Scale-Size Dependency of Intact Rock under Point-Load and Indirect Tensile Brazilian Testing

The problem of scale-size dependency of the mechanical properties of intact rock has been extensively investigated with a particular focus on uniaxial compressive strength, whereas a limited number of studies have included point-load and tensile testing. Such mechanical properties, in particular tensile strength, are of fundamental importance in the hydraulic fracturing design process. Despite advancement in such experimental researches, analytical investigation has not attracted the same amount of attention. In this paper, the scale-size dependency of intact rock under point-load and indirect tensile testing conditions was studied experimentally and analytically using different rock types with various geological origins, including sedimentary, igneous, and metamorphic. From the comparison of the experimental results, it was concluded that all rock types follow the generalized size-effect concept where strength decreases with an increase in size. Also, it was confirmed that under indirect tensile testing with an increase in size, the failure of rock transfers from pure tension to a combination of shearing and tension. In addition, the applicability of different size-effect models (statistics, fracture energy, and multifractals) to experimental data was assessed. It was thus concluded that the models with two constants fit the point-load strength data more accurately. Also, it was found that the size-effect model based on fracture-energy theory provided the best fit to the tensile strength data, whereas the statistical size-effect model provided the least best fit.