Development of damage and permeability in deforming rock salt

Permeability of undisturbed rock salt, or of rock salt which has been sufficiently compacted during laboratory testing, is very low (less than 10(-20) m(2)). Therefore, rock salt structures are used as host rocks for storage caverns (oil, hydrocarbons) and are considered for the long-term storage of radioactive waste. Rock salt deforms plastically without the formation and propagation of dilating cracks as long as the state of stresses remains within the non-dilatant stress domain. This paper presents results of deformation experiments focusing on the transport properties of rock salt and describes the conditions for the transition from non-dilatant to dilatant deformation at the so-called dilatancy boundary. Combined measurements of ultrasonic wave velocities (V-p, V-s) and permeability are used to determine the state of stresses at the dilatancy boundary, in addition to the more usual measurement of the volumetric strain. The results confirm the dilatancy boundary as described by the equation of Cristescu and Hunsche (1998). Loading in the dilatant stress domain gives rise to the generation and propagation of cracks and to the increase of damage with strain. The evolution of damage is continuously monitored by combined ultrasonic and permeability measurements. Damage (porosity) and permeability are controlled by several parameters. Most important are minimal principle stress and loading geometry. Pore pressure is found to affect only the mechanical properties of damaged and dilated rock salt. Implications for the large-scale mechanical behavior of rock salt are discussed. (C) 2001 Elsevier Science B.V. All rights reserved.