The fundamental problem in rock mechanics is accounting for the dscontinuous and anisotropic nature of the jointed rock mass. It is this problem that makes rock mechanics distinct from the mechanics of steel, glass, or concrete. Existing mathematical models explain much of the behavior of rock cores in the laboratory but, in civil engineering at least, the rocks that matter are the ones that are already broken. On the other hand, the basically descriptive methods now used in engineering geology do not lend themselves easily to mathematical analysis.

A set of indices of geological properties related to the strength and deformation characteristics of jointed rock is needed. Like the index properties used in soil mechanics these will probably not have precise mathematical meaning, but once correlations are established with large scale tests1 they can form a basis for the transfer of test results, obtained in one area under one set of conditions to the prediction of rock behavior in another area under another set of conditions. Indices of rock properties are even more important in rock mechanics than in soil mechanics as the cost of testing is much higher. To be useful, these indices must be easily and quickly measured and give reproducible results when measurements are made by different people.

In this paper the word “joint” means all naturally occurring rock Fractures with less than 1/4 in. of clay or other soft coating material.


A comprehensive review of indices of rock properties for intact rock is given by Deere and Miller2. They provide charts “... for estimating the strength or modulus properties for intact rock from the numerical indices obtained by either the Schmidt hammer, the Shore Scleroscope, or the sonic pulse velocity, all used in conjunction with the unit weight of the rock...” They suggest that “... because of the discontinuous nature of a rock mass, appropriate reduction factors should then be determined for application to the 'upper-limits' defined by the intact rock...”

Indices of rock mass quality now in use include fracture frequency,1,3core recovery and modified core recoveries such as the rock qualitydesignation, 1field seismic velocity,1the extent of joints,2 and indices of weathering 4 and alteration.5 Methods of measuring irregularities on joint surfaces have been recently determined. 1,6


The strength across joints is determined by the interaction of many factors including the pattern, spacing, apertures, extent, irregularities and coatings of the joints. One of the practical ways miners have long estimated the total effect of these factors is by observing the proportion of joint faces that occur in the sides of an excavation or tunnel. A high percentage of joint faces is an indication of low strength across joints and is accompanied by overbreak and the need for rock support and lining.

As shown in Fig. 1, rock failure may occur in two ways:

  • If the strength along the joints is weak joint blocks will be plucked out of the face leaving the joint faces exposed.

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