The objective of the model studies described in this paper is to determine the effect of planar discontinuities on the strength and deformability of a rock mass. A model material was used because it satisfied concurrently three requirements that available natural-rock samples did not: (1) low strength, which was desirable for minimizing the loads that had to be applied in the laboratory; (2) ease of producing test specimens with geometrically regular joint patterns; and (3) uniformity of the test specimens.

The model tests were performed on specimens made from a single modeling material and tested under triaxial stress conditions with s2--s3. Results are presented in this paper for tests on specimens that had (1) a single joint set inclined at several angles to the major principal direction, including 0° and 90°, and (2) two joint sets that were mutually perpendicular, one set perpendicular to and one set parallel to the major principal direction. Several different joint spacings were used in each series. Future tests will include a series with two sets of joints that make oblique angles with each other and with the major principal direction.

The results of model tests and tests on cores of jointed rock by several investigators (e.g., MÜller and Pacher,1 Moore, 2 Krsmanovic and Milic, 3 Hayashi4, Rosenblad5, Jaeger6, and Lane and Heck7) have contributed significantly to the design of these experiments, which are believed to be the first in which a comprehensive study has been made of the combined effects under triaxial loading of joint spacing and joint orientation (Fig. 1).

Selection of Model Material

A substantial investigation was made to select an appropriate modeling material. That investigation has been described in detail by Nelson and Hirschfeld,8 and the most important aspects of it are reviewed in the following paragraph.

Selection Criteria

SIMILITUDE REQUIREMENTS--The strength of a jointed rock mass can be represented by tile function (Mathematical Equation)(Available in full paper)

This function can be written in terms of dimensionless factors expressed in terms of the foregoing variables; for example (Mathematical Equation)(Available in full paper) Each of the dimensionless factors (¿-factors) must be the same for model and prototype ((¿model=(¿prototype) if the model is to fulfill the requirements of similitude. It was not the purpose of this study to model any particular prototype rock but rather to ensure that the model correspond so the general range of brittle rocks that are most commonly encountered in civil engineering.

The first two ¿-factors (Mathematical Equation)(Available in full paper) were combined into a single ¿-factor (Mathematical Equation)(Available in full paper), which, together with the shape of the stress-strain curve, the modulus of deformation, and the failure mode, is an indicator of ?brittles.? Typical values of (Mathematical Equation)(Available in full paper) for brittle rocks are between 10 and 20, occasionally even higher. Values that are higher than 20 have usually been determined in investigations in which the tensile strength was determined by means of point-load tests, which tend to give lower tensile-strength values than carefully conducted tension tests, and correspondingly higher apparent values for the ratio (Mathematical Equation)(Available in full paper).

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