The assessment of the shear strength of rock joints is essential for many rock engineering projects. The shear behavior of rock joints is usually investigated in the laboratory by using a direct shear apparatus, where the normal load is kept constant (CNL) during the shear process. However, they may be quite inappropriate for the situations where the normal stress on the joints surfaces changes considerably during the shear process. In this study, a high-performance direct shear apparatus and computer control system have been developed to accommodate the change in normal stress with dilation under constant normal stiffness (CNS) condition. The shear test results of the artificially prepared joint specimens show that the normal stiffness has a significant influence on the shear strength of joints.
A rock mass of a site is not continuum and its mechanical behavior is significantly affected by the existing discontinuities, such as bedding, joints, faults and fractures. Therefore, the correct evaluation of the shear strength of rock joints plays an important role in the cases of design of deep underground openings and stability analysis of rock slopes. The shear behavior of joints is usually investigated in the laboratory by using a direct shear apparatus, but mainly under constant normal load (CNL) conditions (Huang, X. et al., 1993). They may be quite inappropriate for the situations where the normal stress on the plane of shear is far from being constant during shearing process, for example, in deep underground openings or in grouted rock anchors. In such cases, a more representative behavior of joints would be achieved if the shear tests were carried out under conditions of constant normal stiffness(CNS). There are several references concerned with the shear test of joints under CNS conditions. However, the normal stiffness is very difficult to be changed according the deformability of the joint surfaces and rock mass because of using the springs or the equivalent beam to apply the amount of stiffness (Brahim,B. & Gerard,B., 1989; Indraratna,B., et al., 1999; Ohnishi, et aI., 1990), and the joint surfaces are easily damaged when the spring is too strong. This study is to develop a new direct shear test apparatus of a joint interface under the conditions of CNL and CNS, and to describe a rational procedure for the determination of the shear behavior of rock joints. The value of the normal stiffness can be set automatically by a computer with the A/D and D/A boards. Shear test of the artificial joint specimens is carried out with the developed apparatus in order to clarify the influence effects of the normal stiffness on the shear strength of joints.
A servo-controlled direct shear apparatus was designed and fabricated for the purpose of testing both natural and artificial rock joints under conditions of CNL and CNS. The loading capacity of machine is 400kN in both normal and shear directions (Fig. 1 and Fig. 2).
