Framework of the Subsidence Problem

Given an underground opening of known vertical and horizontal dimensions, one wants to know: How much vertical subsidence will there be at the surface? What will be the lateral extent of surface subsidence? When has (or will) subsidence occur? 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 extraction of coal has been completed and a continuous unsupported void has been created. These conditions do result where longwall mining methods are employed in essentially horizontal strata.

In other places, mining was done by the room and pillar method in dipping strata and the underground conditions created cannot easily be made to fit simple subsidence models.

Subsidence Model

Recent 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 observations 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; 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.