Development of a Large-Scale Direct Shear Testing Machine for Unfilled and Infilled Rock Joints Under Constant Normal Stiffness Conditions

In the past, numerous studies have been undertaken to understand the shear behavior of unfilled and infilled rock joints by conducting tests on conventional direct shear apparatus, where normal load is kept constant during the shearing process. This is suitable for planar joints, but rock joints are seldom planar, and, in reality, normal stress on the shearing plane is not constant and testing such rock joints by conventional direct shear apparatus will give inappropriate results. The estimation of correct shear strength is important for safe and economical design of underground openings in jointed rocks, stability analysis of anchored rock slopes, risk assessment of underground waste disposal repositories, design of foundations on rock, and socketed piles in rock. Hence, in the present study, a servo-controlled large-scale direct shear testing machine is designed and fabricated to test the rock samples under varying normal load, i.e., constant normal stiffness (CNS) conditions. Direct shear tests have been performed on physically modeled unfilled and infilled rock joints with asperity angle 30°–30°, at different initial normal stress (Pi). The test results indicate that the CNS conditions greatly influence the shear behavior of rock joints.