Officials don’t think events are related
Just another bump in the road in swampy, sinking south Louisiana or a more ominous early warning from the growing Assumption Parish sinkhole?
State highway officials say they are trying, in an abundance of caution, to get to the bottom of a newly discovered rough patch on La. 70 near the 25-acre Bayou Corne-area sinkhole.
Early indications are that patch may not be anything out of the ordinary, officials said.
“It’s very common to have a bump in the road throughout the state, but because of the location and the concerns there, we’re going to do some investigation, but we have got that whole area monitored and don’t see any indication of the highway being compromised,” Rodney Mallett, state highways department spokesman, said Friday.
Louisiana Department of Transportation and Development officials reported in a parish government blog that the surface of La. 70 is deformed about a half-mile east of the Bayou Corne bridge.
The eastbound traffic lane has a raised circular area an inch high and about 2 feet in diameter while two half-inch-tall ridges about 10 feet apart cross the westbound traffic lane.
DOTD officials said they do not know yet what caused the deformation but possible sources are many, including heat, drought conditions and geological formations.
John Boudreaux, director of the Assumption Parish Office of Homeland Security and Emergency Preparedness, said the distorted area was noticed late Wednesday and state highway officials evaluated it Thursday.
Since the sinkhole was discovered in swamps near Bayou Corne on Aug. 3, 2012, and forced nearby La. 70 to be shut down for about 18 hours while the emergency was evaluated, residents and officials have worried that the sinkhole could eventually compromise the key connector highway.
The sinkhole is about 1,100 feet south of La. 70 and recently has been growing southward, away from the highway.
Scientists think a massive, underground Texas Brine Co. salt dome cavern failed and has been filling with surrounding rock, ultimately causing the sinkhole to form and continue to grow.
DOTD is making plans for a temporary detour and a longer term bypass, should the sinkhole compromise La. 70.
Scientists had originally predicted the sinkhole would stabilize when reaching about 1,400 feet in diameter and not endanger La. 70, but they also are currently recalculating their estimates.
Although DOTD has set up an array of monitors and performs physical inspections of the highway and its bridges to check for subsidence, the agency has not yet found worrisome signs, officials said.
Mallett said the sensors detect minute shifts of the underlying earth and in the highway during sinkhole burp events and tremors, but the highway and earth have always gone back to their original positions.
“On a hot day, we can see our bridges expand and contract,” he said. “That’s how precise it is.”
In light of the new discovery, DOTD officials said they plan to resurvey elevations of measurement points along La. 70 to see how recent events compare with data already collected.
They also plan to set up new data collection equipment near the rough area to check for subsidence in the future and also will recheck how rough the La. 70 pavement is in order to compare it with the deformation area.
Highway officials also plan to mow down roadside grass to aid in closer inspection and are trying to find all the underground utility lines in the vicinity.
DOTD officials said that in regard to the long term, they are trying to better coordinate their data with the seismic data collected around the sinkhole.
DOTD officials issued a statement citing movements on Aug. 22 during a seismic event that displayed an “elastic” property during which La. 70 shifted minutely but returned to its original position.
In that case, DOTD officials have not directly connected the movement of the pavement to the tremors.
Boudreaux did say seismic monitors around the sinkhole picked up a new kind of tremor on Wednesday but asserted the tremor — a single event that came and went — is not related to La. 70’s rough patch.
Other documented tremors have come in two varieties that each build in frequency to a crescendo and then ebb to a low baseline, often after the sinkhole spawns a burp, an edge collapse or some other kind of event.
One kind of tremor has been associated with fluid or gas movement underground while the other has been associated with shifting rock in a zone of shattered earth deep under the sinkhole.
Boudreaux said scientists are still trying to determine the makeup of this new kind of tremor.
“They are investigating if there is something different that they are going to start seeing, now that they saw this one,” Boudreaux said.