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302 <br /> <br /> <br /> lo" <br /> <br /> W <br />V <br />Z <br /> W <br />y <br />H <br /> <br /> <br /> <br />0 I 2 3 4 5 6 <br />Years <br />TIME•SUBSIOENCE CURVES <br />Figure 3. S<ttlemen[ of five reference points located near sinkhole. <br />sudden subsidence was probably as much as two <br />feet, but not more than three fee[. <br />Figure 5 shows successive profiles through the <br />subsiding area, based on the annual surveys of ref- <br />erence points. Reference points surveyed only in <br />1948 and 1950 are not included. <br />Fieure 6 shows changes in the annual rate of <br />subsidence from 1949 to 1953. In the zone of zero <br />change, subsidence took ~p[ace at about one inch <br />per year. It may be assumed that this settlement <br />represents slow creep in the remaining "pillars" of <br />salt located at depth beneath the peripheral por- <br />tions of the bowl of subsidence. <br />FORMATION OF "SINKHOLE" <br />On February 19, 1954, several mcnths after the <br />fifth annual survey of the reference points, an <br />alarming series of events took place at the site of <br />`rineliNd Subsidence •r Windsor, Onrerio <br />•o <br />1.0 <br />30 <br />s°~ <br />.n \ <br />Origina/edge <br />afsinkhole -3__` <br />~~ <br />i ~.~ <br />/ <br />I ~a <br />1 ~~c <br />t ~c <br />I ~6C <br />1 <br />1 <br />1 <br />t <br />\~~ <br />`` ~__ <br />I~~ <br />PLANT NORTH IOOff. <br />Figure {Total subsidence in inches, measured from Octob<r 1948 <br />to Oc[obcr 1953. <br />the plant. The follo~+•ing account of these events is <br />based on. eye-++•itness reports. <br />The first precursors of trouble consisted in <br />rumbling noises and minor vibrations noted be- <br />tween 8 a.m. and 9 a.m. These became more severe <br />after 9 a.m. Next, distortions and leakage of vari- <br />ous ++'ater and steam lines ++'ere observed, and by <br />10:30 a.m. it was apparent that a depression was <br />forming in the originally nearly level surface of the <br />ground. About noon, the depression began to fill <br />with +vater from a nearby s+vamp and later from a <br />ruptured water main and a brine line. Another <br />source of water +vas a sftort-lived jet of v,~ater spurt- <br />ing from a fissure which had opened in the ground <br />within the depression. At about 1:45 p.m., a rapid <br />subsidence, amounting to approximately three <br />feet, took place around one side of the rim of the <br />depression. This event +vas accompanied by the ap• <br />pearance of a jet of +vater about four feet high, <br />issuing from the Cissure along +vhich subsidence had <br />i <br />