Groundwater is not a friend of folks who dig tunnels. And just like with a pesky person you don’t want around, the best way to deal with the problem is to freeze them out.
Crews from Skanska USA are embarking on a project to bore a 2,700-foot-long, 20-foot-diameter tunnel 80 to 160 feet below the Washington, D.C., neighborhoods of Bloomingdale and LeDroit Park. When complete, the project will provide nine million gallons of water runoff storage to alleviate the flooding that’s all to common in this dense residential area with a sewage and drainage system designed in the late 1800s. And to complete the DC Water First Street Tunnel Project, they’re going to freeze the ground and drill through the chill.
In the case of underground infrastructure projects, freezing ground not only keeps groundwater away from a worksite, but also creates a more efficient and quieter project—good news when you’re boring below a neighborhood. Scott Hoffman, Skanska project manager, tells PopMech that various jobs will use the century-old technique of ground freezing. “What makes this job unique is the way we are applying it in the middle of a densely urbanized area. We have a lower profile operation and can run the freeze sites remotely.”
There’s no getting around it: Working underground mean dealing with groundwater. Traditionally, a tunnel project like this might us massive pumping systems—think large and loud equipment—or grouting techniques to keep water from pushing up and flooding a newly bored tunnel. But those methods can prove bulky and unreliable. For example, Hoffman says, a grouting system has trouble solidifying soft ground behind rocks.
Not so with freezing all the ground in the area. “The cooling of the freeze pipes will pervade through everything in the area,” he says. “It is a secure way of excavation.”
To get started, Skanska subcontractor Moretrench, with the design help of Parsons Brinckerhoff, sunk 331 four-inch-diameter steel pipes to a depth of about 150 feet. The drilling locations were chosen to ensure total coverage of the excavation area. The project requires the drilling of a series of 90-foot-shafts ranging from 21 to 23 feet in diameter. Two “drop” shafts will convey overflow water from diversion chambers to the tunnel, and a smaller shaft will connect to the pump station and the existing sewer system. So the steel freeze pipes drop to almost twice that depth, hitting rock at the bottom to form a seal and keep groundwater from pushing up.
Hoffman says freezing around the perimeter and down to the rock creates a double protection and eliminates the power of deep-water pressure. “Once you get an inflow,” he says, “it can be difficult to stop.” Each pipe can freeze the ground six to eight feet around it, if it’s near the surface, and up to nine feet around it if it’s lower down. Crews use foam insulation near the surface to maintain the cold temperatures.
Getting the ground cold requires the use of a giant refrigeration system. Three tractor-trailer units, each 45 feet long, park nearby and use ammonia to cool a brine-saltwater mixture—the same mixture used to treat roads in the winter—to a range between -15 and -22 degrees Fahrenheit. The system moves 1,000 gallons of solution every minute via 50 horsepower pumps, all controlled remotely by a mobile app.
It took six to eight weeks to form the initial freeze and the closed-loop system maintains the temperature while it is “monitored continuously” by mobile app without the need for intrusive equipment.
With the ground frozen, crews can mine these shafts without the fear of groundwater seeping up and overtaking the tunnel or messing with the tunnel-boring machine. This project’s TBM was named Lucy. As Lucy pushes forward, crews apply shotcrete—a spray concrete—in a sequential excavation method that has the project currently about one-third completed, and on track for an April 2016 finish.
As Lucy churns through the clay-filled frozen ground, the hardness of the soil requires a mixture of tools on the cutterhead face, including using 16 disc cutters that can more easily handle the solid ground, two rippers, 78 scraper knives, and 28 bucket lips. Hoffman says that even though the ground was frozen, the disc cutters have kept the project moving along nicely, even through frozen ground.