The massive sinkhole in northern Assumption Parish had another burp and edge collapse Thursday morning after tremors had remained at an elevated frequency and halted work in and around it the past few days, parish officials said.
But parish officials said that by Thursday afternoon, the micro-earthquakes near the Bayou Corne and Grand Bayou communities known as sharp tremors had calmed down and precautions halting work were eased.
While there was no formal estimate of how much swampland collapsed Thursday, John Boudreaux, director of the parish Office of Homeland Security and Emergency Preparedness, said the new collapse area is roughly 25 feet by 100 feet.
The formerly forested swatch is on the east side of the more than 13-acre, 160-foot deep sinkhole, north of a well pad being used for sinkhole access and seismic monitoring, he said.
Boudreaux said trees went into the sinkhole Thursday morning and he was able to see and record — from the well pad — one of the trees going “straight down” into the hole.
“It doesn’t fall. It sinks,” he said.
A few more trees toppled over later in the day, Boudreaux said.
Scientists believe the failure of a Texas Brine Co. salt dome cavern led to the formation of the sinkhole last year. About 350 residents have been evacuated more than eight months due to the sinkhole and oil and gas venting from deep sources unleashed by the cavern failure.
Boudreaux said earlier this week that an increase in micro-earthquakes from about 10 per day to 50 per day prompted a halt in work inside the sinkhole berm beginning Sunday. The micro-earthquakes indicate rocks breaking deep underground.
The berm contains 71 acres that includes the sinkhole and surrounding swamp.
By Thursday afternoon, once tremors subsided, work was only prohibited on the sinkhole surface, Boudreaux said.
He said that in addition to the tremors, slough-off and burp, water was moving in the sinkhole but that movement stopped later Thursday when tremors subsided.
In a presentation to an expert panel on the sinkhole earlier this month, CB&I hydrogeologist Gary Hecox described for scientists how water in and around the sinkhole moves during burps, “sloshing” with a tidal-like action.
“So essentially during one of these burp events, the water inside the berm is active,” Hecox said. “It moves back and forth, which requires a lot of energy to do that, and we think that’s why the bottom is constantly changing.”