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By Sue McCraven
It looks like they’ll all die.
Inside dark caves, a creeping white death blankets the tiny faces and furry wings of bats while these warm-blooded animals sleep or slip into hibernation. Come springtime, the weakened bats emerge to face a hideous death of thirst and starvation while the insidious fungus marches on to new killing fields, leaving behind a permanently destroyed habitat and a species headed for extinction.
In just five years, with its 90% kill rate, this cold-loving, cannibalistic fungus called White Nose Syndrome (WNS) has wiped out 6 million bats in the eastern United States and is swiftly spreading cross-country. Wildlife biologists and scientists are stunned by the exterminating power and rapid spread of Geomyces destructans(1), first found in New York caves in 2007. There is no known treatment or cure.
The Little Brown Bat, particularly devastated by WNS, is the most common bat species in North America. With accurate projections based on recent mass fatalities, experts say the Little Brown Bat will vanish in 15 to 20 years. We will be witnesses to the most rapid mammal extinction in North American history(2) if no method to combat WNS is found. It turns out that the best hope for the Little Brown Bat may be artificial hibernacula(3): precast concrete bat caves.
Custom, precast concrete concept to the rescue
Concrete is so much more than the best infrastructure material. In World War II, concrete bunkers protected soldiers in battle(4). Today, at dangerous Middle East checkpoints, terrorism barriers made of heavily reinforced precast concrete continue to bulwark U.S. soldiers.
Concrete is, in essence, man-made rock with tremendous design flexibility for innumerable applications. So it makes sense that a precast concrete solution could be designed to help defend the Little Brown Bat from WNS with a cleanable cave. Wildlife biologists hope that precast concrete caves can be effectively cleaned of WNS and its fecund spores when bat colonies leave the structure in warm weather.
David Phillips at Summit Constructors Inc. first contacted Larry Taylor at Oldcastle Precast in Lebanon, Tenn., about their ideas for a precast structure to help save bats. “Summit got the job with Panattoni Construction Inc., The Tennessee Nature Conservancy’s (TNC’s) contractor,” says Spencer Jones, plant manager.
Jeff McKinley, Oldcastle Blue Grass regional sales manager, believed that the bat cave project was good for the environment and the precast industry, “even though we’d likely lose money on the job,” says Jones. “All of us here at Oldcastle thought this whole idea to help save the bats was pretty cool and a really unique challenge. You don’t get many chances to help the planet, to give back. We went at it with the best of our ability.”
At Oldcastle, the designers and production crew started hashing out feasible ideas, continues Jones. “We knew there was an above-ground cave in Texas, but could we build an underground cave? We had to consider the specific habitat needs of bats, including cave temperature, humidity, water source, ventilation and human access for the bat scientist,” he says.
A bearded spelunker and wildlife scientist, Cory Holliday is TNC’s cave and karst(5) program manager, the project’s chief technical consultant and a renowned bat expert. “This artificial cave not only has the potential to save a large colony of bats from White Nose Syndrome,” says Holliday, “but also serves as a model that could be replicated anywhere WNS threatens to destroy a significant colony of hibernating bats.”
Cost limitations & finicky fliers
Because bats hang upside down by their feet when sleeping and hibernating, the precast top slab’s inner ceiling had to simulate the varied surface of a natural cave. Oldcastle used 4 ft by 8 ft pieces of polyurethane forms with an irregular surface on one side to create a roosting grip for bats on the ceiling.
“Because this project was one-off on a limited budget, Tony Ziend and Nicholas Krug, Oldcastle designers, had to minimize fabrication costs by working with our existing forms. Summit Constructors provided the ceiling form liner,” says Jones.
“We thought a 6 ft by 12 ft, three-sided culvert mold could be used. We’d stack one atop another and lay them together lengthwise to make a long precast tube,” adds Jones. “It had to have an air chute and a 4-ft by 6-ft opening cut into the box for the bats, and another entryway for the scientists.”
Bats are very finicky creatures. “From all the required specifications Cory gave us, we figured out the overall dimensions and openings of the precast cave,” says Jones. “We designed 1-ft-thick end walls and top slabs to resist the high earth loads. Internal baffles were fabricated to create variations in internal temps for the – hopefully – various bat species.”
In Tennessee’s Montgomery County, cranes set the U-shaped precast concrete pieces in an excavation not far from an existing bat hibernation cave on property owned by the state’s Wildlife Resources Agency. At 78 ft in length, the precast “hibernaculum” is 16 ft wide and 11 ft high and was completed in September 2012.
The result? Over the 2012-2013 winter, “Several bats checked out the cave, and a few even roosted there for a short time,” says Holliday. But a bat colony didn’t select the manmade cave for overwintering. “Bats are very particular about temperature for hibernation and the cave was backfilled during a hot Tennessee summer,” says Jones. It is thought that residual heat from the backfill soils kept the precast cave from dropping to a bat-preferred temperature.
Holliday expects better results in the future, saying, “It did take several months for our concrete cave to cool down, but by January we were observing stable temperatures between 41 and 48 degrees. This is already colder than the natural cave next door. We are very optimistic about our ability to save bats with this cave.”
Sue McCraven, senior NPCA technical consultant and Precast Solutions editor, is a civil and environmental engineer.
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