A re-examination of slenderness ratio effect on rock strength: Insights from DEM grain-based modelling

Uniaxial compressive strength (UCS) is often an indispensable parameter for the engineering geological assessment. The specimen geometry has a significant influence on such determination in the laboratory. Previous laboratory test results confirm that the laboratory-determined rock strength generally decreases with the increase in the slenderness ratio, which has been shown to be attributing to the end friction effects. In the present study, by numerically investigating the effect of slenderness ratio (i.e., shape effect) on the strength and deformation behavior of an intrusive crystalline rock based on the discrete element method, the key underlying microscopic factors contributing to this slenderness effect are examined in detail. A loading scheme is carried out on a grain-based model to simulate a series of uniaxial compression tests on specimens possessing different slenderness ratios. The findings reveal that the Young's modulus remains almost unchanged with the increase in the slenderness ratio. The UCS gradually decreases, which is in good agreement with previous laboratory test results. The numerical results further reveal that the change of rock strength in response to the slenderness ratio is mainly associated with the stress distribution and micro-cracking pattern inside the model. As the slenderness ratio gradually increases, the stress in the model becomes more uniformly distributed and the micro-cracking becomes less prominent but more homogeneous. The chance of finding an effective failure pathway (i.e., a macroscopic fracture) is higher, thus resulting in a lower rock strength. Our simulation results not only offer a plausible microscopic explanation for the progressive decrease of laboratory-determined rock strength with the increase in the slenderness ratio, but also provide insights on practical applications such as hard rock pillar design of underground openings.