There are nearly 50,000 acres undermined along the Front Range corridor, in Boulder, El Paso, Jefferson and Weld Counties, as well as Fremont, Huerfano and Las Animas Counties. Coal mining took place from the late 1860s until the 1930s, and in some cases into the 1950s.
More than 7,500 homes have been built over mined lands and 25,000 residents have been affected so far.
Hard rock mining is only a small part of the actual mining that has occurred throughout the Front Range. Prospectors packed up and starting digging throughout the Front Range in hopes of getting rich quick from coal.The problem with the coal mining boom was that no one kept an accurate map of who was digging where. This means that we ended up with abandoned mine shafts in some pretty unlikely areas. Additionally, there has been (and still is) extensive large-scale undermining done in our mineral rich backyard. This has lead to a problem called subsidence.
Subsidence is soil sinking to fill voids in the strata (or soil layers). These voids can be caused by water erosion, uneven soil compaction, or the collapse of mine shafts.
Coal is the decomposed and compressed remains of primordial swamps. It's abundant here because millions of years ago, this part of the North American Continent was covered with oceans. As the oceans receded, fresh water swamps were formed that flourished with life. Trees and other plant and small animal life died; decomposing and becoming peat. Between Ice Age melting and glacial shifting, the peat became buried under tons of sand and water, which compressed under that pressure into coal.
Sometimes, vast amounts of coal are discovered in a single deposit. It is often the result of an ancient lake or other body 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 timbers 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 tapped out the remaining coal in the area, these pillars would eventually be 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.
Check back next week for helpful checklists and answers to the most commonly asked questions about subsidence!
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