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subsurface. Once again, however, these effects would diminish <br />• quickly as surface tension cracks began to heal. <br />It should also be noted that in the existing environment of <br />instability the development of tension cracks at the surface is a <br />common phenomenon which occurs at various locations on the slope <br />virtually every year,- pa rziculady ~ahen_cli.mactic conditions are <br />i <br />in a wet cycle. Therefore, the effects of subsidence represent <br />only one more source for the development of these tension cracks <br />but does not represent a major change in the existing <br />environment. <br />8.3.3 Long-Term Effects <br />The long-term, or permanent, effects of subsidence are primarily <br />• associated with the rubblization of rock in the immediate roof <br />area and for some distance above the mine level permanently <br />altering the shear strength characateristics of those materials <br />in the geologic profile. In areas having low overburden <br />thickness where chimney subsidence might result in the <br />development of sinkholes on the surface, a permanent surface <br />drainage modification might result which would increase <br />permanently the amount of moisture introduced into the subsurface <br />from surface runoff. The caving of the immediate roof and the <br />progressive failure of rock higher in the profile will create <br />zones of much weaker rock having strength characteristics <br />associated more with waste rock material than with a good quality <br />. jointed rock mass. <br />26 <br />