This study deals with the comparison of the results of shear tests conducted under conditions of constant normal load (CNL) and constant normal stiffness (CNS). The aim was to investigate whether the shear strength results of the two test procedures are comparable and whether the results from one test under CNS conditions are sufficient to describe the shear behavior of a rock joint. The experiments were carried out on artificial samples with identical material properties. To ensure that the geometry was identical, replicas of natural rock joints were prepared using silicone molds. Fifteen tests under CNL and CNS trial procedures were conducted with a servo-controlled direct shear apparatus. The mechanical parameters friction angle and dilatancy were derived from the results of both experimental procedures and compared subsequently.
Rock joints (fractures, faults) are the weakest point of a rock mass. As such, they determine the deformation characteristics and stability of rock engineering structures. With various tests, the shear behavior as well as the most important mechanical properties of rock joints can be determined. Some of these tests are field trials, like the tilt test or the direct shear test conducted with a portable shearing device, or laboratory tests such as triaxial and shear tests (Schubert 2007). However, this paper only deals with direct shear tests carried out in the laboratory. Two main stress conditions under which the direct shear device simulates conditions occurring in nature and under which the shear force of rock joints can be determined are CNL and CNS.
1.1 Boundary conditions
The direct shear device was configured to simulate conditions that occur in nature during civil engineering works, such as above and below ground excavation, tunnels or caverns. As mentioned above, two main experimental stress conditions under which the shear force of rock joints can be determined, are the Constant Normal Load condition, CNL, and the Constant Normal Stiffness condition, CNS.