Joint spacing criterion for equivalent continuum model

Currently, the Yucca Mountain Site Characterization Project is investigating the feasibility of the disposal of high-level radioactive waste in the unsaturated Topopah Spring formation. The Topopah Spring formation is a heavily fractured, predominantly vertically jointed, welded tuff, and the potential disposal area is cut by the Ghost Dance fault and bounded by several other faults structures. The joints in the tuff and the faults may have an impact on the emplacement drift or borehole stability, as well as on the movement of fluids through the rock mass. The design of the repository drifts and layout, the waste emplacement scheme, and the thermomechanical performance of the rock mass will be analyzed using various numerical models. These models may be based on different assumptions regarding the representation of the fracture behavior under given applied stresses, and will range from discrete models where individual mechanically active fractures are treated distinctly, to continuum models where the joint behavior is smeared over a representative volume. There is always the question of applicability of a model with respect to a given material domain to be analyzed. For the mechanical analysis of the rock mass response around a repository drift, the applicability of an equivalent continuum model is dependent on the joint spacing in the rock mass. Considering the joint spacings that may be encountered at the potential repository site, a ratio of joint spacing to the planned drift diameter may be adopted as a criterion for evaluating the applicability of the Compliant Joint Model (CJM) in the Yucca Mountain Site Characterization Project. In this paper, this criterion is established by reviewing basic continuum concepts and numerical approximation implications used to build the CJM and by examining rock mass conditions that may be encountered at the potential Yucca Mountain repository site