The state of Colorado has a rich legacy of mining. The search for gold and silver in the mid 1800s and the development of mines throughout the Colorado Mineral Belt to extract that wealth drove the exploration and settlement of Colorado, and was an important factor in Colorado becoming a US territory and ultimately achieving statehood in 1876. Extensive coal resources were also discovered and exploited early in the state history to help drive the continued growth and expansion.
While hard-rock mining has diminished in Colorado, its citizens and government agencies continue to wrestle with the physical hazards and environmental impacts caused by mine drainage. For more information on mine drainage, click here. However, Colorado has huge coal resources and coal mining remains an important part of the Colorado economy today, click here.
Hard Rock Mine Subsidence
Underground hard-rock mining generally follow ore-bearing veins and mineralized ore bodies. The entrances to underground workings are by adits and shafts. The mine workings tend to be smaller and follow the ore by narrow tunnels, drifts, and stopes. Roof failures of these workings can cause subsidence at the ground surface or leave dangerous surface openings. The Colorado Division of Reclamation Mining and Safety has an Abandoned Mine program that is tasked to identify and safely close, or seal dangerous mine openings in Colorado.
On right is a 2006 subsidence hole that opened on I-70 west of the mining town of Idaho Springs. This photo shows the hole has been filled with flow fill. Note the yellow dashed line that shows the alignment of the tunnel towards a rock opening, partially obscured by the guardrail, in the highway cut slope across the westbound lanes.
Coal Mine Subsidence
Colorado coal has been mined since 1864 out of 1,738 coal mines across the state. Mines that were closed prior to 1977 are considered to be historic coal mines by the Office of Surface Mining (OSM) and pose particular risks to surface development. Mine subsidence is one of these risks.
Mine subsidence is the movement of the earth's surface caused by the collapse of underground mine voids or entries. Over time, gravity and the weight of the rock overlying the mine cause the layers of rock to shift and fall downward into the mine area. As one layer falls down, the void moves upward toward the ground surface where it can cause holes, cracks, tilting, and sags. Even a few inches of settlement can cause significant damage to structures, roads, and utilities. Mine subsidence can occur abruptly or gradually over many years.
Many factors contribute to the risk of coal mine subsidence. Some of the more common factors include the depth of the mine workings, the geometry of the mine, how much coal was extracted, the overlying geology, and groundwater fluctuations.
Much of the Front Range, including El Paso, western Jefferson and southern Boulder and Weld Counties have problems with subsidence as a result of abandoned mine tunnels that snake under developed areas.
The Graden Coal Mine shaft collapsed in 2007 south of Dacono in Weld County. Note the land development activities in the background. Photo by Celia Greenman.