nce at the surface.
����The vertical component of subsidence decreases with increasing depth or thickness of overburden, especially bedrock. As the roof rock sags, ruptures, and eventually collapses into a mined-out area, the roof rock rotates, twists [16], splinters [17], or crumbles [18] as it falls, resulting in incomplete compaction. In other words, the mine void is not completely filled during a mine-roof collapse. Because bedrock collapses with incomplete compaction, the deeper the extraction area, the smaller the vertical component is at the surface.
����Mining subsidence is related to the strength or competency of bedrock, which is a measure of a rock's load-bearing capacity [19]. Sandstones and limestones are capable of withstanding greater loads than are shales and mudstones. Therefore, sandstones and limestones can span larger unsupported distances or support thicker amounts of overburden before failing.
����Mining subsidence is affected by water circulation [20] or the fluctuation of water level in a mine. Some underground mines remain dry after abandonment; many others fill with water. Circulating water in an underground mine can deteriorate roof support or the roof rock. Because of its incompressibility, water provides support to the roof of a mine that is filled with water. However, the likelihood of roof collapse may be enhanced or accelerated in mines where the roof rock is repeatedly saturated [21] then left unsupported by fluctuating water levels (either by seasonal weather conditions or intentional pumping) and where the pillars of coal are eroded by flowing water.
����The likelihood of subsidence increases where fractures (joints) intersect the mine roof. Fractures or joints are natural planes of weakness where collapse of the mine roof is likely to occur. Fractures also may allow the subsidence to extend beyond the limit of the mined area.
����The length of time for mining subsidence to occur increases with increasing depth of mining and increasing competency of overburden. The type and amount of roof support in addition to pillars of coal left in the mine also affect subsidence. Most early underground mines used wooden timbers [22] as additional roof support. Steel I-beams [23] were used mines as roof support beginning in the early twentieth century. By the mid-twentieth century, roof bolting [24] was another type of roof support being used in mines. With time following abandonment of an underground mine, these types of roof support eventually rot or deteriorate, allowing subsidence to occur. Because of the complexity of the variables which contribute to mine-related subsidence, no acceptable system exists which is capable of accurately predicting the time or amount of subsidence in a variety of geological settings.
NOTES TO THE TEXT [1] mining subsidence�����ɳ��� [2] earthquake������ [3] damage���ƻ����� [4] collapses�����䣬���� [5] potential risk��DZ�ڵ�Σ�� [6] mecha ��һҳ[1][2][3]��һҳ
����¼�룺mkaq ���α༭��mkaq |
|
