Texas Brine Co. is building a westward extension of a 1.5-mile containment berm encircling the Assumption Parish sinkhole because a 200-foot stretch of the earthen wall is sinking, authorities said Friday.
The berm has been under construction for months to keep oil, salt and other contaminants from escaping the 13-acre sinkhole and harming surrounding freshwater swamps, bayous and aquatic life.
Scientists think the sinkhole surfaced in August after a subterranean Texas Brine salt dome cavern broke open, forcing the nearly nine-month evacuation of 350 people in the Bayou Corne and Grand Bayou communities.
John Boudreaux, director of the parish Office of Homeland Security and Emergency Preparedness, said Texas Brine contractors began work on the berm expansion Friday. The V-shaped “reroute” is shown on a company map jutting out from the western part of the berm.
“They started clearing trees,” he said.
Boudreaux said Texas Brine officials have said it should take a couple of weeks to put down the first layer of sand used for initial containment.
Still, Patrick Courreges, spokesman for the Louisiana Department of Natural Resources, said the additional work would likely delay the expected completion date, although containment of the sinkhole can be maintained by the existing wall until then.
“It’s not a situation where they’ve got to get this done tomorrow. They can manage it with fill until they can get the new part built,” Courreges said.
The existing western berm will be partially maintained for access.
Before the latest developments, Texas Brine officials had said the berm, which is being designed to last at least 20 years, is due for completion by July 1.
In a statement late Thursday, Louisiana Office of Conservation officials attributed the subsidence to continued filling of the failed Texas Brine cavern with unconsolidated sediments and compaction in a zone of broken rock under the sinkhole.
Courreges added Friday that the decision to move the berm was not related to sinkhole growth but to the subsidence and need to keep containment.
Boudreaux said the berm has not been breached. It gained containment on Feb. 22 of a 71-acre area that includes the sinkhole, Texas Brine officials have said.
Conservation officials have said the berm was designed to hold the sinkhole even at the worst-case size predicted by scientists, about 1,400 feet in diameter. The sinkhole now measures about 1,000 feet across.
Conservation officials said Thursday the sinkhole’s edge is still more than 100 feet from the western berm.
Boudreaux said depth surveys show the subsiding area along the western berm has slumped about 3 feet since January.
He added that recent video from overflights of the sinkhole show a patch of dead cypress trees along the inner wall of the western berm. Boudreaux attributed the dead patch to the fact that the trees are standing in water too deep for them to survive.
The western berm tracks an oilfield access road known as the Rig Road.
Boudreaux said there were indications last summer suggesting that part of the road is low. Later, when building the berm was proposed, concerns arose that the Rig Road was close to the sinkhole. He said Texas Brine officials agreed to move the berm west if needed.
Sonny Cranch, spokesman for Texas Brine, said Friday that because of the subsidence, the company is playing it safe and building the new berm.
Scientist have said since last year that the sinkhole showed up after a sidewall failure in the Texas Brine cavern at a depth of more than 5,600 feet. Millions of cubic yards of sediments flowed into the cavern as a result.
In addition to the sinkhole, the shift in earth left behind a zone of fractured rock running up outside the cavern and alongside the Napoleonville Dome, scientists have said.
In a presentation April 5 at LSU, Gary Hecox, a CB&I hydrogeologist, described how other rock movements also have been increasing the sinkhole’s size and creating subsidence in the surrounding area.
The sinkhole has a 25-acre settlement area around it, he said then.
Hecox said that sands from the Mississippi River Alluvial Aquifer, which runs in a band about 120 feet to as much as 600 feet deep, are flowing down into the zone of broken rock under the sinkhole.
Also, the sinkhole’s volume has more than doubled since it was calculated in October, rising from 550,000 cubic yards to 1.2 million cubic yards, Hecox said then.
“The material is flowing out of the bottom (of the sinkhole) down the disturbed rock zone,” Hecox said.
Texas Brine released results Monday from a new seismic survey suggesting the rock zone is far shallower than Hecox and others had theorized and that the zone is connected to the cavern by a narrow pathway.
State officials say they are checking Texas Brine’s seismic data.
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