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6 ~ ~ M, L. JEREMIC <br />s a result of solution mining, the thick beam will deform. <br />he deformation slowly increases. <br />2. Slow lateral movement at an approximately con- <br />tant rate permits gradual bending of the postulated thick <br />eam. It is possible that during tl~e extraction period <br />here will be formed not one but more cavities in salt <br />ormation controlled by individual wells. It mighC be <br />upp~ that more or less similar sagging takes place <br />n all cavities. As a function of time, creep deformation <br />111 be prolonged before rupture occurs (Fig, lb). <br />,r avi[a tional body For cr. <br />Overlying strata <br />a <br />Rock Sal[ Formation <br />Intercalated <br />Rock Salt Layers <br />• <br />Fhaar Ctress <br />1'c nsilc ~f r~ss <br />b -~ <br />//////// <br />Indivi d~>al Caves <br />Uniaxia] Corn?r~:ssion <br />~ i ~ <br />C ~ d ~ I ~ <br />Cave" <br />"Ar cti of Draw" <br />FIGURE 1 - The hypothetical model of the subsurface <br />subsidence with stress field development. <br />SUBSIDENCE FROM SOLUTION MINING OF SALT 207 <br />3. Dr. Ilays (1) suggested that soon or later most <br />salt wells became connected through the solution channels <br />to each other and finally to a "general cavern". At this. <br />moment, the strength of the overlying sediments might be <br />overcome and rupturing initiated. The rupturing will be <br />progressive, and arches in the overlying rock will be <br />formed (Fig. lc). <br />4. When the rock salt remnants can no longer <br />carry their own weight and the weight of unconsolidated <br />overlying rock in the area of a "general cavern", they <br />will collapse: At this moment a "general cavern" will t>e <br />' totally filled in by fragmented rock and the arch will be <br />extended at an accelerated rate. Probably, when the para- <br />bolic slope of the arch of drove intersects the ground sur- <br />face, a sink hole will be formed, with vertical ground <br />i <br />displacement of 5 to 25 feet (Fig. ld). <br />' Surface subsidence obviously results from the sub- <br />, surface deformations. From many published data of surface <br />subsidence caused by solution mining, several common <br />features as this mechanism can be inferred as suggested <br />below: <br />1. The surface effects are associated with verti- <br />cal and lateral ground movements. Probably all lateral <br />surface displacements are toward the center of the caves. <br />Vertical surface displacement starts well beyond the <br />