sewer capacity of six South Shore communities took a major step early this morning when contractors finished mining a deep rock tunnel, which runs 2.7 miles underground from North Weymouth to the Nut Island Headworks Facility in Quincy.
The deep rock tunnel is a major part of the MWRA’s Braintree-Weymouth Relief Facilities Project, a $217 million series of projects that will significantly reduce the risk of sewage backups and overflows in the region. The capacity of the regional sewer system, which serves Braintree, Weymouth, Randolph, Holbrook, and parts of Quincy and Hingham, will be increased by about 19 million gallons per day during wet weather conditions.
The deep rock tunnel, which is 12 feet in diameter and 250 feet below the earth’s surface, is a unique engineering achievement because it will have two-way piping and serve two purposes. The tunnel will transport wastewater for eventually treatment at the Deer Island Treatment Plant. It will also transport treated sludge back from Deer Island, to the MWRA’s sludge-to-fertilizer plant in Quincy.
Within the tunnel there will be three separate force mains.
One 42-inch main will convey sewage from the new Intermediate Pump Station under construction in King’s Cove out to the Inter-Island tunnel that begins at Nut Island, Quincy. The 4.8 mile Inter-Island tunnel then transports the wastewater to Deer Island for treatment. Two smaller 14-inch force mains, also within the larger tunnel, will be transporting sludge back, in the opposite direction. With the sludge pipelines in operation, MWRA will be able to eliminate the barging of sludge across Deer Island.
Construction of the tunnel by Modern Continental Construction of Cambridge, Massachusetts began in June 1999. A large access shaft in North Weymouth near the Fore River Bridge has been the center of construction activity. The tunnel has been excavated using a tunnel boring machine similar to those used in other MWRA water and wastewater tunnels.
The Braintree Weymouth Relief Facilities Project includes the construction of two new pumping stations and also includes the repair or replacement of a number of inadequate sewer system siphons and interceptors in the area.
"The Braintree Weymouth project represents a part of a series of improvements that we have undertaken to rebuild the South Sewer System," said MWRA Executive Director Fred Laskey. " No one enjoys sewage backups and overflows, and without these improvements, that system wouldn’t meet the demands put on it in the future. The completion of the Deep Rock Tunnel, as well as the rest of this project will also help continue the progress we’ve made in restoring the health of Boston Harbor and it’s tributaries and bays."
Under a Memorandum of Agreement, the fire departments of Quincy, Braintree and Weymouth have provided tunnel rescue and fire support services for the project. MWRA’s contractors have operated the project in accordance with state regulations for noise and vibration levels.
MWRA projected spending on the tunnel construction project is expected to be within the $73 million construction contract and an associated 15% contingency set aside for difficult tunneling projects.
Wastewater is slated to flow through the new tunnel by January 2004.