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Curtain grouting is a leak-mitigation technique used to treat an entire area that is leaking by adding a rubberlike membrane on the outside of the concrete tunnel wall. To do this, holes are drilled in the ceiling of the existing tunnel until the exterior of the tunnel is reached. From there, a proprietary polymer-based emulsion (PBE) grout is injected into the hole at high pressure, which begins cascading down the curved exterior of the tunnel (like the way chocolate syrup cascades down an ice cream sundae). Two holes are drilled every 10 feet for the injections. The holes are then sealed at the conclusion of the process. The injected material forms a rubberlike impenetrable membrane, or "curtain," between the exterior of the tunnel wall and the surrounding ground medium.
The contractor has successfully used this solution in the mining industry to seal ground water inflows - some with flow rates of 50 gallons per second.
Metro plans to test this technique in the two different environments that exist along the Red Line segment - one in a linear bored tunnel and one in a blasted-rock cavern. The linear tunnel segment that will be used for the pilot will be a 2,000-foot section of the inbound track between Medical Center and Bethesda. For the second test location, Metro plans to use the entire Medical Center interlocking area, which is a cavernous space that was constructed out of blasted rock.