The shear characteristics of rock mass are subjected to not only to the shear force but also to the boundary conditions given by neighboring rock mass. The boundary conditions of the rock mass can be classified into four categories according to the stress state of the rock joint. The constant normal load (CNL) is the most widely used for shear test and produces the lowest shear strength and different behavior. In this study, the shear behavior under constant normal stiffness (CNS) condition is replicated by graphic method normalizing the test results under constant normal load (CNL) condition.
The discontinuities in the rock mass reduce their resistance capacity to shear loading and provoke the displacement of the rock mass. In order to understand the behavior and stability of rock structures, such conditions highlight the importance for the engineer to apply analysis and test methods that comply with circumstances of the field subjected to discontinuities. Leichnitz (1985) studied and stressed the importance of the boundary conditions in underground openings which is constrained by the surrounding rock masses using numerical approach. Ohnishi (1990) attempted to simulate rock joints conditions by installing a spring with stiffness at the top of the rock joints, and Mouchaorab et al. (1994) developed an experimental device which is able to simulate the rock joint behavior using a hydraulic servo control system. This study investigates a predictive method for the behavior of rock joints under constant normal stiffness (CNS) using the results of shear test that is performed under constant normal load (CNL). The study processed by performing a series of tests on saw-toothed specimens with constant angle, and then predicting the shear behavior of CNS from the prior CNL test results using partially modified graphic method of Saeb & Amadei (1990).
Apart from the structural features of the rock joints, the shear characteristics of rock mass are subjected not only to the shear force but also to the boundary conditions in the neighboring Figure 1. Example of boundary conditions. (Mouchaorab & Benmokrane, 1994). the rock mass. For example, the boundary conditions of masses of behaving freely in a slope differ from that in an underground opening constrained by the surrounding rock masses. Mouchaorab et al. (1994) classified the two boundary conditions of rock masses into CNL and CNS conditions, as illustrated in Figure 1. As shown in the figure 1, the rock is not constrained under CNL conditions and normal load acting on the rock joint is constant, while CNS conditions stand for underground openings, rock-socketed piles or grouted rock anchors where normal load acting on the rock joint increases due to the stiffness (K) of the surrounding rock mass when normal displacement develops at the rock joint. The latter exhibits larger shear strength than CNL conditions since normal load increases due to the stiffness of the surrounding rock mass. Numerous researchers made efforts to examine the behavior of rock joints regarding the boundary conditions.
