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Blinerield Subsidence at Windsor, Onrarir~ SETTLEMENTS PRIOR TO MAJOR SUBSIDENCE Variuus observations indicate that differential settlement of the surface of the brine field had become appreciable by the late 1940's, The earliest oC these observations was recorded in 1948, when it teas noticed that cracks had become conspicuous in a number of the plant buildings. At this time, the management initiated an investigation of the cause of the cracking. On the advice of consultants, some 80 reference points were established within the area of the plant. All were located in the west- erly part of the area involved in what appears to be a roughly circular bowl of subsidence. In October, 1948, the elevation of these points was determined with reference to a bench mark located at the east end of the plant (B\I 92, shown near the bottom of Fib. 2). The T`tarch, 195-1, sur- e I O5~ ;3 2 :l / \ /n1 t0 \5 2 / v •} 34 S~ 6 ,d !s t.a •2d ~ I:0 -'---- 12 ?o I.5 U - ';! ' A 5 .\ ORIGINAL , i EDGE OF J, I ~, SINKHOLE ~' ` ~ ; 7 • w --- •9 •Id •20 21 •tt •10 •q PLANT NORTH BN 32 • BRINF WFLLS ~-Sao I~ Figure 2 Location of brine srxlls and total subsidence in inches, measured from Oct. 1938 to Oct. 1950. - 301 vcy shos4rd that~had not participated in the gen- eral subsidence since 1948 (Peck, 1954). Annual sun•eys t.•erc carried out on the points located in the northwesterly quadrant of what later became the bow•I of subsidence, Points located in the southwesterly part of the area were not rc-surveyed until 1950; owing to a change of ownership, obser- vations on these points w•cre discontinued anti[ march, 195.1, after the major subsidence. Since 1954, changes in elevation of most of these points have been determined at regular intervals, whereas observations on the points in the northwesterly quadrant were discontinued after 195-1. Hence only Figure 2 showing settlement Erom 1948 through 1950 represents data for all reference points. Levels were carried out with ordinary rather than with precision instruments. Concerning their accuracy, Dr. Peck (1954) stated, "Studies of the data suggest that the elevation of a given reference point at the time of any one sun•ey ma}' have been determined with an error not exceeding one-half inch. Variations of this order of magnitude in suc- cessive readings are probably not significant." After the program of observations had been set up in 1943, the advice of consultants was not again sought until February, 194. The following account is based on data t~hich became available at that time. At the end of the first }'ear (October, 1949), measured settlements ranged between zero and 1.5 inches and exhibited no clear pattern. By the end of the second year (October, 1950) settlement ob- sen•ations indicated that a bo4vl-shaped depression was developing. As shown in Figure 2, maximum observed subsidence in 1950 had taken place in an area several hundred feet west of [he site of the future sinkhole. The reference points ]ocated in this area were trot main sun eyed until 195-1, and it is therefore not kttotsn whether this area continued to experience the maximum rate of subsidence. In the following }ears, (1951 through 1953), settlement increased at an accelerating pace, as il- lustrated by Figure 3 which shows measured settle- ment of Cive reference points [ocated near the site of the sinkhole formed in 195-1. Fiwre 4 shows the total settlements obsened in October, 1953, at the occasion of the fifth annua] sunev. It should be noted that the maximum set- tlement of 16 inches refers onh to that which took place after October, 19-18. Observations of the tilt of plant buildings carried out under the direction of Dr. Peck in 1954, indicated that the maximum total settlement which had taken placz prior to the