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<br /> <br /> <br />Subsidence <br /> <br />Ground SUBSIDENCE Is the sinking of the land over man-made or <br />natural underground voids. In Colorado, the type of <br />subsidence of greatest concern is the settling of the ground <br />over abandoned mine workings. <br /> <br />Characteristics <br /> <br />Subs I dence may occur abrupt I y--v I rtua II y I nstant I y--or <br />gradually over many years. It may occur uniformly over a <br />wide area as local depressions or pits separated by areas <br />which have not visibly subsided. In Colorado, It Is most <br />common in the sedimentary rocks over abandoned coal and clay <br />mines. The crystal I lne rocks in which most metals are mined <br />have greater strength and are I ess II ke I y to sett I e or <br />collapse. Subsidence can also occur where underground water <br />has dissolved subsurface materials or has been withdrawn by <br />wells. Although serious In other western states, these <br />latter types of subsidence are less common In Colorado than <br />sinking caused by the caving In of underground mine workings. <br />Subsidence caused by collapsing 50115 Is discussed under the <br />heading Collapsing Soils. <br /> <br /> <br />,.' <br />, ".. <br />@>' ~:I;> <br />---r <br />~ <br /> <br />This cutaway view of an underground coal mine shows the progression of <br />subsidence with time and Its affect on the surface topography and Improvements. <br />During operation. the mine roof In the working areas Is supported to protect the <br />miners. After abandonment, the underground voids may suddenly or slowly <br />collapse or fi II with rubble as the roof caves. This in turn can cause <br />subsidence at the surface. <br /> <br />45 <br />