Abandoned Underground Mine
An is any large excavation in the earth formerly used to extract ore, coal, or mineral, which is no longer in production.
Gases from abandoned underground mines can be hazardous. In 1967, four people were overcome by gas at an old tunnel entrance, near Issaquah. All were rescued and recovered. Seattle has several abandoned coalmines that have collapsed and damaged surface structures.
Hazard Identification and Vulnerability Assessment
Underground coalmines present the largest abandoned mine hazard in Washington State. This is because of the extent of the mines and the urban development that has occurred around them. Most residents of cities such as Renton and Bellingham are not aware that they live in former "mining towns," with abandoned mines still present. Almost all coal production from underground mines was in the populated counties of Whatcom, King, Pierce, Thurston, Lewis, and Kittitas. Metal mines, in contrast, tend to be remote from population centers. The Washington State Department of Natural Resources (DNR) has maps of almost every abandoned mine in the state.
Hazards are related to mine shaft openings, the mouths of tunnels and airways, or where mining operations were conducted close to the surface. Openings in developed areas are commonly plugged with mine waste, land clearing debris, or car bodies. These "unengineered" caps may eventually fail, especially where the original slopes are steep. Plugging of mines in which water flows may cause unexpected and sudden outbursts of water at unanticipated locations.
The most obvious effect of abandoned mine cave-ins is the sudden appearance of a hole in the ground. Due to the size and location of the underground opening, holes may be very difficult to plug permanently and after "filling," may reappear unexpectedly. Repairs are costly when holes appear under or near structures, homes, land developments, transportation routes, and utilities. Also, the costs of geological and engineering services to locate, map, and evaluate the safety of sites are expensive.
Land subsidence or ground settling can be the result of underground mining of coal or other minerals, groundwater and petroleum withdrawal, and drainage of organic soils. Earthquakes near abandoned mines may cause cave-ins, surface faulting, and liquefaction. Construction near abandoned underground mines should be regulated and avoided to prevent injury and save property. In some cases, the location of hazards can be assessed by site analysis. When the costs of assessing the hazard and mitigating its effects are too great, it may be inappropriate for development.
The Washington State DNR and local engineering, building, or planning agencies can often provide information regarding the location of abandoned mines.