Investigations were performed to explore the possibility of developing correlations between rock mass properties and fracture tensor parameters. The first invariant of the fracture tensor is suggested as an index of rock mass quality. Another fracture tensor parameter is suggested as an index for rock mass joint anisotropy. Possible correlations between rock mass mechanical (and/or hydraulic) properties and fracture tensor parameters are shown.
Discontinuous rock mass mechanical and hydraulic behaviour depend strongly on the discontinuities present in the rock mass. Discontinuities may be divided into major (large extent features) and minor (small extent features) types. For most civil, mining and petroleum engineering projects, rock masses contain only a few major discontinuities. For such projects, major discontinuities can be considered as single features and their geometry may be represented deterministically. On the other hand, due to its inherent statistical nature, the geometry of minor discontinuities should be characterized statistically. A discontinuous, large rock mass may be discretized into a few major discontinuities and a large number of minor discontinuities and intact rock elements. Note that the microcracks are included under the intact rock. The following procedure is suggested for representing a large rock mass for stress, deformation and fluid flow analysis: (a) Divide the rock mass into statistically homogeneous regions (structural regions) (e.g. Kulatilake et al. 1990, 1996). (b) For each homogeneous region, obtain a representative elementary volume (REV) with respect to the rock mass property considered for representing the combined influence of intact rock and the minor discontinuities; use these representative elementary volumes to replace the rock mass; properties of these elementary volumes should be expressed in statistical terms in order to reflect the statistical nature of minor discontinuity geometry and uncertainty of estimated geomechanical properties of intact rock and joints, (c) Add major discontinuities as deterministic geometric features to describe the complete discontinuous structure of the rock mass. A rock mass which is represented as above can be subjected to stress, deformation or fluid flow analysis using a discontinuum numerical technique. The major hurdle for the above representation of a large rock mass is the satisfactory completion of item (b) above. Recently, Kulatilake and co-workers studied the influence of minor discontinuity geometry on equivalent mechanical properties (Kulatilake et al. 1993 a) and equivalent hydraulic properties (Panda and Kulatilake 1996) of rock blocks which contained intact rock and minor discontinuities. To describe the joint geometry pattern in a statistically homogeneous rock mass, it is necessary to specify the number of joint sets, and for each joint set, the statistical distributions for joint density, orientation, and location. The fracture tensor (Oda 1982) is a combined measure of joint density, orientation and size. Therefore, fracture tensor parameters have the potential of describing the rock mass quality with respect to the aforementioned three joint geometry parameters, as well as describing the anisotropy of the joint geometry system. In this paper, fracture tensor parameters are suggested as indices of rock mass quality and rock mass joint network anisotropy.
