Based on direct shear tests performed on smooth and rough surfaces, one may conclude that the mechanism of failure and sliding on the planar surfaces may be determined to an acceptable degree with the Mohr-Coulomb criterion. The failure mechanism of rough surfaces, however, is difficult to evaluate and predict because of the irregularity, the roughness and the complex stress field causing failure of such asperities. Practical difficulties in measurement and interpretation of the influence of these roughnesses led the investigators to develop the physical and analytical models for the idealization of rough surfaces. Comparison of the failure curves using various shear failure criteria led in remarkable results. This paper deals with consideration of different shear failure criteria and compares the results of direct shear tests performed on artificial specimens containing rough surfaces with the criteria and evaluates compatibility of the results with these shear failure criteria. It has been concluded that in civil engineering projects in which the normal stresses are relatively low, the empirical criterion suggested by Jaeger has better compatibility with the results of direct shear tests conducted on artificial rough surfaces prepared in the laboratory.


The behavior of rock joints (including all discontinuities in rock mass) under shear loading depends upon not only the friction between joint surfaces and the joint material strength, but also the geometrical profile (roughness) of the joint surface. Two mechanisms usually occur during shear failure of a rock joint: (i) dilation by sliding over the asperities of the joint surface, and (ii) shearing through the asperities. The shear strength of a rock joint is the combination of the shear resistance provided by these two mechanisms. The shear resistance in the first mechanism depends on the basic friction resistance between joint surfaces, and the dilation during shearing. The shear strength of the joint material provides the shear resistance in the second mechanism. The dilation of a rock joint depends on the roughness of the joint surfaces, and is also a function of the joint material strength and the normal stress on the joint surface. The strength of the joint material and the basic friction resistance between planar joint surfaces can be readily obtained through shear strength tests in the laboratory and in the field [1]. On the other hand the relationship between the roughness and the shear strength of rock joints is usually expressed by empirical relations, for example, the joint roughness coefficient (JRC) proposed by Barton (1973), or the inclination angle by Dight and Chiu (1981).



2.1. Smooth Surfaces

In direct shear test, when discontinuity surfaces are smooth, clean and in direction of applied shear load, the relation between peak shear strength and normal stress is as follows:

(mathematical equation available in full paper)

In whichtp is peak shear strength, sn is normal stress and f is friction angle of discontinuity surfaces.

2.2. Rough Surfaces

Surface roughness is one of the most important parameters which affects the shear strength of discontinuity surfaces and brings near the concept of shear strength of the joints to real conditions.

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