Ground penetrating radar and a falling weight deflectometer was used to determine the location of recesses and cavities beneath the airport, their size and what it would take to patch them.
Contractors are working night and day at Monroe County Airport in Bloomington, Ind., where airport officials determined they had to “obliterate” their longest airport runway in order to solve a geographic problem that became apparent when sinkholes appeared two years ago.
The airport closed its 6,500 ft. (1,981 m) long runway on Sept. 16, and is open now only to planes able to land on its remaining runway, which is 3,800 ft. (1,158 m) long. Overall, said airport manager Bruce Payton, this has meant diverting 30 to 40 percent of its total air traffic to Indianapolis International Airport, 50 mi. (80.5 km) to the north. The Part 139 Class IV airport operates almost 40,000 flights a year.
The $11 million project is scheduled to be completed in November.
“This is a substantial project,” said Darrel Berry, a spokesman in the Springfield, Ill., office of Hanson Professional Services, which produced the geotechnical engineering studies for the project, and continues to serve as project manager. “They are obliterating their runway to solve this problem. The work we’re doing is not a band-aid. It’s a long-term, environmental solution.”
The problem first became evident during a routine runway inspection in May 2011, a few days after a very rainy weekend. Airport maintenance staff noticed that a large hole about 6 ft. (18.3 m) in diameter and 6 ft. deep had appeared next to the airport’s longest runway. Two more sinkholes were also discovered later.
The sinkholes may have been unwelcome, but their appearance was not a total surprise.
The airport sits atop karst topography, common in southern Indiana and much of Kentucky. Formed by underground water that over time slowly dissolves the soluble limestone in the bedrock, karst topography is characterized by a mixture of caves and underground channels with rough, uneven areas at the surface.
When enough of the limestone erodes, a sinkhole can develop. Mammoth Cave, located about 200 mi. (321.9 km) south of the airport in Kentucky, is an unusually large example of karst topography.
Payton, who has worked at the airport for 35 years, said he recognized the problem because similar sinkholes appeared near the same runway in the early 1990s. At that time, he said, “sizable” repairs were made.
“We dug down and poured 52 yards of concrete on the crack,” he said. “So when I saw the sinkhole two years ago, I knew that if we just tried to repair every little sink as it appeared, we’d be behind the curve.”
A special concern for Payton was the airport’s heavy use by sports teams, dignitaries and others traveling to and from Indiana University, which is located about six miles from the airport. This traffic helps account for many of the approximately 115 regional sized jets, such as Boeing 737s and McDonnell Douglas MD83s that use the airport each year.
The airport also is used frequently by visitors to the U.S. Navy’s Naval Surface Warfare Center in Crane, about 32 mi. (51.5 km) south of the airport. Two fixed base operators (FBOs), Cook Aviation and BMG Aviation, also operate out of the airport.
“Because of the college, we’re high profile,” said Payton. “I had a great concern about whether we would have a collapse in the runway. You never want that to happen, and certainly not with a 170 passenger jet carrying a college football team.”
Payton said when the first sinkhole was discovered, the airport contacted the Federal Aviation Administration (FAA) about applying for funding from the Airport Improvement Program (AIP) to help with repairs. At the same time, Hanson began doing geotechnical engineering to determine the extent of the problem before recommending ways to fix it. Initial tests, said Payton, determined that the runway was not in imminent danger of collapse and so it stayed open, although some companies chose to divert flights even before it closed in September.
From the beginning, said Berry, Hanson knew it would not be proposing a solution that involved pouring concrete into cracks in the bedrock.
“You have to understand that the flow of water never stops,” he said. “So there’s always the potential to erode more bedrock because the water continues to flow around the plug. The plugs loosen, and then they fail.
Another problem, he said, is the concrete itself.
“It’s very hard to quantify how much concrete you have to use to fill a hole to stop the water,” he said. “And it’s not a very environmentally conscientious way to fix the problem, since you always have concerns about water quality.”
Hanson used ground penetrating radar and a falling weight deflectometer to determine the location of recesses and cavities beneath the airport, their size and what it would take to patch them. After confirming their analysis with the U.S. Geologic Survey, Hanson recommended filling the voids in the limestone with a type of rock designated as “Clean 53.”
Berry said the stone compacts tightly, matches the local geography and will allow the water to continue to flow naturally through the bedrock and also keep the groundwater safe.
An added benefit is that the airport was able to purchase the stone from Rogers Quarries, which is located near the far end of the runway, less than two mi. (3.2 km) from the airport entrance.
“This is probably one of the greenest large projects ever,” said Berry. “The trucking route is incredibly short.”
Hanson also prepared an environmental information assessment, and in July the proposed project received a finding of no significant impact (FONSI) from both the Environmental Protection Agency and the FAA.
Once the FONSI was in hand, the FAA awarded the airport a $10.9 million AIP grant. Payton said the balance of the project funding has come from the Indiana Department of Transportation and Monroe County. Payton said a request for bids was put out within 30 days, and three firms responded.
The contract was awarded to Bloomington contractor, Crider and Crider in September and work began almost immediately.
“It was a tremendous undertaking, even at the best times of the year,” said Payton. “But we got started late in the construction season. We looked at delaying this until next year, but with safety paramount, we chose to move quickly to make these repairs.”
From the first day the contractor proposed running two 10-hour shifts seven days a week, with 30 workers per shift, said Payton, adding that a 1,600 ft. (488 m)-long section, or almost a quarter, of the runway, has been excavated. The slope-sided pit is 260 ft. (79 m) wide at the top and just a bit wider than the 150 ft. (45.7 m) wide runway at the bottom.
Sixty to seventy percent of the pit, which has been excavated to bedrock, is 30 ft. (9.1 m) deep. Another 20 percent is about 5 ft. (1.5 m) deep, and the balance is 16 to 18 ft. (4.9 to 5.5 m) deep. Payton estimated that 430,000 cu. ft. (12,176 cu m) of dirt, which is being stored on site, has been removed.
The construction site has been a popular destination for local visitors, who tell Payton they can’t believe it cost so much money to dig a hole until they saw how large it was.
“It’s been estimated that the hole could hold several large office buildings,” he said. “But it has to be this large because we wanted to address everything under the runway for many years into the future.”
Payton said once the site was fully excavated, a 4 to 5 ft. (1.2 to 1.5 m) high layer of basketball sized shot rock was laid down, followed by a fine layer of 53 stone.
Next, a layer of geotextile fabric was laid over the gravel. Finally, the excavated dirt was compacted back on top of the geotextile fabric, followed by another layer of the 53 stone to a depth of about 16 in. (40.6 cm).
“During the construction, we’ve found several instances of sinkholes and karst formation that confirmed all our concerns,” said Payton. “Clearly, this was a project that had to happen and the sooner the better.”
After the hole is filled, the entire length of the runway is slated to be repaved. That work will be done by Dave O’Mara Construction of North Vernon, Indiana.
Berry, at Hanson, offered kudos to the airport for the way in which the project has developed.
“The impressive part is how big it is, and the timeline in which it is being done,” he said. “Overarching everything is the fact that they are being environmentally conscientious while working with the public and various government agencies that all had concerns. They fixed and addressed everyone’s concerns, and I consider it a feather in their cap that they found a solution everyone is comfortable with.”