A sinkhole formed over the former salt mine used for crude oil storage by the U.S. Strategic Petroleum Reserve at Weeks Island, Louisiana. This created a dilemma because in-mine grouting was not possible, and external grouting, although possible, was impractical. However, environmental protection during oil withdrawal and facility decommissioning was considered critical and alternative solutions were essential. Mitigation of, the sinkhole growth over the salt mine was accomplished by injecting saturated brine directly into the sinkhole throat, and by constructing a cylindrical freeze curtain around and into the dissolution orifice at the top of the salt dome. These measures vastly reduced the threat of major surface collapse around the sinkhole during oil transfer and subsequent brine backfill. The greater bulk of the crude oil was removed from the mine during 1995-6. Final skimming operations will remove residual oil trapped in low spots, concurrent with initiating backfill of the mine with saturated brine. Environmental monitoring during 1995-9 will assure that environmental surety is achieved.

The initial sinkhole at the Weeks Island SPR site that was first observed in May 1992 gradually enlarged and deepened, concurrent with the increasing dissolution of salt over the mined oil storage area below. Beginning in 1994 and continuing to the present, the injection of saturated brine directly into the sinkhole throat some 76 in (250 ft) beneath the surface essentially arrested further dissolution, buying time to make adequate preparation for the safe and orderly transfer of crude oil to other storage facilities. A second and much smaller sinkhole was first noticed in early 1995 on an opposite edge of the SPR mine, but with a very similar geological and mine mechanics setting. Both sinkholes occur where the edges of upper-152 in (-500 ft) and lower-213 m (-700 ft) storage levels are nearly vertically aligned. Such coincidence maximizes the tensional stress development leading to fracturing in the salt. Such cracking takes years to develop, perhaps 20 or more. The cracks then become flowpaths for brine incursion, wherein after time it is released into mined openings. Undersaturated ground water gradually enlarges the cracks in salt, leading to further dissolution and eventual collapse of the overlying sand to form sinkholes. Other geologic conditions may have been secondary factors in controlling both mining extent and sinkhole location. An en echelon alignment of sinkholes over other mine edges has been observed. Thus most likely areas of future occurrence at Weeks Island are adjacent to the existing sinkholes; surface inspections are now concentrated at those locations. Although neither timing nor location is predictable with precision, the study of numerous sinkholes elsewhere shows that progression is inevitable, provided that relevant conditions and enough time exists for development. These principles should provide mine designers and operators the knowledge to minimize the occurrence of sinkholes, and to plan for their progression when they occur.

The initial sinkhole at the Weeks Island SPR site that was first observed in May 1992 gradually enlarged and deepened, concurrent with the increasing dissolution of salt over the mined oil storage area below. Beginning in 1994 and continuing to the present, the injection of saturated brine directly into the sinkhole throat some 76 in (250 ft) beneath the surface essentially arrested further dissolution, buying time to make adequate preparation for the safe and orderly transfer of crude oil to other storage facilities. A second and much smaller sinkhole was first noticed in early 1995 on an opposite edge of the SPR mine, but with a very similar geological and mine mechanics setting. Both sinkholes occur where the edges of upper-152 in ( -500 ft) and lower-213 m ( -700 ft) storage levels are nearly vertically aligned. Such coincidence maximizes the tensional stress development leading to fracturing in the salt. Such cracking takes years to develop, perhaps 20 or more. The cracks then become flowpaths for brine incursion, wherein after time it is released into mined openings. Undersaturated ground water gradually enlarges the cracks in salt, leading to further dissolution and eventual collapse of the overlying sand to form sinkholes. Other geologic conditions may have been secondary factors in controlling both mining extent and sinkhole location. An en echelon alignment of sinkholes over other mine edges has been observed. Thus most likely areas of future occurrence at Weeks Island are adjacent to the existing sinkholes; surface inspections are now concentrated at those locations. Although neither timing nor location is predictable with precision, the study of numerous sinkholes elsewhere shows that progression is inevitable, provided that relevant conditions and enough time exists for development. These principles should provide mine designers and operators the knowledge to minimize the occurrence of sinkholes, and to plan for their progression when they occur.

A sinkhole measuring 11 m (36 ft) across and 9 m (30 ft) deep was first observed in alluvium overlying the Weeks Island, Louisiana, salt dome in May 1992, but it was about a year old, based on initial surface appearance and subsequent reverse extrapolation of growth rates. A second and much smaller sinkhole was identified in early 1995, nearly three years later. Their position directly over the edges of the SPR oil storage chamber, a former room-and-pillar salt mine, caused apprehension. The association of sinkholes over mines is well established and this occurrence suggested that groundwater influx undoubtedly was causing salt dissolution at shallow depth, and associated collapse of soil at the surface. Leaks of groundwater into other salt mines in Louisiana and elsewhere led to flooding and eventual abandonment (Coates et al., 1981). Consequently, much attention has been and continues to be given to characterizing these sinkholes, and to mitigation. This paper summarizes current engineering geologic concepts, and briefly describes diagnostic and risk mitigation efforts being conducted by the US Department of Energy, operator of the Strategic Petroleum Reserve (Bauer et al., 1994).

A sinkhole discovered over the edge of the Strategic Petroleum Reserve storage facility at Weeks Island salt dome, Louisiana, led to decommissioning the site during 1995--1998, following extensive diagnostics in 1994. The sinkhole resulted from mine-induced fractures in the salt which took may years to develop, eventually causing fresh water to leak into the storage chamber and dissolve the overlying salt, thus causing overburden collapse into the void. Prior to initiating the oil removal, a freeze wall was constructed at depth around the sinkhole in 1995 to prevent water inflow; a freeze plug will remain in place until the mine is backfilled with brine in 1997--8, and stability is reached. Residual oil will be removed; environmental monitoring has been initiated and will continue until the facility is completely plugged and abandoned, and environmental surety is achieved.

A sinkhole was first observed in May 1992 over the edge of the two-level former salt mine that was converted for oil storage by the US Strategic Petroleum Reserve (SPR). Diagnostic studies that included geophysical, geochemical, drilling, and hydrological methods suggest a direct connection exists between the surface collapse area and the underground mine as shown by correlative measurements of sediment slump rates and brine influx 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 may be required before tensional 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 freeze grouting to restrict hydrologic flowpaths.