WMATA begins pilot program to combat water infiltration

Written by Maggie Lancaster, assistant editor
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Alarm bells for public transit?

Washington Metropolitan Area Transit Authority (WMATA) approved a pilot program that it says could have long-term benefits for Red Line riders.

WMATA and its contractor will test the use of a “curtain grouting” technique to add a waterproof membrane to the exterior of the tunnel walls using a proprietary polymer-based material.

This solution comes after a recent issue that suspended Red Line service in two separate areas due to arcing insulators caused by water infiltration during the morning commute. WMATA says the incidents led to widespread delays, crowding and inconvenience for tens of thousands of its 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,” said WMATA General Manager and CEO Paul J. Wiedefeld.

Curtain grouting is a leak-mitigation technique used to treat an entire area 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, then 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. Two holes are drilled every 10 feet for the injections. The holes are sealed at the end 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.

WMATA plans to test this technique, beginning on July 10, 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, WMATA plans to use the entire Medical Center interlocking area, which is a cavernous space that was constructed out of blasted rock.

WMATA says it will evaluate the pilot during the autumn rainy season, when hydrostatic pressure builds and water infiltration is more prevalent. By early 2018, WMATA says its engineers should be in a position to judge the effectiveness of the curtain grouting process.

WMATA says that the pilot area only represents less than three percent of the affected area. Wiedefeld says 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 WMATA Board Chair Jack Evans. “Fixing this problem will not be cheap or easy, but it is absolutely necessary and the right thing to do.”