Model pillar tests for evaluating the structural stability of openings in rock salt utilized for the disposal of radioactive wastes

Abstract

Perhaps the most promising geologic environment for the disposal of large quantities of heat-generating, high-level, radioactive waste is subterranean rock salt deposits. In order to design an actual disposal facility in salt, it is necessary that the effects of elevated temperature, load, and stratigraphic relations on structural openings be determined. To predict flow in rock salt, scale models of salt pillars and their surrounding rooms have been fabricated from cores taken in mines at Lyons and Hutchinson, Kansas; Retsof, New York; Detroit, Michigan; Grand Saline, Texas; Cote Blanche, Louisiana; and Asse, Germany. Tests have been conducted at temperatures of 22.5°, 60°, 100°, and 200°C for axial loads of 2,000, 4,000, 6,000, 8,000, and 10,000 psi at each temperature. From these tests, it has been observed that the deformation of the pillars increases markedly with increasing loads. However, even more significant is the greatly accelerated creep rates of the salt at elevated temperatures. At all axial loads and constant temperatures, there is initially a high creep rate that decreases with time. Creep rates have been observed to continue to decline even after 3 1 3 years of testing. Laboratory studies show that thin shale beds, which commonly occur with bedded salt deposits, cause greatly accelerated rates of deformation in excavated cavities when they occur in the pillars at the roof and floor interfaces. Under these conditions the shales, which are effective friction reducers, serve to reduce significantly the transfer of lateral confining stresses from the roof and floor of the openings into the pillars.