Sinkholes

The U.S. Geological Survey states: “Sinkholes are a natural and common geologic feature in areas underlain by limestone and other rock types that are soluble in natural water. The term sinkhole is used for closed depressions in the land surface that are formed by surficial solution or by subsidence or collapse of surficial materials owing to the solution of near-surface limestone or other soluble rocks…”

The formation of sinkholes is the result of the long-term dissolution of underground rock formations (usually limestone or dolomite) by acidic water. As groundwater percolates through the soil, it reacts with decaying organics and carbon dioxide and forms a weak carbonic acid. Over time the acidic water dissolves the susceptible stone. Sinkholes are a product of the underlying geology, and are generally limited to areas where the water table is below the limestone surface.

Sinkholes may cause structural damage because they can contribute to or cause a loss of soil support beneath a building. The manifestation of sinkhole damage in a structure is often very similar to other causes of significant settlement. These include the presence of organics or loose soil, both of which may indicate the improper preparation of the subgrade prior to the original construction of the building. Significant long term settlement damage is discussed here.

Aside from the most dramatic of cases, the potential for sinkhole activity at a subject property usually cannot be determined with certainty without a geotechnical investigation. Such an investigation involves procedures such as standard penetration test (SPT) borings, ground penetrating radar (GPR), excavations, etc. Even if potential sinkhole activity is determined, this does not necessarily mean that the sinkhole activity has or will contribute to building damage or loss.