Numerical simulation and interpretation of the grain size effect on rock strength

Although the grain size effect on rock's physical-mechanical properties has been widely studied, little attention has been paid to the reasons for this phenomenon. In this paper, an attempt is made to interpret the effect of grain size on uniaxial compressive strength (UCS) from the point of view of mechanics. Based on the microscopic images of rock with different mineral grain size, actual mineral types and grain sizes were identified and characterized using digital image processing techniques and microstructure analysis. By mapping this real rock microstructure into a well-established rock failure process analysis code, numerical specimens were built to model the grain size effect on the UCS. The results showed that considering only mineral type and grain size, the numerical method can reveal the grain size effect on UCS. Further, an ideal conceptual model is proposed to clearly interpret and discuss the grain size effect on strength. It was found that grain size results in different stress concentrations within rock. Under identical loading conditions, the bigger the grain size is, the stronger the stress concentration is. This different mechanical response can eventually affects the UCS of the rock, resulting in the grain size effect on rock strength. Moreover, fractal geometry is also introduced to quantify rock microstructure with different grain sizes. The results show that there exists a good relationship between the fractal dimension and the grain size, which can describe the grain size effect on rock strength both quantitatively and effectively.