Chapter 1 - Size effect of rock samples

Abstract

A unified size effect law (USEL) was introduced for intact rock in order to model both the ascending and descending strength zones. It was shown that there was a good agreement between the model outputs and the experimental data. The sample length-to-diameter ratio was found to have a significant impact on the failure mode and the validity of the failure modes for both axial and diametral point load testing. A new bounding surface plasticity model for intact rock has been presented. Using a single set of equations, the complete stress-strain behavior from initial loading to large shear strains was simulated well. A suite of point load and indirect tensile (Brazilian) tests were conducted on six different rock types having various geological origins over a range of sizes. It was demonstrated that all rock types follow the generalized size effect trend where an increase in size leads to a decrease in strength. In addition to the statistical size effect model, the fracture energy and multifractal size effect models can suitably predict the size effect behavior of point load results. A modified multiaxial failure criterion including scale effect was developed and the modified multiaxial failure criterion was calibrated against the experimental data.