That sinkhole at the National Corvette Museum is no longer a gaping hole in the ground.

It's been filled with about 4,000 tons of manufactured sand, and engineers started Feb. 9 to drill the second round of micro piles for additional support of the circular Skydome building above the hole in Bowling Green, Ky.

Scott, Murphy & Daniel, the Bowling Green contractor, started the fill work in November, and Hayward Baker, the geotechnical contractor, is installing the micro piles, like those added to stabilize the building soon after the incident.

The sinkhole—actually a collapsed underground cave roof—fell before dawn Feb. 12, 2014, taking with it eight vintage Corvettes on display. The museum is located in one of the nation's highest concentrations of karst, where caves and sinkholes abound.

"There are two approaches to dealing with structures in karst like this," says Michael J. Marasa, senior engineer at Hayward Baker. "You can fix the problem or isolate yourself from it."

Fixing meant filling "any and all cavities we found with stiff grout" and not knowing what it might cost, he says. Investigation after the building was stabilized showed it sits above a C-shaped cave with 20-ft-wide "arms," each extending about 30 ft from the 40-ft-wide hole.

Isolating the problem "is more finite than grouting. We knew we could go in and stabilize the structure," Marasa says.

The 23 micro piles in the initial phase were placed at regular intervals around the exterior of the octagonal footing under the 138-ft-dia building.

The next 46 micro piles will be in the interior, on an approximate 20 x 25-ft grid, says Zach Massey, project manager. Some piles will extend 4 in into the 2 x 3-ft concrete slab beams in the new 15,000-sq-ft floor that will cover the hole.

Before work started, workers removed about 600 cu yds of dirt and rock from the 80-ft-deep hole. Then came a mud mat, creating a work-surface base, which was covered with a double layer of sheet pile. Parts were cut out to allow installation of 4 x 2-ft concrete plugs where some of the micro piles will be installed.

The concrete plugs will allow clean drilling for the micro piles, which will go from between 130 and 200 ft to the bedrock below. They will have a capacity of 50 kips to 150 kips and help transfer load-bearing from the building to the bedrock.