THE MECHANISM OF SINKHOLE FORMATION IN GLACIAL SEDIMENTS ABOVE THE RETSOF SALT MINE

The Retsof Salt Mine in Livingston County, New York was successfully operated for 109 years by extracting approximately 3.7 m of salt from a gently dipping layer within the Vernon Formation. A collapse and inrush of brine occurred at mine level on March 12, 1994. The brine gradually changed over to fresh water and flowed in at a sufficiently high rate to result in the complete flooding of the 26 km2 mine in 21 months. Although the initial collapse occurred into a room with a nominal height of 3.7 m, a 4.6 m deep sinkhole eventually developed at the land surface 335 m above the mine. This initial sinkhole destroyed a portion of a state highway and bridge. A second sinkhole formed over an adjacent panel and resulted in 21 m of surface subsidence. These sinkholes penetrated through geologic materials consisting of 183 m of rock overlain by 152 m 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 bedrock support, and 2) loss of support for the glacial sediments by undermining and downdropping of the bedrock. The resolution of these theories was important in order to assess whether similar sinkholes were possible over other portions of the mine or whether the existing sinkholes would stabilize sufficiently to allow reconstruction of the state highway and bridge. Drill hole data, seismic activity records, subsidence measurements and seismic reflection data were analyzed. The data indicate that the sinkholes formed by the downdropping 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.