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212 M. L, JEREMIC <br />3. However the phase of accelerated subsidence <br />takes place wi thrn a period of a couple of years. During <br />this time the ground surface deformation forms a bowl- <br />shaped depression, which might subside up to 15 inches. <br />Accelerated subsidence in Windsor occured during the <br />period between 1953 and 1954. <br />SUBSIDENCE FROM SOLUTION M1N1NG OF SALT 213 <br />4. Abrupt subsidence can occur within several <br />days or hours. Nor example, aC wind sor (February 17,' <br />'~ 1954) the catastrophic subsidence happened with an interval <br />of only several hours. In Lhe mo rniny precursors of <br />abrupt subsidence consisted of rumbling noises and minor <br />vib r.,Liulu; Lhcsu Ix cauiu muru suvuru aL LUr unc hour. <br />After several hours, the bow-shaped depression began to <br />Lill wiCh waLCr and rapid subs id eucc formed a sinY. hole. <br />' The sink hole appeared along one side of the rim of the <br />bowl-:;hoped dupres:;iuu and iL was up Lu 10 .u fiche:: deep. <br />Dy mid-afternoon, movement had virtually ceased. In a <br />similar nia nner, Lhe sin Y, hole at '1'uzla, Yugoslavia, was <br />' formed but the proces of abrupt subsidence lasted through <br />several Boys. <br />5. Post subsidence settlement was immediately <br />Lollowed by irregular movemcn Cs oL small magnitude. AC <br />Windsor, the average annual rate of settlement over the <br />14-year period Following the sink hole was up to 1/2 inch <br />per year. <br />If the curve of the subsidence deformation as a <br />function of time is analyzed, many similarities with a <br />generalized creep curve can be noticed. A typical three- <br />stage creep is developed in which the deformation rate <br />accelerates with time. In time (decades and years), a <br />rock salt formation with some ductile properties may be <br />deformed by the creep mechanism with three distinct <br />. FIGURE 3 - Diagram of subsidence deformation <br />as a function of time at Windsor, <br />Ontario (after published data by <br />R D. Terzaghi). <br />