Thursday, August 16th, 2012 by Cassi Sherman Henes
Coal is primarily mined in two ways: either surface mined or underground. Surface mining is generally easy to detect, since the layers of earth are stripped away revealing the coal underneath.
Underground mining begins with vertical or diagonal shafts dug into the earth. From there, horizontal tunnels called "streets" are dug until a coal seam or ribbon is discovered. If possible, the ribbon is removed without disturbing the earth around it. This leaves empty space where the coal used to be. The empty spaces collapse as pressure from the surface, including the weight of a house, press down. This is subsidence. Ribbons of coal crisscross throughout the Front Range and as they were mined out, subsidence risks rose.
During the mining process, larger deposits of coal are found. These areas are marked and recesses called "rooms" are created off the original tunnel. These rooms become larger voids and are usually supported from collapse by timbers will rot away.
In some cases, vast amounts of coal are discovered in a single deposit. It is often the result of an ancient lake or other body of water becoming completely filled with peat and turning to coal. These are called "fields". In order to remove a field, vast excavations occur. This means that timber beam systems are constructed to keep the soil crust above from caving in. During the mining process, portions of the coal, or other earth, would be left in pillars to help reduce the risk of cave in. As the mine taps out the remaining coal in the area, these pillars are eventually removed as well and replaced with rubble, or timber. Naturally, the pillars do not last forever. When the soil crust above the field collapses, large areas of subsidence occur. This subsidence usually happens quickly with dramatic results.