A large hole sits to the side of Hooper road near Greenwell Springs road. The hole extends under the roadway making it hollow underneath and subceptible to caving in on May 12, 2019.

The hole looked deceptively small from the outside. But when Justin Kozak applied pressure to its edges, it opened to reveal a cavity two feet wide by two feet deep.

He filled the hole up with dirt, but it came back.

“After a heavy rain, we noticed flies, and it smelled like a sewer line,” said Kozak, who lives on Olive Street in Garden District. “When (a worker) came out to fix it, I said, ‘Does this happen often?’ The guy nodded and said, ‘All the time.’

“I said, ‘That can’t be good.’”

Baton Rouge residents are no stranger to sinkholes. So far this year, 950 people have called its 311 telephone line to report sinkholes and cave-ins. That’s a 7.3% increase from the same timeframe in 2020, when 885 people called reporting sinkholes, and 20% over 2019, when 792 people called.

Since the beginning of 2016, the city-parish has documented 11,682 sinkholes, according to city-parish data. Of those, 10,324 have been addressed, while 1,358 remain in need of service, according to the data.

As a massive influx in federal aid from the American Rescue Plan heads to the city-parish, local officials have already earmarked $5.5 million of the $167 million pot specifically toward addressing “roadside drainage cave-ins,” or sinkholes.

The funding will allow maintenance crews to address more than 1,100 of the outstanding sinkholes, city-parish spokesman Mark Armstrong said.

Sinkholes reported to 311 lie in in both old and new parts of Baton Rouge, indicating the troublesome holes in the ground are not caused by aging infrastructure, said Marty Horn, a professor within LSU’s Louisiana Geological Survey research center. The calls for sinkhole repairs are most heavily concentrated in the Capitol Heights, City Park, Garden District, Glen Oaks, Hundred Oaks, Mid-City and Village St. George areas.

Baton Rouge’s unique soil formation is conducive to sinkholes, Horn explained. The city is built on a deposit of loess: wind-borne glacial sediment finer than sand. Around 10,000 years ago, glaciers ground rocks into dust, and wind and water transported that dust down the Mississippi River Valley. Loess deposits ranging from three to 30 feet deep blanket the east and west banks of the Mississippi River Valley from Louisiana to Illinois.

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In some ways, loess is stable because its irregularly shaped, angular grains fit together “like a pile of popcorn,” Horn said. “It has the remarkable quality of holding up, even though it isn’t cemented together,” he said.

Loess is porous, so water can trickle through it and form tunnels in a phenomenon known as piping. Because of the nature of loess’s interlocking particles, the tunnels are stronger than those found in other types of soil.

“Moles and trees and animals that burrow underground love it,” Horn said. 

However, piping isn’t so welcome when it comes to human inhabitants. When underground pipes leak or surface water trickles down, it can also cause pathways for piping to exploit and expand. Those pipes from sewage leaks grow in size, and they weaken as they grow larger. Eventually, a sinkhole forms, collapsing sidewalks, streets or whatever happens to be above the piping.

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“It is no coincidence that a lot of sinkholes that get reported are out in the streets where there are utilities under the streets,” said Horn, who has reported multiple sinkholes in his neighborhood Hundred Oaks. “Sewer and water pipes carry water, and if they leak or seep a little bit the water can get into sediment around them and enhance erosion.”

City-parish data concurs with Horn’s assessment, labeling all 11,682 sinkholes since 2016 as either sewer- or drainage-related.

“I’ve made my share of calls to 311 about sinkholes in the servitude adjacent to my backyard and am happy with the city’s response to address the issue,” said Byron Townsend, who lives in Capitol Heights. “In my case, the sinkhole was due to soil erosion occurring because of a leak in the sewer line located underground in the servitude.”

But leaks may not be the only driver of piping.

Horn hypothesizes that a mostly dry fall last year, followed by several days of heavy rainfall (more than an inch a day for three days) at the end of November, created piping that may be driving the current peak in sinkhole-related 311 calls.

“Prolonged dry conditions may have caused cracks in the ground, serving as downward pathways for the sudden heavy rain,” Horn said in an email. “Subsequent erosion along those paths may have enhanced piping in the sediment (loess).”