Sinkhole progression at the Weeks Island, Louisiana, Strategic Petroleum Reserve (SPR) site Page: 4 of 20

Solution Mining Research Institute Fall Meeting
San Antonio, TX; 23-24October, 1995
Sinkhole Progression at the Weeks Island , Louisiana,
Strategic Petroleum Reserve (SPR) Site
James T. Neal, Stephen J. Bauer, and Brian L. Ehgartner
Underground Storage Technology Department
Sandia National Laboratories
Albuquerque, NM 87185-0706
ABSTRACT
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
storage area below. Beginning in 1994 and continuing to the present, the injection of saturated
brine directly into the sinkhole throat some 250 feet beneath the surface essentially arrested fur-
ther dissolution, buying time to make adequate preparation for the safe and orderly transfer of
crude oil to another storage facility. This mitigation measure marked the first time that such a
control procedure had been used in salt mining; previously all control had been achieved by in-
mine and surface grouting.
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 oc-
cur where upper (-500 ft) and lower (-700 ft) storage levels are nearly vertically aligned. Such
coincidence maximizes the tensional and dilatant stress development, leading to fracturing in the
salt. Such cracking takes years to develop, perhaps 20 or more. The cracks then become pas-
sageways for brine incursion, and after time find their release into mined openings. Undersatu-
rated ground water gradually enlarges the cracks, leading to further dissolution and eventual col-
lapse of the sand overlying the salt to form sinkholes.
The en echelon alignment of sinkholes elsewhere over mine edges is commonplace. Thus
most likely areas of future occurrence at Weeks Island are adjacent to the existing sinkholes; sur-
face 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 prin-
ciples should provide mine designers and operators the knowledge to minimize the occurrence of
sinkholes, and to plan for their progression when they occur.
INTRODUCTION
A sinkhole measuring 11 m (36 ft) across and 9 m (30 ft) deep was first observed in allu-
vium 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.
This work was performed at Sandia National Laboratories and supported by the U. S. Department of Energy under Contract DE-ACO4-94AL85000

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Neal, J.T.; Bauer, S.J. & Ehgartner, B.L. Sinkhole progression at the Weeks Island, Louisiana, Strategic Petroleum Reserve (SPR) site, article, November 1, 1995; Albuquerque, New Mexico. (https://digital.library.unt.edu/ark:/67531/metadc620914/m1/4/ocr/: accessed April 19, 2019), University of North Texas Libraries, Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.

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