Like a chocolate soufflé removed too soon from the oven, fractured rock and sediment rising up for months inside a failed salt dome cavern tied to the Assumption Parish sinkhole has begun to drop, if ever so slightly.
The fall could be an indication that stability is increasing around the 15.1-acre sinkhole and Texas Brine Co.’s failed cavern that is suspected of causing the sinkhole to form, company officials and regulators agreed Friday.
The sinkhole, which has grown in spurts for months and is beset with periodic tremors, is one reason 350 people in the nearby Bayou Corne and Grand Bayou communities have been under evacuation orders for just short of 10 months.
Seeping natural gas also triggered by the cavern failure is another reason for the order, parish officials have said.
Workers have been idled inside the sinkhole’s earthen containment levee since early last week because of increasing numbers of tremors pointing to a possible sinkhole burp of natural gas or an edge collapse, parish officials said.
John Boudreaux, director of the parish Office of Homeland Security and Emergency Preparedness, said none such had occurred by Saturday morning.
During a period of nearly three decades, the Texas Brine cavern was mined with freshwater under pressure into a roughly cylindrical shape deep in the interior of the Napoleonville Dome, a vast solid salt deposit reaching tens of thousands of feet underground.
Scientists think a breach at a depth of 5,600 feet underground, near the bottom of the suspect cavern, has allowed sediments outside the salt dome and under intense pressures to flow into and rise up inside the 2,258-foot-tall cavity in the salt dome.
This subterranean movement of earth into the cavern undermined shallower layers of rock and sediment and triggered the swampland sinkhole, now 174 feet deep, scientists have said. The flow of rock has continued, fueling growth of the sinkhole and certain types of earth tremors.
Boudreaux said that cavern depth measurements going back to last fall have shown the rock had risen inside the cavern, on average, about 3 feet per day.
But, on Friday, Texas Brine measured a 29-foot drop during one week. Boudreaux added that never before had the level of rock slumped inside the cavern since the sinkhole emerged.
Texas Brine officials speculated Friday the drop could be an early sign that an awaited point of equilibrium is happening.
“It is compaction, and we might be reaching a point of equilibrium where in-filling may be coming to an end,” said Sonny Cranch, Texas Brine spokesman, citing the reasoning of company experts.
Scientists have said that the sinkhole would begin to stabilize after the cavern gets filled up to its roof with rock.
Once completely filled, the cavern eventually would reach an equilibrium, or balance point, between the pressures pushing sediments into the cavern and pressures working against that kind of movement. This balance would bring a probable end to the instability of both the cavern and the sinkhole.
If Texas Brine experts are right, equilibrium could be established sooner than predicted. The sediment and rock were measured at level of 129 feet below the cavern’s roof after the recent drop, according to parish figures.
State regulators, however, said it was too soon to make claims about arriving at a state of equilibrium, though they said stability may be improving.
“Pending further analysis, consultants indicate that the unconsolidated material that has been filling the cavern may be starting to settle and compact under its own weight and the subsurface pressure,” Patrick Courreges, spokesman for the Louisiana Department of Natural Resources, said in an email.
The sediment is called “unconsolidated” because it is new geologically and has not undergone the necessary eons of time to compress and harden.
The sediment is believed to have expanded once the rock was pushed inside the cavern and relieved of the underground pressures outside the salt dome. Scientists have speculated that the expanded sediments would re-compact once inside the cavern.
While indications of increasing stability may be emerging, Texas Brine has been hit by lawsuits in both state and federal court over the sinkhole and its side effects. At the same time, the company has been put under public pressure to offer out-of-court buyouts for residents covered by the evacuation order.
Through Friday, Texas Brine officials said they made settlement offers to 38 residents who have not filed suit among the 111 people eligible and interested in receiving the offers.
“We’ve had several acceptances,” said Texas Brine spokesman Cranch. He declined to provide any numbers.
He said others who received offers are considering them or have made counteroffers.
In a fact sheet issued Friday, the company said it is operating under a 30-day window for direct negotiations ordered May 22 by the federal judge overseeing four class-action lawsuits seeking damages as a result of the sinkhole.
That period ends at 5:30 p.m. June 24, according to the judge’s order.
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