The performance of seal systems in rock salt formations can be dominated by the increased permeability of the damaged rock adjacent to an excavation. Under favorable stress conditions, this damaged region is expected to and return to a condition comparable to intact rock salt. A simple method is presented for first predicting excavation-induced damage of rock salt, and then the potential for its subsequent healing. This method avoids the necessity for developing complicated, coupled models for damage and healing processes, and is appropriate for locations where conditions favorable for healing are expected to develop quickly such as adjacent to a relatively stiff seal. Predictions are consistent with field and laboratory measurements, and suggest that healing will occur within a few years after emplacement of a cementitious seal.
An abundance of measurements and observations reveal that a Disturbed Rock Zone (DRZ) develops around excavations in rock salt. In the DRZ, the rock mass becomes more permeable to both gas and liquid. The performance of seal systems - designed to help isolate the waste by limiting flow in, through and out of the facility - will depend to a large extent on the DRZ permeability. The DRZ may, however, experience a reversal of the damage. This process, referred to as healing, can return the rock to a condition comparable to its pre-disturbed state. When a relatively stiff inclusion (such as concrete immediately after emplacement and crushed salt after it appreciably consolidates) is located in an opening in rock salt, the tendency of the rock to creep will cause the stresses in the vicinity of the inclusion to approach a uniform state of stress near the lithostatic value. These stresses are expected to reverse the disturbance (including a decrease of permeability) in the adjacent rock by literally forcing the rock back together. In this paper, the evidence of a DRZ in rock salt is briefly reviewed, and its impact on sealing effectiveness is considered. A simple approach to first predict the extent of the DRZ and then the potential for healing of the DRZ is given, followed by an example for a shaft seal located in a rock salt formation.
Numerous measurements and studies indicate that rock salt will dilate in a limited region surrounding excavations. Many gas permeability measurements have been made in rock salt surrounding excavations of various dimensions (Borns and Stormont, 1989). The test results indicate that a dilated, partially saturated zone extends between 0.25 and 0.5 times the borehole radius into the formation. The effective permeability of the rock salt in this region varies from 10-15 to 10-18 m, substantially above the assumed value for intact rock salt of about 10-²¹ m². Measurements of electrical resistivity and seismic tomography indicate a dilated, partially saturated zone surrounding excavations in rock salt (Borns and Stormont, 1989). Ultrasonic velocity measurements from a shaft also indicate a dilated region confined to less than ¹/² an excavation radius (Munson et al., 1995).