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;' <br />' Figure 2. <br />3.0 Cround response to solution mining. <br />Formation of underground openings in the process of conventional or solution mining results in <br />deformations of the surrounding rock mass caused by load redistribution that follows removal of <br />ground support. Although the ground response is always immediate, ground deformations would <br />continue long after mining, even on a geological time scale, most likely until the opening is per- <br />manently closed. Closure of openings taould inevitably be reflected in ground surface move- <br />ments. Such movements may not be noticeable if their rate is comparable to the rate of natural <br />changes in the ground elevation. Alternatively, ground subsidence may be rapid and may lead to <br />formation of sinkholes. Such time-dependent processes are determined by time-tiependerrt prop- <br />erties of geologic materials. <br />All physical substances continue to deform under sustained loading to one degree or another. If a <br />cylindrical sample of rock salt, for example, is placed under the type of vertical stress that exists <br />in the ground at a depth of, say, 1000 ft. and be left rmconfined laterally, it would reduce its height <br />by half in something like 2000 years. If this load is tripled, the time to reduce the height of an <br />unconfined cylindrical sample by half maybe only 10 years. This example reflects complex creep <br />properties of rock salt. Only when stresses are equal in all directions creep ceases. An under- <br />ground opening is always unconfined at its face and creep never ceases until the opening is <br />closed. There are other t}~pe oftime-dependent properties of rocks, also related to the difference <br />in imposed stresses: strength of rock may degrade with time if a stress difference, also called <br />sheer stress, is acting over a long period of time. Strength degradation may be particularly rapid <br />tinder tensile stress. Development of tension in formations surtounding salt cavities is the key <br />mechanism that controls design of openings in salt rock. Tensile stresses in overburden strata <br />develop and increase gradually as a result of their sagging caused by creep of salt. <br />3.1 Sb•ess redish•i6ution mechnnisms <br />Removal of ground support as a result of mining causes thz overburden to transfer the weight of <br />6 <br /> <br />