WASHINGTON — Water infiltration into Metro’s tunnels is historically the leading cause of insulator fire incidents, but the numbers are down this year despite record rainfall.
According to a news release, Metro encountered only 22 insulator-related or fire incidents in 2018, down from 65 in 2016 — a 66 percent drop.
Metro credits this sharp drop in incidents to waterproofing of tunnels, particularly the vulnerable stretch of Red Line tunnel between Friendship Heights and Medical Center stations.
In recent years, that three-mile stretch of tunnel — which accounts for less than three percent of the whole system — was the site of nearly half of reported incidents. In 2016, there were 29 insulator fires along that stretch. In 2018, there were three.
The Red Line tunnel connecting Friendship Heights to Medical Center is among the deepest in the Metro system and was constructed between the late 1970s to early 1980s, before the common practice of a tunnel-waterproofing technique called New Austrian Tunneling Method, or NATM.
Tunnels built during this era were designed to channel water toward designated drains and pumping stations. But the unintended side effect of this design was that the water carried mud and sediment into the tunnels, deteriorated metal rail segments and created a damp atmosphere where it was easy for electrical currents to run astray.
In 2017, following a series of Red Line incidents, Metro approved an innovative waterproofing pilot program known as “curtain grouting” to mitigate water infiltration into the tunnels. This involved drilling hundreds of holes into the tunnel ceiling and injecting “a proprietary polymer-based emulsion into the holes at high pressure.” The grout then spread over the tunnels’ exterior and hardened, providing much-needed protection against rain and runoff.
The effectiveness of this program is reflected in the sharp drop of fire and insulator-related disturbances.
Metro said it plans to continue waterproofing an additional section of the Red Line, near Forest Glen, in 2019.
