Meeting the demands of projected growth required years of precise planning and execution for the Washington Suburban Sanitary Commission and its partners
As population, business and industry continue to grow in Montgomery and Prince George’s counties, Maryland, the area’s water system will be prepared to deliver.
That’s due in large part to a 5.3-mile, 84-inch-diameter underground pipeline recently completed by the Washington Suburban Sanitary Commission (WSSC). The line, known as the Bi-County Water Tunnel, connects two existing water mains that transport water from WSSC’s Potomac water plant to the eastern sections of Montgomery County and most of Prince George’s County.
The new line, operational since February, has a capacity of 100 million gallons a day and ensures that constant and reliable water supplies and pressures can be maintained throughout the system, now and well into the future.
WSSC Project Manager John Mitchell says that while the counties are not experiencing water system issues currently, the projected growth in the area would have created situations where the existing 54-inch pipeline could not have supplied water at sufficient pressures or volumes to the developing areas.
According to planning documents and census reports, the combined population of the two counties is expected to grow from 1.8 to over 2.1 million in the next 25 years.
“The old line would not have met the expected demand,” Mitchell says. He adds the new line not only assures adequate supplies system-wide, but enables WSSC to take the old line down for maintenance or other purposes. “The new line gives us redundancy,” he says.
Depending on terrain, the new line runs from 90 to 200 feet underground from an existing water main near I-270 and Tuckerman Lane at its western end, and another main at Stonybrook and Beach Drives in Rock Creek Park at its eastern end. Supplied by Northwest Pipe, the pipe consists of 50-foot spiral-wound steel sections, lined with cement and butt-welded at the joints.
The tunnel bore is 10 feet in diameter, and the annular space between the pipeline and the tunnel walls has been completely grouted with concrete. Grout was batched on site by Pacific International Grout Company and pumped through ports in the interior pipe walls. “It’s pretty solid,” Mitchell says.
The pipeline planning and public input process was completed in 2005, and the joint venture RSS-JV (Oscar Renda Contracting, Southland Construction, and SAK Construction) began digging the main entry shaft at Connecticut Avenue in August 2009. That was followed by the eastern shaft at Stonybrook, and finally the western shaft at Tuckerman Lane.
The tunnel-boring machine (TBM), owned and operated by SAK, was lowered into the shaft and began excavating the tunnel through solid rock in July 2010. By November, it had completed the first 0.8 miles (eastern section). A small railroad was built inside the tunnel to facilitate the removal of tunnel muck and then delivery of the 50-foot pipe sections. The rolling stock was supplied by Durango Mining Equipment.
The TBM was temporarily sidelined in February 2011 for a few months, until digging of the western section could be resumed that August. The long length of the western section tunnel bore and the extremely hard ground conditions overwhelmed some of the parts and affected the main bearing of the TBM, according to SAK spokesman Scott Linke.
Pipe and grouting installations began in the eastern section during spring 2013 and was completed in April. The TBM was returned to service in August 2011 and “holed through” in early 2013, completing the western section excavation. Pipeline installation and grouting followed, and the pipeline began carrying water in February 2015.
Mitchell says among several key challenges facing the pipeline, the most difficult was the length of the western bore and the positioning of the shafts.
“Of the three working shafts, two were at the ends, and the third was just a mile from the eastern end,” he says. That made for a 4-mile long western section tunneling run, and impacted tunneling and pipe section transport, Mitchell says. Crews worked around the clock to keep the project on schedule.
Jim Delmonte, project manager for Southland Construction, says the task was difficult because it required the piping sections to be inserted through the middle shaft and transported over 4 miles to the western end and installed back toward the middle shaft. Workers walked back through the pipeline to gain access to the sections and weld them together.
Once the pipeline was complete, tie-ins to the existing water mains took little more than a day, Mitchell says. “We started shutdowns after peak demand on a weekend night. We had our systems and tanks full, and worked through Saturday and Sunday to complete the connections.”
He adds that longer shutdowns earlier in the contract allowed installation of isolation valves on the existing line. “Once they were in place, and once alignment was complete, the connections could be made in one weekend,” he says.
Delmonte explains that crews from Southland Construction brought the 84-inch pipeline to the surface at both ends, and connected the new line to the existing water mains in valve vaults containing 72-inch gate valves.
“We also installed a pretty extensive cathodic protection system,” Delmonte says. In addition to the three shaft locations, four intermediate shafts were drilled to allow cables to be connected between the pipeline and read-out stations on the surface.
The project was the result of diligent planning and public input sessions. Mitchell says the tunneling approach was compared against open-cut installation, and found to be more cost-effective. “We looked at a number of alternatives early on,” he says. “When we added in all the environmental and community impacts in addition to cost, tunneling became the best option for us to use.”
Cost of the project was about $113 million and was funded through WSSC’s “systems development charge.” Mitchell explains that the fund consists of a development fee the utility charges for new connections. No federal or state funds were used for the project, and the utility incurred no additional debt.
Mitchell says WSSC still has some site restoration work to finish up. “We’ll be back in this fall to plant some trees,” he says. “Overall, we’re very pleased with the results. The new line has already provided us with an immediate increase in supply and pressure as well as the redundancy we needed.”
More information about:
Washington Suburban Sanitary Commission (www.wsscwater.com)
Southland Construction Inc. (www.southlandconstruction.com)
SAK Construction (www.sakcon.com)
Oscar Renda Contracting Inc. (www.oscarrendacontracting.com)