'Wet weather being blamed for sinkholes'
This headline in the March 17, 2011, issue of the Commonwealth Journal newspaper described a series of soil collapses along busy Monticello Street. One of the most noticeable collapses occurred near the entrance to Somerset Village Apartments. While definitely grabbing your attention, it did not impact the roadway. Today this sinkhole has become somewhat of a landmark. Its size has remained relatively stable since it first appeared.
Karst Topography
Pulaski County is located in what is considered a karst region. According to the U.S. Geological Survey, karst is described as "A terrain, generally underlain by limestone or dolomite, in which the topography is chiefly formed by the dissolving of rock and which may be characterized by sinkholes, sinking streams, closed depressions, subterranean drainage, and caves."
Natural sinkholes are depressions or holes that are formed when the land surface gets dissolved by groundwater. They can be shallow or deep, small or large. There are also several ways sinkholes can be formed by man: 1) When natural water-drainage patterns are changed and new water-diversion systems are developed. 2) When the land and surface is changed, such as when storage ponds are created. 3) When groundwater pumping for urban water supply and for irrigation is done in sinkhole-prone areas.
Schematic diagram illustrating the development of a cover-collapse sinkhole.
Upper left, the soil arch develops; upper right, the arch begins to fail; lower left, the arch has collapsed and soil has been transported underground; lower right, the walls of the collapse slump and the depression takes on the classical bowl shape of the sinkhole until the next collapse. Source: Kentucky Geological Survey, University of Kentucky
According to the Kentucky Geological Survey: "Sinkholes that suddenly appear form in two ways. In the first way, the bedrock roof of a cave becomes too thin to support the weight of the bedrock and the soil material above it. The cave roof then collapses, forming a bedrock-collapse sinkhole. Bedrock collapse is rare and the least likely way a sinkhole can form, although it is commonly incorrectly assumed to be the way all sinkholes form."
"The second way sinkholes can form is much more common and much less dramatic. The sinkhole begins to form when a fracture in the limestone bedrock is enlarged by water dissolving the limestone. As the bedrock is dissolved and carried away underground, the soil gently slumps or erodes into the developing sinkhole. Once the underlying conduits become large enough, insoluble soil and rock particles are carried away too."
The type of sinkhole seen at this location is called a "cover-collapse" sinkhole. The general theory is that surface water from two months of heavy rains became diverted from the control system in place and contributed to undermining of this area.
Sinkholes are so common in Pulaski County that the City of Somerset has produced a brochure listing "Facts and Environmental Issues Associated With Sinkholes." An obvious threat caused by sinkholes is the potential for damage to property or infrastructure, and therefore a threat to public safety. A less obvious, but equally important, potential threat created by the presence of sinkholes is that pollutants on the ground can be carried quickly and unfiltered with surface water runoff to underground aquifers. It is, therefore, important to consider topography when undertaking construction within a karst region.
To claim credit for this Earthcache, email or message me answers to the following questions:
1. What is this type of soil cave-in called?
2. If you were standing in the center of the sinkhole, what is the height of the highest side (parking lot side) and the height of the lowest side (Monticello Street side)?
3. What is the approximate width and the length of this feature?
4. Name one potential threat caused by the presence of sinkholes.
5. On the south side of the sinkhole there is a manmade feature. Describe this feature and tell what relationship you believe it has to the sinkhole?
6. Although optional, I always appreciate it when cachers post photos of themselves with their log.