Beginning July 10, D.C. Metro and its contractor will 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 GM/CEO Paul J. Wiedefeld.
Last Friday, for example, Red Line service had to be suspended in two separate areas due to arcing insulators caused by water infiltration during the morning commute. The dual incidents led to widespread delays, crowding and inconvenience for tens of thousands of riders.
What is curtain grouting?
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.
Tunnel designed to leak
The nine-mile long deep-tunnel segment between Farragut North and Grosvenor is the oldest bored section of the Metrorail system and was designed and constructed prior to the widespread use of NATM, the New Austrian Tunneling Method, which provides tunnels with a waterproof membrane. Engineers designed the Red Line tunnel to leak, knowing that it would be well below the groundwater table and under high hydrostatic pressure. The tunnels have leaked, by design, since they were constructed and massive drainage pumping stations remove millions of gallons of water every week.
The lack of a waterproof membrane has posed significant maintenance challenges for Metro. First, the presence of moisture and dripping water is a leading cause of arcing insulators. More than two thirds of all arcing insulators occur along this section of the Red Line. Second, water corrodes the track bed, rails, fasteners communication cables, pipes and other components, all of which must be replaced more frequently in this area. Finally, the water entering the tunnels carries mineral deposits that accumulates as a “muck like” substance in switches and other components that are needed for train movement. (By contrast, all other deep Metro tunnels were designed using NATM and are quite dry by comparison to the western Red Line.)
For years, Metro has relied on a strategy of “negative side grouting,” where individual leaks are addressed from the inside of the tunnel. This approach has limited effect, because the hydrostatic pressure outside the tunnel is strong enough that the water will simply find another path. For several years prior to 2011, Metro stopped trying to keep the interlocking (crossover location) at Medical Center in service because it required daily visits by maintenance crews to remove the muck.
Evaluating the Pilot
Metro has moved quickly to have the work associated with the pilot occur during the summer months when ridership is lighter. Evaluation of the pilot will occur during the autumn rainy season, when hydrostatic pressure builds and water infiltration is more prevalent. By early 2018, Metro engineers should be in a position to judge the effectiveness of the curtain grouting process in the Metro environment.
Wiedfeld cautioned that the pilot area represented less than 3 percent 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.