Sinkhole progression at the Weeks Island, Louisiana, Strategic Petroleum Reserve (SPR) site Page: 5 of 20
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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 sink-
holes, and to mitigation. This paper summarizes current engineering geologic concepts, and
briefly describes diagnostic and risk mitigation efforts being conducted by the U. S. Department
of Energy, operator of the Strategic Petroleum Reserve (Bauer et al., 1994).
LOCATION, OCCURRENCE, AND CHARACTERISTICS
The Weeks Island salt dome is located 23 km (14 mi) south of New Iberia, Louisiana, and
is the central dome in the Five Islands chain, along with Belle Isle, and Cote Blanche, Avery, and
Jefferson Islands. All five have been mined because of their near-surface salt, and their logistical
advantage near the Gulf of Mexico and the Intracoastal Waterway. Belle Isle and Jefferson Island
are now closed to mining because of deliberate and inadvertent flooding, respectively.
The sediment cover consists of deltaic alluvium of the ancestral Mississippi River and is
about 56 m (185 ft) thick over the top of salt, which is 30 m (100 ft) below sea level at the-sink--
hole. The water table conforms generally with sea level over the dome but fluctuates somewhat
with topography and frequent torrential rains.
Weeks Island sinkhole #1 occurred over the southern perimeter of the Upper Level of the
two-level SPR mine (Figure 1), which has contained 73 million barrels of crude oil since 1981.
The mine was originally opened in 1902 and salt was extracted commercially until 1977, at which
time Morton Salt developed a new mine immediately adjacent to the northwest while the older
workings were converted for oil storage. Minor leaks of water had been noted at various times
during the 75 years of active mining, but in-mine grouting controlled inflow (Acres, 1987).
The nearly vertical sidewalls in the surface sediments surrounding the sinkhole caused
some perplexity initially, but were readily explained geologically as being typical of the Pleisto-
cene less mantle which caps the island. The sinkhole was also directly beneath a former resi-
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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/m1/5/: accessed April 26, 2019), University of North Texas Libraries, Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.