Choosing a proper loading rate for bonded-particle model of intact rock

Bonded-particle model (BPM) is widely used to model geomaterials, in which calibration against the results from uniaxial/triaxial compressive tests, Brazilian tensile tests and shear tests have been commonly conducted. However, since different loading rates were used, it is difficult to assess the numerical results of these studies if the effects of the loading rate are ignored. This paper discusses the loading mechanisms associated with different loading rates in the BPM and examines the numerical outputs under these different rates. The results indicate that the time step is an important factor controlling the loading rate of the BPM and should be considered in addition to the velocity of the loading platen. The strain rate, which is usually employed to describe the loading rate in a physical test, cannot be used for the direct comparison of different numerical tests in PFC2D due to the time step. A proposed "step strain rate", which considers the time step, is found to be more appropriate for comparing the loading velocity on specimens of varying sizes. Six different loading rates (0.005, 0.01, 0.02, 0.08, 0.2 and 0.6m/s) are employed in uniaxial compressive tests and Brazilian tests during this study. After comprehensive examinations, a maximum step strain rate of 1.1 x 10(-8) step(-1) is considered to be appropriate for quasi-static uniaxial compressive tests and Brazilian tests using the BPM.