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Posted September 9, 2013 | comments Leave a comment

Expert: Rezoning site poses sinkhole risk

Sinkholes, contamination of ground water among problems of building on karst topography, geologist says

By Alex Bridges

Land outside Strasburg considered for industrial use lies on land troubled by sinkholes, and a state geologist warns that motorists and residents can expect more sinkholes in the area -- with or without development.

Shenandoah County stands poised to rezone 92 acres of that land at 1095 Oranda Road after the Planning Commission endorsed the request, despite concerns raised by adjacent property owners over increased traffic on underdeveloped roads as well as dust and dirt.

Later this month, the Board of Supervisors will consider the Department of Community Development-initiated request to rezone the land owned by Glendale Properties that lies west of a quarry operated by Carmeuse Lime & Stone and north of Interstate 81.

State Geologist David Spears explained Monday that large swaths of the Northern Shenandoah Valley, including the Oranda Road property, contains karst topography.

At least twice in the past four years, the Virginia Department of Transportation has had to close Oranda Road when a sinkhole opened in the same spot in the undivided state route.

"And it will probably happen again," Spears warned. "Often with construction you're altering the draining of an area and very often highway construction can actually cause sinkholes to form."

The Oranda Road site lies on the Beekmantown Formation, Spears explained. The formation contains limestone and dolomites and remains susceptible to sinkholes. Regardless of the potential for sinkholes, residential, commercial and industrial construction continues to occur.

"If you're in the valley, that's pretty much what you've got," Spears said.

Construction firms usually fill in the underground voids with reinforced concrete or other material to prevent sinkholes.

"We see a lot of times that, say, in new subdivisions when they put in a new road or during highway expansion like on the I-81 corridor, a few weeks or months after the highway work you'll see new sinkholes," Spears said.

On Thursday, speakers at the commission's public hearing, held jointly with the Board of Supervisors, voiced concern about the potential industrial development that would come with the rezoning. Neighbors noted that Oranda Road already experiences heavy traffic from the quarry. County officials said Oranda Road likely would be the main entrance to an industrial development at the site. Development could spur improvements to the road.

Spears warned that neighboring properties located close to Oranda Road could experience sinkholes.

"Whenever you do any kind of ground disturbance there's always a chance of creating new sinkholes," Spears said.

Restricting development on karst would eliminate the use of land along the interstate as well as U.S. 340, U.S. 11, Spears noted.

"It's something we have to live with," Spears said. "There are the hazards of living on karst ... highways can collapse and buildings can be swallowed, but that's not as much of a problem here as say in Florida where these giant sinkholes open up."

Sinkholes remain small in Virginia, though larger pits occur in the southwestern part of the state, Spears said.

"We can't avoid the hazards of sinkholes entirely unless we just stay off of the karst, and we really can't considering how widespread it is, so we have to decide what level of risk we want to live with and that's basically what your zoning people are facing," Spears said.

Building on karst comes with other threats.

"There's the other danger of actually contaminating the groundwater because when a sinkhole opens that's a direct, unfiltered pipeline directly into the local groundwater," Spears said. "To me that is probably the greater concern."

A developer likely would conduct tests and study the topography and soils before constructing a building on the property, Spears said. Building designs would reflect what the studies uncover in the site investigation.

"I wouldn't be too concerned about the stability of an industrial building," Spears said. "But what the county really should be concerned about is contamination of groundwater."

The geologist noted that contamination remains an issue for either industrial or residential development and on any similar karst land.

"Septic systems, residential, suburban pesticides and motor oil and things like that are just [as much] of a concern in residential areas as they are in industrial areas," Spears said.

Contact staff writer Alex Bridges at 540-465-5137 ext. 125, or abridges@nvdaily.com


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