Several Seattle suburbs sit on coal mines. What's the risk?
Ours is a region full of hazards, including earthquakes and slides.
It’s also a region where the rising cost of housing has been pushing people to the edges of the region to look for homes. But as people go farther out, they encounter a hazard not seen in the city: abandoned coal mines.
Abandoned coal tunnels and mine shafts run under Black Diamond, in south-east King County — just like they run under more developed placed such as Issaquah, Renton, Bellingham and Newcastle on the Eastside.
It was in Newcastle, according to the state Department of Natural Resources, that a sinkhole caused by a collapsing coal tunnel created the most damage. The hole opened up beneath a home off the Coal Creek Parkway in Newcastle in the 1980s.
The hole was not big enough to swallow the whole house, said Tim Walsh, assistant state geologist at Washington’s DNR. “The hole when it opened up was probably no more than 10 feet in diameter.”
It damaged a corner of the house — which was very expensive, but not fatal to the house or its occupants.
Walsh said there are suburban neighborhoods where coal-related sinkholes will open up in the yard. Sinkholes from coal mining tend to be limited in size, because the coal tunnels were around eight feet across.
The Palmer Coking Coal company still operates in Black Diamond, though it hasn’t mined in the area for years. Manager Bill Kombol said people should not be overly concerned about a coal tunnel collapsing and wrecking their house.
He said retreating mining companies tended to take out supporting structures on their way out, precipitating tunnel collapses. He said time also reduces the potential hazard.
"Most collapse happens in the first couple of years after abandonment,” he said. “So we're now on year 90, right here."
The depth of the tunnel is also an issue. The closer to the surface, the more of a hazard it poses.
But it's hard to get specific information. The state keeps a database of old mining maps. Black Diamond puts out an information sheet and has a map that provides a rough idea of the hazard area.
Above is a general coal mine hazard area map for Black Diamond provided by the city. See a detailed map of past mining activity by KUOW and learn more about the city's "Covenant not to sue."
Andy Williamson, director of community development in Black Diamond, said people who build or renovate in a known risk zone must find out the actual risk on their property. “They need to go out and hire a geologist and then they survey the land and they give us the report,” he said.
But people like Judy Baxley bought a house that was already there. She said no one told her about the hazard when she bought her house. She said she found out by accident at a social event. But now that she's there, Baxley says she plans to live with it.
It's been decades since the state has recorded any real danger from a tunnel collapse around Black Diamond. Geologist Walsh said the last time was in the 1980s.
“A tunnel that was under the Green River Gorge Road burst,” he said. “It could have been serious.”
But again, people were fine. A woman driving with children saw a puddle in a new place. She stopped the car. “That was just before the edge of the road failed. And water sort of explosively came out of the mine across the road.”
Walsh said coal mine tunnels tend to be too small to consume an entire house. So there's probably greater danger from our region's natural threats, like earthquakes, mudslides and volcanic mudflows — the ones we share, whether we live in the city or the distant suburbs.
Carolyn Adolph can be reached at cadolph@kuow.org. Have an idea about a community or a growth issue we could cover? Tell us here.
