ABSTRACT:

  • on rock specimens with polished flat and irregular joint surfaces. The rock is an ordovician fine grain limestone which originates from St-Marc's quarry in Quebec. Its mechanical properties are illustrated in Table 1.

  • on specimens made from a rock-like material with polished flat and triangular or rectangular regular interface surfaces. The rock-like material used in this investigation is a cement mortar (mixture of portland cement, sand, water and chemical additives). This material was described elsewhere (Djellouli, 1989). Its mechanical properties are illustrated in Table 1.

A direct shear device capable of shear testing of a joint interface under constant normal stiffness conditions was built. Thereafter, a testing program including static shearing tests on rock specimens and on cement mortar (as rock-like material) specimens was carried out. The results show that the normal stiffness can have a large influence on the shear strength of the joint interface wherever displacement in the direction normal to the interface accompanies the shear motion. 1 INTRODUCTION The shear behaviour of rock joints is usually investigated by using the classical shearing method with the aid of direct shear apparatus. In this method of testing, the specimens are sheared under constant normal lead conditions. If these procedures of testing seem to be appropriate to many practical situations, that is whenever it is reasonable to assume that the force acting normal to the shearing plane remains constant during the shearing (a typical example of such a situation is the case of slope stability problems). They may be quite inappropriate for other situations involving shear displacement where the forces perpendicular to the plane of shear are far from being constant. Such cases are encountered during the development of shear side resistance, for example, in rock socketed piles, in grouted rock anchors or in planar joints for deep underground openings (Lam, 1983). Indeed, when such joint interfaces are loaded and because of the roughness of the joint interfaces and the relative stiffness of the surrounding rock mass, the relative shear displacement may be accompanied by a dilation of the adjacent joint interfaces; therefore, the forces normal to the joint surfaces are not constant but increase as dilation increases. In such cases, a more representative behaviour would be achieved if the laboratory tests were carried out under conditions of constant normal stiffness (Obert et al., 1976; Natau et al., 1980; Lam et Johnston, 1982; Leichnitz, 1985). There is a reasonable amount of reported investigations in the literature on the shear behaviour of rock joints under constant normal lead conditions; on the other hand, the investigations for constant normal stiffness conditions are very limited. In view of this, a laboratory study has been undertaken to evaluate the effects of the normal stiffness on the shear behaviour of joint interfaces. The testing program, with the aid of a direct shear device, capable of shear testing of a joint interface under constant normal stiffness conditions, was carried out. The program included static shearing tests:

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