Coal Mine Subsidence Theory

Basic Framework of Subsidence Problem

The basic questions asked with regard to subsidence are few in number and seem deceptively simple to answer. Given an underground opening of known vertical and horizontal dimensions, one wants to know:

  1. How much vertical subsidence will there be at the surface?
  2. What will be the lateral extent of surface subsidence?
  3. When has (or will) subsidence occur?
  4. Has the process of subsidence been continuous from beginning to end or has it been episodic?

Generally, the subsidence theories used to answer these questions are based on the assumption that extraction of coal has been complete and that a continuous unsupported void has been created. These conditions do result where longwall mining methods are utilized in essentially horizontal strata, as in Europe. However, in Jefferson County, most mining was done by the room and pillar method in dipping strata and the underground conditions created cannot easily be made to fit European subsidence models.

Subsidence Model

Modern attempts to relate surface subsidence to underground mining make use of the concept of the subsidence trough. The concept takes into account one of the most important observed facts regarding subsidence; namely, that the surface area affected is larger than the mined out area.

The extraction of coal removes support from the overlying strata causing them to sag into the void space created. The sag is propagated upward to the surface, and, it follows, that the maximum surface subsidence can be no greater than the thickness of the coal bed mined. (In beds that dip significantly subsidence can exceed the thickness of the coal bed mined.) However, the lateral extent of subsidence at the surface is greater than the extent of underground mining. The surface position of the boundary between areas of subsidence and no subsidence is defined by the "angle of draw." This is the angle between a vertical line drawn upward to the surface from the edge of the underground opening and a line drawn from the edge of the opening to the point of zero surface subsidence. The angle of draw varies from 25 to 35 degrees in most instances. The larger the angle of draw the wider will be the zone on the surface in which subsidence should occur. By using the largest of several possible angles of draw a greater margin of safety is established for those areas lying outside the boundary of possible subsidence.

Subsidence usually occurs gradually when it is concurrent with mining. After cessation of mining, subsidence may continue to occur in a steady, gradual manner, or it may stop for a period, to be followed by failure at some later date.

Subsidence Hazard Zones

Subsidence hazard zones are those areas where ground subsidence may occur as a result of past mining activities. The term "mining activities" is meant to include open pit mining as well as underground mining since the subsidence hazards that arise from open pits while not so obvious, can be equally as dangerous as those associated with underground mines. The extent of the subsidence hazard zones near Golden is shown below.

Subsidence hazard map of Golden area. Shaded areas represent subsidence hazard zones.

Subsidence Hazard Boundaries

The purpose of the Subsidence Hazard Map is to represent those areas where the land surface may possibly be affected by subsidence or collapse related to underground or surface mining of coal and clay.

The determination of the limits of hazardous areas varies with the type of mine and with mine geometry. The following criteria were used to delineate the subsidence hazard areas over the mines of Golden.

Surface Mines

The hazard zone boundary will lie at a distance beyond the outer limits of the bottom of the pit at a distance which is equal to three times the nominal depth of the pit plus an additional 25.

Underground Mines

  1. Relatively low-dip mine workings (horizontal to 30 degree dip) - The hazard area boundary will lie beyond the mapped boundaries of the mine at a distance which is determined by a 35 degree angle of draw plus a 200 safety zone.
  2. Steeply dipping mine workings (30-70 degree dip) - If the ground surface over a mine in steeply dipping beds is nearly horizontal then the subsidence hazard boundaries on the footwall side of the mine will lie 100 beyond the extension of the footwall to the surface. The subsidence hazard zone boundary on the hanging wall side of the mine will lie beyond the mapped workings a distance equal to the maximum depth of the mine plus a 100 safety zone.
  3. Near-vertical and vertical mine workings (70 degrees to vertical dip) - If the ground surface is nearly horizontal then the subsidence hazard zone boundaries of a mine in nearly vertical or vertical beds will lie beyond the mapped boundaries of the mine at a distance equal to the maximum depth of the workings plus a 100 safety zone.

For surface and underground mine workings where the deepest part of the mine is at an elevation lying above much of the surrounding topography, the topographic contour of that elevation becomes the hazard boundary.

Note: The data shown on this map is not intended to be used for site-specific planning and development. When a specific site is being considered for development a detailed study should be undertaken which considers not only subsidence and geologic factors but also the planned use for the site.

All of the geologic information contained on this page was taken directly from Coal and Clay Mine Hazard Study and Estimated Unmined Coal Resources, Jefferson County, Colorado by Amuedo and Ivey, 1978