Axial Stiffness Evolution of Rock Observed During Uniaxial and Triaxial Compression Strength Tests

ABSTRACT: Elastic properties, including Young's modulus and Poisson's ratio, are commonly reported parameters estimated from standard compression strength tests of intact rock core. Although it is recognized that axial stiffness is not constant during loading, Young's modulus is often estimated at a pre-defined portion of the stress-strain curve, typically at approximately 40-50% of the peak strength. Poisson's ratio, which is an indication of the axial to lateral strain response, is also highly non-linear and varies throughout the loading cycle as damage is initiated and accumulates in the rock specimen. While this method of interpreting elastic properties is sufficient for some rock materials under specific test conditions, plotting the instantaneous Young's modulus, or tangent modulus, as a function of axial stress reveals that the maximum stiffness and/or most linear region of elastic response does not necessarily fall within the typically applied fixed range of the stress-strain curves often used for interpretation. A series of uniaxial and triaxial compression tests were completed on Westerly granite, Stanstead granite and Indiana limestone to investigate the relationship between axial stiffness and stress during loading. It was found that while the maximum tangent modulus (i.e. maximum stiffness) for both the Westerly and Stanstead granites is reached between 45-55% of the peak axial strength during UCS tests, as confinement is applied the range decreases to approximately 20-25% and 30-40% of peak axial strength under 30 MPa confinement, respectively. Therefore, applying a standardized fixed range as a percentage of peak stress for calculating elastic properties under various confinement levels can be problematic. An improved strategy is to determine the maximum tangent modulus and select a range of stress or percentage around the stiffness inflection point to estimate Young's modulus. 1. INTRODUCTION An understanding of the physical and mechanical properties of rock material is critical to safely designing structures involving earth materials. This includes surface or near-surface geo-engineering projects such as open pit mines or foundations, and deeper underground projects such as tunnels, mines, or waste repositories. Furthermore, knowledge of material properties is important to effectively design borehole drilling programs and model subsurface behavior for large scale subsurface storage operations/projects.