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The Retsof Salt Mine in Livingston County, New York, was successfully operated for 109 years by extracting -12 ft of salt from a gently dipping layer within the Vernon Formation of Silurian age. A collapse and inrush of brine occurred at mine level on March 12, 1991. The brine gradually changed to fresh water and flowed in at a sufficiently high rate to result in the complete flooding of the 6,500 acre mine in 21 months. Although the initial collapse occurred into a room with a nominal height of 12 ft, a 15-ft-deep sinkhole eventually developed at the land surface 1,100 ft above the mine. This initial sinkhole destroyed part of a U.S. highway and bridge. A second sinkhole formed over an adjacent panel and resulted in nearly 70 ft of surface subsidence. These sinkholes penetrated geologic materials consisting of 600 ft of rock overlain by 500 ft of glacial sediments. Two distinct theories were postulated for the mechanism of sinkhole formation: (1) piping of glacial sediments through open fractures in the bedrock without actual loss of bed rock support, and (2) loss of support for the glacial sediments by undermining and down dropping of the bedrock. The resolution of these theories was important in order to assess whether similar sinkholes were possible over other parts of the mine, or whether the existing sinkholes would stabilize sufficiently to allow reconstruction of the U.S. highway and bridge. Drill-hole data, seismic-activity record, subsidence measurements, and seismic reflection data were analyzed. The data indicate that the sinkholes formed by the down dropping of the bedrock and glacial sediments into voids created by the dissolution of salt and the slaking of salt-bearing shale upon exposure to fresh water.

Authors:

Gowan, S. W. and Trader, S. M.
Oklahoma Geological Survey Circular /09, 2003

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