Performance of Reinforcement Bar in Jointed Samples under CNL Direct Shear Tests

Abstract

Reinforcement of disturbed or discontinuous rock strata by installing reinforcement bars (bolts) has always been the primary measure for stabilizing the ground. Bolts are generally installed perpendicular to a discontinuity plane to obtain the maximum resistance against shearing. However, in practice, rock bolts are installed at any suitable orientation with respect to the shearing direction. In such cases, few examples exist in the literature to quantify the shear strength of the reinforced joints as well as the efficacy of the installed bolts. This paper presents an extensive experimental work to establish the relationships between shear resistance of the reinforced joint surface and its deformation behavior with the applied normal stress and bolt orientation. Concrete jointed samples of 10 cm cube size are casted and a reinforcement bar is inserted with cement grout at an angle of −30°, 0°, and +30° with respect to vertical axis. Five different normal loads are applied with three repetitions in each case. Altogether 45 direct shear tests are conducted at a constant strain rate of 0.2 mm/min and results are obtained in terms of shear stress versus shear displacement. The results suggest improved frictional behavior of the joint with reinforcement and also confirm that normal orientation of the bolt provides the best performance in terms of peak shear strength and post-yielding behavior. A closer look into the bending of the bolt rod reveals that at certain orientations, the bolt is subjected to shear loading, behaving as a Timoshenko beam, and it provides higher resistance to shear movement. In some other orientations, the bolt rod behaves like an Euler-Bernoulli beam and, hence, shear resistance is low. A procedure to estimate bolt performance for different orientations based on normal stress and bolt orientation is presented in this paper. The results provide qualitative and quantitative insights as to why the normal orientation bolts perform better. The study also demonstrates that positive orientation angle with respect to the shearing direction can also be the second choice if normal bolts cannot be installed.