Development of an automated servo-controlled direct shear apparatus applying a constant normal stiffness condition

Correct assessment of the shear strength of rock joints is essential for several rock-engineering projects. The shear behavior of rock joints is usually investigated in the laboratory using a direct shear apparatus, wherein the forces or stresses acting normal to the direction of shear displacement are maintained constant during the shear process. However, this apparatus may be quite inappropriate for situations in which the normal stress on the joint surfaces changes considerably during the shear process. In the present study, an automated servo-controlled direct shear apparatus and computer control system have been developed to automatically accommodate the change in normal stress with dilation under the constant normal stiffness (CNS) boundary condition. A new LabVIEW-based system is used to described the performance of instrument control, data acquisition and storage operations of the new apparatus. The results of the shear tests conducted using the artificially prepared joint specimens show that the normal stiffness has a significant influence on the mechanical behavior of joints during the shear process.