It is generally recognized to date that a rational stability analysis of slopes and foundations in rock masses is a desirable but still rather remote goal of rock mechanics1 In principle, a slope analysis is possible provided the original state of effective stress in the ground is known and the shear strength of the earth material can be clearly defined. In a macroscopically continuous earth medium such as soil, the two basic ingredients of the stability analysis in most cases can be assessed with sufficient approximation.
A much less favorable situation exists when the slope is composed of a macroscopically discontinuous medium such as rock mass containing one or more sets of geological planar structures. In such a ease, both the original effective stresses and the overall shear strength of the mass are much more difficult to determine. As for the former, it is only recently that certain calculation methods and solutions concerning the stress distribution2 and the seepage forces3 in a discontinuous medium have been proposed.
As the strength behavior of the rock mass is concerned, much valuable effort has been made in recent years toward reaching a proper understanding of the phenomenon of rock mass failure under stress. Probably one of the most complete investigations of the problem to date has recently been made at the University of Illinois. The study resulted in establishing a first useful working hypothesis of the shear behavior of a regularly jointed rock mass, proposed by Patton in 1966.4
The main purpose of the work described in this chapter is to establish a more general framework for the shear behavior of rock mass, one that would be directly related to some basic rock parameters and would also be able to take into account the effect of previous deformations. The strength behavior predicted by the proposed model is compared in the paper with the results of some direct shear tests intended to simulate the behavior of a jointed rock mass in laboratory scale.
Newland and Allely,5 in a study of shear behavior of a granular mass, have shown that the shear strength of such a mass depended not only on the intrinsic frictional properties of the particles, but also on the average angle of deviation of particle displacements from the direction of the applied shear stress.
By assimilating the overall sliding surface to a plane containing interlocked sawtooth irregularities, the foregoing authors developed from static considerations the following equation for the shear strength of a granular mass (Mathematical Equation)(Available in full paper), in which Fµ denotes the angle of frictional sliding resistance along the contact surfaces of the teeth, i is the angle of inclination of the teeth with respect to the general sliding surface, and s and t are the conventional normal and shearing stress, respectively, acting upon that surface, Fig. la.
When considering the shear behavior of irregular rock surfaces, Patton4 observed that Eq. 1 represented fairly well the shear strength along such rock surfaces in the range of low normal loads when there is practically no shear of asperities.
