Scale effect on the shear behaviour of rock joints based on a numerical study

• The shear behaviour of rock joints was numerically simulated using PFC2D.
• The effect of joint length on the shear behaviour of rock joints was investigated.
• The smallest samples had a wide range of peak shear strength.
• As the joint length increases, the peak shear strength decreases.
• Mobilisation of different asperities at different scales results in scale effect.

A large number of studies have been carried out to investigate the effect of scale on the shear behaviour of rock joints. However, the nature of scale dependency of rock joints is still unknown and remains an ongoing debate. This paper investigates the effect of joint length on the shear behaviour of rough rock joints using PFC2D. In this numerical approach, the intact material is simulated by a dense packing of bonded particles and the joint is represented by micro-scale slip surfaces at contacts between particles. The validity of this numerical approach in reproducing the shear behaviour of rock joints has been studied in previous studies of authors against the physical experiments, analytical and empirical models. In this study, numerical direct shear tests on a natural tensile joint and subdivided joints were carried out and the shear behaviour of these joints at different scale was investigated. Results of this study show that as the joint length increases, the peak shear strength, peak dilation rate and shear stiffness decrease while the peak shear displacement increases. In order to find out the reason of observed scale dependency, the mechanism of asperity degradation at different scales was investigated and it was found that the mobilisation of different sized asperities at different scales resulted in the observed scale effect.