Sinkhole Assessments

Sinkholes are a common, naturally occurring geologic feature and one of the most obvious landforms throughout Northampton and Lehigh County, Pennsylvania.  Sinkhole collapse poses hazards to property and the environment.  It is often important to be able to evaluate the risk of sinkhole collapse for a specific site, for insurance purposes, for pre-purchase evaluation, for land planning purpose, and for development of the site.  Even after a road or a building is in place, such a risk analysis can help determine whether the ground may collapse.

 

Subsidence is the sinking or collapse of a portion of the land surface.  The mechanisms of collapse, and sometimes the conditions existing before the collapse, result from natural physical processes.  Groundwater facilitates ground collapse through dissolution of limestone and other carbonate rocks.  These rocks are nearly insoluble in pure water but are readily dissolved by carbonic acid, a common constituent of rainwater.  The weathering attack occurs mainly along fractures and other partings and openings in the carbonate bedrock.  The resulting features include caves, sinkholes, and karst topography.  In many cases the conditions leading up to a subsidence event are exacerbated or even created by human actions.

 

A sinkhole is a large dissolution cavity that is open to the sky.  The formation of sinkholes can be due either to the sudden wholesale collapse of the roof of a cave or by a more gradual downward movement of unconsolidated material into an open, chimney-like passageway.  The downward movement of material eventually leaves the roof materials unsupported; surface fractures begin to develop and the roof eventually collapses.

 

Sinkholes are enlarged whenever groundwater levels are high, but the pressure of the water in the caverns and passageways helps support the weight of the overlying rocks.  Collapse results from the lowering of the water table due to drought and/or excessive pumping of water wells which leaves underground spaces and passageways unsupported, facilitating the collapse of the overlying rocks.

 

The terrain that is characterized by the presence of caverns and sinkholes is called karst topography. The most common type of karst terrain is sinkhole karst, a landscape dotted with closely spaced circular collapse basins of various sizes and shapes. Several factors control the development of karst landscapes:

 

  • The topography must permit the flow of groundwater through soluble rock under the pull of gravity.

  • Precipitation must be adequate to supply the groundwater system

  • Soil and plant cover must supply an adequate amount of carbon dioxide (to make carbonic acid from rainwater)

  • Temperatures must be high enough to promote dissolution.

New sinkholes are widely viewed as random, unpredictable geologic phenomena. However, BlackRock's experience suggests that the general probability of having one sinkhole develop is dependent on the area of the site, new sinkhole distribution and the sinkhole size.  BlackRock’s staff of licensed Professional Geologists performs sinkhole risk assessments in karst using qualitative analysis based on physiographic, geomorphic, geologic and hydrogeologic information.