The presence of asperities can significantly affect the mechanical behavior of rock joints during the process of shear by inducing dilatancy and consequently contributing shear resistance to rock joints. Dilatancy is the bulk volume increase in jointed rock during the process of shear resulting from the separation of the two contacting sides of each joint due to sliding along asperities.

Many attempts have been made to investigate the effect of joint surface asperities on the peak shear resistance of rock joints, and several models for peak shear strength were proposed. Among these, Barton's model [1] is most widely accepted. In his model, the contribution of joint surface asperities to the peak shear strength of a rock joint is incorporated by the JRC parameter which has a value of 0 (the smoothest) to 20 (the roughest). In addition, attempts have also been made to derive constitutive models to describe the relationship between stress and displacement during the process of shear. Piesha [2] developed an advanced elastic-plastic incremental model by analogy of frictional slip to plastic flow. The model successfully simulates dilatancy and strength behavior of rock joints by idealizing the joint surface to be of "saw-tooth" shape [3, 4]. The model uses the asperity angle rather than the JRC as in Barton's model to describe the joint surface roughness.

This paper presents the results of some 60 direct shear tests we conducted on artificial joints of saw-tooth shape asperities molded of hydrostone, based on which, a correlation between the JRC and the asperity angle was established and validated by tests on natural joints in Niagara dolomite. Thus, given the JRC value, the correlation provides a means of specifying the joint surface asperity angle for use in constitutive laws such as those proposed by Piesha [2], or vice versa.

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