ABSTRACT
Subsidence usually refers to downward motion of a ground surface, “settlement,” but may also refer to below ground motion or internal subsidence. The most common causes of subsidence are: (1) withdrawal of fluids from pores and fractures and (2) disturbance of the ground by excavation processes. Water well pumping and consequent drawdown of the water table is a very common cause of surface subsidence. Withdrawal of oil, and to a lesser extent, withdrawal of gas from wells is also a significant source of subsidence in some areas. Superincumbent loads on saturated, porous ground may also cause settlement of the ground surface by expulsion of pore fluids, a consolidation process. Surface and underground excavation cause subsidence through induced changes in stress, strain, and displacement fields in adjacent soil and rock masses. Sometimes excavation is a natural process, for example, formation of solution cavities by circulating ground water in limestone formations. If the induced displacements from underground excavation extend to the surface, then surface subsidence as well as internal subsidence result. In cases where damage to surface structures or underground utilities occurs, there is considerable importance attached to the determination of the cause of damage in the presence of subsidence and the actual subsidence mechanism.
Caving is an often observed phenomenon associated with underground mining and begins with collapse of the back, roof, or hanging wall. Caving may occur unexpectedly or be deliberately induced in conjunction with the mining method (block caving, sub-level caving, longwall mining, room and pillar mining with pillar extraction). Caving begins because locally stress tends to exceed strength, loading progresses beyond the elastic limit, fracture and failure occur, and broken rock is liberated from the parent rock mass and falls to the floor. Failure of the rock mass is greatly assisted by the presence of joints, of course, especially if the intact rock, between joints is strong, but both mechanisms of rock mass failure (slip and separation of joints, fracture of intact rock, and extension of such fractures) contribute to caving. Cessationof cavingmayoccur because an equilibriumshape forms as the cave evolves (natural “arching”), a strong rock formation is encountered during the upward progress of the cave or the void becomes filled with broken rock (swell).
