CHICAGO – When an interstate bridge between Springfield and Champaign began to sink three years ago, the cause was quickly identified: An old coal mine 220 feet below the surface was starting to give way. Engineers backfilled the mine to prevent the bridge from collapsing.
In 2009, a 7-year-old elementary school in southern Illinois that was built over an old mine developed large cracks and the floors buckled, displacing hundreds of students until a new school could be built on another site – after tens of thousands of cubic yards of backfill was pumped hundreds of feet below the surface to ensure it was stable.
The incidents underscore the potential – and literal – pitfalls of building throughout much of Illinois, where more than 100 years of coal mining carved out vast swaths underground.
Now, the Illinois Department of Natural Resources is seeking the public's help to find old maps of long-closed and sometimes-forgotten mines to help prevent similar problems.
The southern two-thirds of Illinois – about 75 of its 102 counties – was mined for coal to various degrees starting in the mid-1800s. Almost all of have been closed or abandoned.
Over time, timbers that support mine roofs can decay and break or the pillars of remaining coal can get pushed into the mine floor, causing holes to form at the surface or the ground to sag – conditions known as "subsidence," said Robert Gibson, the emergency section supervisor at the DNR's Department of Mines and Minerals.
Pits are holes that form over shallow mines, and usually are no more than 6-12 feet deep and about the same width. Sags are bowl-shaped depressions about 350-500 feet in diameter, often with the ground sinking 2-3 feet. That can cause houses and other structures to move toward the center of the sag, causing cracks to form or walls to bow, Gibson said.
"You can get homes that are extremely damaged and can be unstable," said Gibson, whose office investigates about 80 subsidence cases a year.
THE OLD MAPS:
Mine maps marked the locations of rooms, pillars and shafts – a necessity then and a valuable tool for state officials, homeowners and developers today.
But the DNR only has about 2,000 maps for more than 4,000 mines, Gibson said, and each map may have multiple segments.
The state is digitizing those in its archives before they deteriorate, but also is searching for missing maps. Some have been discovered in historical societies, libraries and mining company archives.
Many may be in people's homes, Gibson said.
"I have been called to look at subsidence where the homeowner had a mine map in their barn that was not in our collection," Gibson said. "Maybe their fathers or grandfathers worked there, and their descendants acquired them."
Gibson said he'd like to borrow and scan the maps. If the owners don't want them anymore, the agency will keep them and give the owner a digital copy.
The most important task, though, is finding and scanning the deteriorating documents before it's too late.
"Some are over 100 years old, handled, dried and brittle," he said.
The Illinois State Geological Survey recently released an updated map collection that includes one in Springfield that officials didn't have before; others there still are missing. Officials know, for example, that there was a mine on the west side of the city because of subsidence occurring over a 10-block area, "and I would love dearly to have that mine map," Gibson said.
THE NEW MAPS:
Once the DNR digitizes an old map, the new version can be paired with geographic information software to identify roads, buildings and other structures that might be at risk. It also can help determine where and how future development occurs.
Developers can tell from the mine footprint where the existing coal pillars are located and if they need to be reinforced, whether a building can go on particular site or whether a subterranean cavity can be filled.
But until – and unless – the state gets the missing maps, "many homes are at risk," Gibson said.
"You cannot develop a community wisely to avoid these things, and you put large buildings at risk because you don't know if there is a mine," he said. "Then you can get a horrible surprise someday."