ABSTRACT
A sinkhole was first observed in May 1992 over the outer edge of the two-tiered former salt mine that was converted for oil storage by the U. S. Strategic Petroleum Reserve (SPR). Results of diagnostic studies which included geophysical, geochemical, drilling, and hydrological methods suggest a direct connection exists between the surface collapse area and the underground mine. The connection was confirmed by correlative measurements of sediment slump rates, piezometric surface deflection, and brine influx rates into the mine. The dissolution of salt below the sinkhole that initiated the leak into the mine was likely caused by several confluent geologic processes, and exacerbated by mining-induced stresses that created fractures which served as hydrologic flowpaths.
Modeling studies of mine stresses show that years of tensional stresses may be required before cracking begins to occur, but once begun can continue to develop, and relieve the stress in that specific regime. The crack regime creates the avenue for incursion of groundwater, very slowly initially, but gradually enlarging as undersaturated groundwater dissolves salt on the sides of the crack.
Mitigation measures include increasing the mine pressurization, slowing the dissolution by injecting brine into the sinkhole throat, and permeation grouting in hydrologic flowpaths.
