An Assumption Parish sinkhole’s edge collapsed following increased underground seismic activity for a third time in 16 days, leaving areas on or near the sinkhole temporarily off limits to response workers, officials said.
An out-of-use sinkhole access ramp, which had been cracked Monday after a previous round of tremors, slumped into the sinkhole Thursday along with trees on either side of the ramp and a corner of the earthen well pad linked to the ramp, parish officials said.
John Boudreaux, director of the Assumption Parish Office of Homeland Security and Emergency Preparedness, said the latest tremors were detected about 5:30 a.m. and 6:30 a.m. Thursday under the sinkhole and near a failed salt cavern suspected of causing the sinkhole.
The tremors were the same kinds of “very long period” events, linked to fluid and gas movement underground, that had been detected earlier this month, he said.
Work was halted only at the slurry-filled sinkhole’s surface, state Office of Conservation officials said in a news release Thursday.
The nearly eight-month-old and more than 13-acre sinkhole near the Bayou Corne area has remained in this fairly steady pattern since March 13, a condition characterized by tremors, work stoppages and slough-ins, with occasional burps of hydrocarbons and debris.
Experts have known for months that tremors precede edge collapses and other side effects, but the pattern this month appears to be cycling at a faster rate than previously.
Patrick Courreges, spokesman for the Louisiana Department of Natural Resources, said experts think the quickened pace indicates the cavern suspected of causing the sinkhole is getting close to getting full of surrounding earth, causing subsurface shifting movements to occur at shallower depths.
“That’s the theory,” Courreges said.
The Texas Brine Co. LLC cavern was carved between 3,400 feet and 5,650 feet deep, but scientists think the cavern was mined too close horizontally to the outer face of the Napoleonville Dome. The cavern is now known to have had a sidewall failure at a depth of more than 5,000 feet.
Millions of cubic yards of material from strata outside the salt dome appear to have migrated into the cavern. After subsurface fracturing during a period of several months or possibly longer, the slurry-filled sinkhole emerged in early August swamplands between the Bayou Corne and Grand Bayou communities, scientists have said.
Deep underground since August, though, material has continued periodically to enter the remainder of the cavern, a 20-million barrel cavity as tall as the highest buildings in the world.
Scientists and Texas Brine officials believe the sinkhole and surrounding earth will stabilize once the cavern is filled. In the meantime, about 350 residents of Bayou Corne and Grand Bayou remain under evacuation orders issued Aug. 3.
Boudreaux said it will probably be next week before work can be resumed following the latest tremors because small parts of the well pad were still breaking off and falling into the sinkhole as of Thursday afternoon.
Since March 13, authorities ordered two day-long work stoppages at the sinkhole and vicinity. During that span, two post-tremor slough-ins swallowed about 1.25 acres and many trees.
In a separate development, experts and Texas Brine officials have opened discussions on whether to drill one or two new, 2,000-foot wells into the western side of the Napoleonville Dome, officials said.
The wells would be used to create places deep inside the salt dome for installation of a seismic array previously deployed in another Texas Brine cavern.
Some geophones, which pick up underground tremors, were removed from the cavern Wednesday because officials want to focus on monitoring it, Boudreaux said.
Recent tests have shown the cavern, Oxy Geismar No. 1, is closer to the edge of the salt dome than once thought. That has raised worries about its structural integrity, since the cavern that failed, Oxy Geismar No. 3, also was close to the dome’s outer surface.
Though officials have said Oxy 1 is currently stable and the cavern’s internal pressure is being maintained, Boudreaux said the geophones can require adjustments that might involve removing them from the cavern, which could risk dropping the cavity’s pressure.
Sonny Cranch, spokesman for Texas Brine, noted that other elements of the seismic array remain in place, including two sets of geophones deep underground. He said removal of geophones from Oxy 1 had been under discussion for a few weeks.
Cranch added the company has made progress on the early phases of homeowner buyouts, signing up 86 homeowners for talks and conducting pre-appraisal inspections on 40 of their homes.