A massive sinkhole could delay a roadway extension project — and tack a lot more money onto the price tag — for the Tallahassee, FL, Department of Public Works.
Surveyors happened upon the sinkhole while staking out the area for the .7-mi. roadway extension in mid-October.
What was then a 5-ft. (1.5 m) depression has since grown into a hole approximately 40 ft. (12 m) deep and 70 ft. (21 m) wide, said Public Works Director Gabriel Menendez.
“This is a big one,” he said.
Tallahassee has called upon its Department of Street Maintenance and Construction, led by Supervisor Tom Lewis, to stabilize the sinkhole so geologists can investigate the site.
During the last week of January, crews used a John Deere 544 endloader to clear out the site, including some light vegetation.
Once cleared out, the city crews then came in with a Caterpillar D4 dozer and Gradall 4200 to cut two ramps into the sinkhole — one entry and one exit on opposite sides for heavy equipment to access the bottom of the sinkhole.
Lewis said safety around the edge of the sinkhole is a priority, because no one is sure it is done growing. To leave a buffer zone, Lewis said they used a Volvo excavator with a 40-ft. boom to remove dirt from the sinkhole.
Five dumptrucks were used to haul the material out of the sinkhole.
Crew members have cleared out an area large enough for on-site storage, as well as provide room for excavation.
A 2,500-ft. (762 m) silt fence surrounds the site.
In the past month, approximately 20 more feet (6 m) of dirt has fallen from the sides and Lewis believes the sinkhole may be shifting to the south.
Then, to stabilize the sinkhole, crew will try to plug it up with flowable fill, which includes some cement to stiffen it up.
The prep work is only the beginning of the solution, Menendez said.
The Tallahassee geotechnical firm of Ardeman and Associates will work to determine the cause of the sinkhole and propose possible solutions.
At the site, a layer of lime rock sits at 97 ft. (30 m) below the surface underneath a large layer of clay, Menendez said.
“What we think is happening is that the water above it has found a void in the lime rock,” he said.
Menendez has been told a sinkhole in this kind of dirt is unusual because clay tends to fill any holes below the surface.
Originally priced at $12 million, the project’s cost could increase significantly.
Menendez said solutions could include plugging the sinkhole, building a bridge over it or realigning the roadway and could cost anywhere from “tens of thousands to well over a million dollars.”
But until the geotechnical analysis is completed, Menendez would not speculate about the future of this project. He said city officials will have to weigh the costs of their options to determine their next step.