EVAPORITE-KARST PROCESSES AND ENGINEERING/ENVIRONMENTAL IMPACTS IN THE UNITED STATES

Evaporites, including rock salt (halite) and gypsum (or anhydrite), are the most soluble of common rocks. They dissolve readily to form the same types of karst features that commonly are found in limestones and dolomites, but evaporite-karst features are produced in a relatively short time. The four basic requirements for development of evaporite karst are: 1) a deposit of gypsum or salt; 2) contact of the evaporite with water unsaturated with respect to CaSO₄ or NaCl, respectively; 3) an outlet for escape of water that is dissolving the evaporite; and 4) energy to cause water to flow through the system. Typical evaporite-karst features observed in outcrops include sinkholes, caves, disappearing streams, and springs, whereas other evidence of active evaporite karst may include surface-collapse structures and saline springs or plumes that result from salt dissolution. All these natural karst phenomena can be sources of engineering or environmental problems. Dangerous sinkholes and caves can form rapidly in evaporite rocks, or pre-existing karst features can be reactivated and open up (collapse) under certain hydrologic conditions or when the land is put to new uses. Many karst features also propagate upward through overlying surficial deposits.

Human activities also have caused development of evaporite karst, primarily in salt deposits. Boreholes (petroleum tests or solution-mining operations) or underground mines may enable unsaturated water to flow through or against salt deposits, either intentionally or accidentally, thus allowing development of small to large dissolution cavities. If the dissolution cavity is large enough and shallow enough, successive roof failures can cause land subsidence and/or catastrophic collapse.

Evaporites are present in 32 of the 48 contiguous United States, and they underlie about 40% of the land area. Evaporite karst, both natural and human-induced, is far more prevalent than is commonly believed. In addition to problems in siting of dams and other construction projects in (or above) evaporite deposits, there are a number of sites where large collapse structures have developed above dry mines, solution mines, and petroleum tests that have penetrated evaporite deposits.