Abstract Extraction of soluble minerals, whether by natural or man-induced processes, can result in localized land-surface subsidence. The subsidence is caused by partial or total collapse of underground cavities resulting from dissolution of salt or other soluble evaporites. In many cases, subsidence is ultimately related to the strength limit of the overlying rocks that form the unsupported roof above the cavity. Downwarping results where strength of roof spans are exceeded. In other cases, collapse of the undermined roof causes stoping of the overburden rocks. If sufficient underground space is available for the loosely packed rock debris to collect, the void can migrate to the surface and produce surface subsidence, or in the extreme, catastrophic surface collapse. Another mechanism is subsurface erosion of susceptible layers (sandstone, silt, loess) overlying salt cavities. Ground water can erode and transport the loose material down subsidence-induced and natural cracks, or drill holes into the salt cavity. The voids formed in the higher eroded beds can then cause surface subsidence.

The Oak Ridge National Laboratory has for several years been conducting tests relating to ultimate disposal of highly radioactive power reactor wastes in salt mines. A number of laboratory and field tests have been conducted on properties of salt which are important in the design and operation of a waste-disposal facility. In situ heat transfer properties are sufficiently close to the values determined in the laboratory that confidence can be placed in theoretical heat-transfer calculations. Most bedded salt contains trapped moisture which is released with violence at temperatures above about 250°C. Radiation exposure doses of 5 × 10 8 roentgens produce some changes in structural properties, but the effect on mine stability should be negligible. Rock salt is approximately equivalent to concrete as a gamma-radiation shield. Stability of a waste-disposal mine at ambient temperature can be predicted from observed conditions in existing salt mines. Elevated temperatures will cause accelerated creep, but the exact effects on structural stability cannot yet be predicted accurately.