Evaluation of sample scale effect on geomechanical tests

Abstract

The size of the rock specimen affects the stress concentrates in the vicinity of the top/bottom of the rock specimen during the evaluation of the geomechanical parameters in the laboratory, which causes unreliable results. However, the appropriate size for geomechanical evaluation is not well understood yet because of limitations in the sampling and analysis. In this study, a series of numerical simulations using a finite element package was conducted to investigate the effect of sample aspect ratio, fluid saturation, and porosity, on the mechanical behavior of the rock under elastic and poroelastic conditions. In addition, two concepts, stress/strain homogeneity index (SHI) and representative elementary volume (REV), were developed to find out the appropriate sample size. The results show that the presence of stiff platens, which are dissimilar to the specimen material, causes significant stress concentration in the two ends of the specimen. The concentration of stress in the specimen reduces when the aspect ratio increases. An optimum aspect ratio (length-to-diameter equal to 3) was observed by SHI analysis which after that the changes in stress concentration are insignificant. The REV size analysis confirms the obtained optimum aspect ratio by SHI analysis. The saturated specimens show a lower magnitude of stress than applied stress because of the presence of pore pressure, which can carry a portion of the stress. The higher void ratio results in lower strength of the specimen. This study could be beneficial for the better design of geomechanical tests to have reliable results.