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Z1O M, L, JEREMIC SUBSIDENCE FROM SOLUTION MINING OF SALT 211 (Windsor, Ontario (1); Tuzla, Yugoslavia (4)). Theoretical analyses of the deformation on the ground surface caused by mining operaCions in bedded de- posits is very well explained by "trough subsidence (5). Unfortunately, there is no similar analysis for sub- sidence caused by solution mining. It is very difficult to represent, from an engineering point of view, the rc- l~n between surface ground deformations and the extrac- tion area. However, in a general sense, subsidence curva- ture of deformation might sometimes be used as a parameter for an approximate estimation of the extension of the "general cavern" and the tip of the parabolic slope of the arch of drove. SUBSIDENCE n5 A FUNCTION OF TIt4~ Surface and subsurface subsidence are time dependent deformations. For example, the surface subsidence of most r~ salt deposits excavated by underground methods is very shallow and regular, without rupture of the ground surface. In those cases, subsidence is not manifested immediately after excavation, and it can only be measured after several years. Usually a couple of years after underground excava- tion ceases, the surface comes to rest. Ilowever, on the contrary, subsidence caused by solution mining may cause an erratic and unpredictable surface deformation. An instructive example of the subsidence progress is published by~R. D. Terzaghi in a paper on "Brinfield Subsidence at Windsor, Ontario" (1). The rate of subsidence at wind sor ~ had many common features with that of other surface defor- ~ mations caused by solution mining in many parts of the world (Fig. 3). At Windsor, the salt formation is about 600 feet thick and consists of shale, dolomite, gypsum anhydrite and rock salt. The thickness of the overlying sediments is about 900 feet. The subsidence deformation as a function of time is summarized below: 1. ~ The period interval before subsidence was evident was much longer for solution mining than for an excavation formed by underground mining methods. Usually subsidence above an excavation due to mining is noticeable after two ~to three years. however, this period for solu- ' tion mining of rock salt deposits, subsidence may perhaps be only noticeable after decades and in some cases after centuries, depending on the intensity of solution mining. For example, at Windsor, solution mining began in 1902, but the first appreciable surface deformation was the late 1940's. 2. The first phase settlement of subsidence for solution mining progresses at a very slow rate, and it can last for several decades. At Windsor, the first phase of surface subsidence was from 1940 to 1952, forming a con- 1 vex shaped depression. t