Metro has approved a pilot program to test the use of a “curtain grouting” technique to add a waterproof membrane to the exterior of Red Line tunnel walls using a proprietary polymer-based material.
“Since this tunnel segment was constructed, Metro has fought a battle against Mother Nature, and Mother Nature has always had the upper hand,” said Metro General Manager and CEO Paul J. Wiedefeld in a press release.
Last Friday, the last business day of the SafeTrack Surge which shut down the Shady Grove and Rockville Metro stations, the Red Line had a suspension of service because of a problem with arcing insulators in two separate areas caused by water infiltration during the morning commute. Metro admits the shutdown led to widespread delays, crowding, and inconvenience for tens of thousands of riders.
“Just as we have addressed the root causes of track infrastructure problems and railcar reliability issues, I want to address the water infiltration problem head on and find a sustainable solution. Our Red Line riders deserve nothing less,” Wiedefeld said.
Curtain grouting will add a rubberlike membrane on the outside of the concrete tunnel wall. After holes are drilled in the ceiling of the existing tunnel until the exterior of the tunnel is reached, here’s the nitty gritty of the procedure.
“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.”
Metro says that the contractor hired to do this work has successfully used this solution in the mining industry to seal ground water inflows, even 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 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.
However, drilling more than 500 holes in the tunnel ceiling and injecting high-pressure grout is a time-consuming process that can only occur when trains are not running, so Red Line riders are facing schedule changes this summer.
Beginning Monday, July 10, 2017, and continuing every weeknight (Monday-Friday) through Friday, August 11, 2017, Red Line trains will single track between Friendship Heights and Medical Center starting at 9 PM nightly.
Free shuttle buses will replace Red Line trains between Grosvenor and Friendship Heights on the following 2017 dates:
Saturday, July 15 and Sunday, July 16
Saturday, July 22 and Sunday, July 23
Saturday, July 29 and Sunday, July 30
Saturday, August 5 and Sunday, August 6
To find out if the process works, the system will be evaluated during the autumn rainy season.
Wiedefeld cautioned that the pilot area represented less than 3% of the affected area of the Red Line, and that any eventual full-scale solution would take time and a significant amount of capital funding to advance.
“This kind of capital project perfectly illustrates why we need a dedicated funding source for our Metro system,” said Metro Board Chair Jack Evans. “Fixing this problem will not be cheap or easy, but it is absolutely necessary and the right thing to do.”