In a large in situ experimental room, circular in cross section, inflow of brine was measured over a five year period. After correcting the measured brine accumulation for initial losses by evaporation into the mine ventilation air, the measurements gave data for a period of nearly three years. Predicted brine accumulation based on a mechanical [snow plow] model of the volume swept by creep-induced damage as calculated with the Multimechanism Deformation Coupled Fracture (MDCF) model was found to agree quantitatively with the experimental results. The calculation suggests the damage zone at five years effectively extends only some 0.7 m into the salt around the room. Also, because the mechanical model of brine release gives an adequate explanation of the measured data, the hydrological process of brine flow appears to be rapid compared to the mechanical process of brine release.
One of the more challenging aspects of the development of the Waste Isolation Pilot Plant (WIPP) is the prediction of the inflow of brine into the underground rooms. Although this inflow may be completely the result of Darcy hydrological flow, it may also be the consequence of (or aided by) the time-dependent mechanical deformation and damage of the surrounding salt. The hydrological approach to the inflow problem is typified by the analysis of Webb (1992). WIPP salt is estimated to contain approximately 1.0% brine by volume (Stein 1985) located in the interbeds, clay stringers, or negative crystals. Because of the influence of the brine on repository performance, it is necessary to be able to predict with some assurance the expected amount of inflow. In this work, although the mechanism of brine inflow has not yet been firmly established, it is assumed for this evaluation that the process is one of mechanical deformation-aided brine release rather than hydrological flow. Thus, the manner in which the prediction of the evolution of brine is carried forward is through the use of a structural prediction of the amount of damage introduced by the time-dependent deformation of the salt as it creeps inward toward an underground opening. The prediction of the damage requires a sophisticated model of creep and fracture, as well as implementation though a powerful numerical calculational method. Once the damage has been predicted, a model that relates the damage to the increase in permeability or to the release of brine is also required. In this case, a "snow plow" brine release model is proposed which relates the volume swept by the damage to the brine release. Formulation of this problem of interactive mechanical and hydrological effects is of value only if the predictions have some adequate degree of validation against in situ measurements. Presentation of the work includes a summary of the constitutive model of creep and fracture, followed by the development of the brine release model. Next, the details of the in situ brine inflow experiment are given. Then, a comparison is made between the calculated and measured brine release. A summary concludes the work.
