The Sinkhole that Swallowed a Forklift

  • March 1, 2011
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How Rembco filled a void and saved a department store

The anchor store for Bessemer, Alabama’s newest shopping center was already under roof. As construction continued, the reach forklift operator stopped for a quick break. When he returned, a crowd was gathered around his forklift which now sat upside down at the bottom of a thirty foot opening in the concrete floor.

Sinkholes – they have the annoying habit of showing up at the least opportune moment. This one, at least according to the forklift operator, had excellent timing, but still caused an immediate halt to construction. It was even possible that the entire project would have to be scuttled. That’s when Rembco got the call.

The solution is the problem

Sinkholes can be caused by several different processes, but sinkholes caused by solutioning of the bedrock are very common in the Karst topography that is found in Bessemer and much of the East/Southeast. That’s because carbonate bedrock such as limestone or dolomite is widespread throughout the Karst region, and carbonate rock dissolves relatively quickly when exposed to ground water.

“This sinkhole was caused by solutioning of bedrock, eventually producing a hole,” says Rembco owner, Clay Griffin. “Once a hole forms in the bedrock, ground water has a drain to move through. As it moves, it carries soil along with it. Over time, more and more soil is carried away and an underground void forms. The void grows until its domed ceiling eventually collapses and that is when a surface depression appears – sometimes dramatically.”

“Additional subsurface investigation revealed karst activity under other portions of the building,” says Griffin. “It turns out this department store had significant problems…well beyond the original dropout.”

The Plan

The plan for a typical deep sinkhole repair is to drill a pattern of holes down to the bedrock and ‘cap’ the solution feature by pumping in a thick, mortar-like grout. After capping the defect in the bedrock, the Rembco team fills any voids in the subsurface and restores bearing capacity to the affected area with compaction grouting. (See animations of a typical sinkhole repair – click here)

“But this was no ordinary sinkhole” says Griffin, “This was a monster sinkhole that eventually required 75 injections to depths of 110 feet.” To create a solid foundation, 50 more injections were placed in other locations throughout the building. In all, 7,000 lineal feet of grout casing was advanced to bedrock, and a total of 2,700 cubic yards of grout was injected.

To meet an aggressive project schedule, drilling and grouting operations were conducted 24 hours a day, using two drill rigs and three pumps. “Due to the widespread sinkhole activity under the building,” says Griffin, “16 columns had settled beyond the allowable tolerances for the structure. We again used compaction grouting, this time to raise the sinking columns as much as 3 inches, bringing them within the required tolerance. To complete the job, we underpinned four columns around the original dropout with 12 drilled steel micropiles.” Mission accomplished.

One-Call Terra Firma

Whether you have a forklift-eating sinkhole, precarious slopes, low bearing capacity, or all of the above…make Rembco your first call for geotechnical challenges. Please call at 865-363-4708.

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