VDOT Quickly Patches Recent Sinkholes.

Thanks to prompt action by Virginia Department of Transportation (VDOT) crews, three sinkholes that suddenly appeared along a 14.5-km (9 mi) stretch of Interstate 81 in Augusta County were patched quickly, and traffic went back to normal. The largest of the recent sinkholes was 6 meters long,

During the past 30 years, almost 350 sinkholes have threatened roads in the district whose geographical center is Harrisonburg. That's an average of 12 per year. But the year 2000 was anything but average; 32 sinkholes were reported in spring alone.

Following long dry spells the previous summer, 17.8 centimeters (6.8 inches) of rainfall in April contributed to the large number of sinkholes last spring, and helped cause 16 landslides along Staunton District roads.

When sinkholes occur, VDOT has to dig them out to assess the full extent of the problem. For example, a sinkhole that occurred near Virginia's Natural Bridge first appeared to be 3.7 m long, 2.4 m wide, and 1.8 m deep (12 ft by 8 ft wide by 6 ft deep), but was actually 6.7 m by 4.0 m by 5.8 m (22 ft by 13 ft and 19 ft deep) -- big enough to swallow a fleet of small cars.

Sinkhole repair typically requires closing a lane of traffic while the work is being done, because VDOT's excavating machine is more than 3.7 m wide (12 ft wide) and requires a strong, flat surface for stability.

Areas prone to sinkholes typically form in karst terrain, which is made up of limestone or dolomite bedrock, and usually contains barren rocky ground, caves, sinking streams, underground streams, springs, and, in some places, an absence of surface streams and lakes. Naturally occurring sinkholes are shaped like a cone and are common where slightly acidic groundwater has dissolved limestone and dolomite bedrock. Karst terrain is formed where surface water enters the ground and moves downward through enlarged fractures to conduits such as caves.

Most sinkholes occur suddenly where soil collapses into a pre-existing void over bedrock, as often happens during floods or prolonged wet seasons, or during prolonged dry seasons. Sinkholes can also result from man's activities, such as excessive pumping of groundwater from karst aquifers, which could lower the water table and cause a sudden loss of the buoyant factor that stabilizes the roofs of caverns.