Determination of crack initiation CI and crack damage CD as well as the understanding of failure mechanisms like spalling of rocks is still an issue to solve. Several methods are available for estimating CI and CD under uniaxial loading. Spalling typically occurs in close distance to the free face of an underground excavation at low confinement. A triaxial cell has been designed to examine spalling behavior of sedimentary rocks under low confinement up to 1 MPa. The triaxial vessel allows to measure axial and circumferential displacements, permeability as well as acoustic emissions during loading. This work reports on results of numerous uniaxial and triaxial tests on sedimentary rocks. The aim is to compare different CI/CD evaluation methods and give a rough assessment which method is suitable for a given rock type. Additionally to stress observations also the strains at failure and strains at CI/CD were studied.
Over the last decades many scientists made suggestions to predict the in-situ rock spalling strength because it differs from the compression strength determined in laboratory. Spalling makes the construction of tunnels more expensive and, more importantly, is very dangerous for the workers. Hence, it is important to define a standard procedure to estimate spalling strength, which is approximately 0.4 ± 0.1 UCS and coincides with the CI stress detected from laboratory experiments (Cai & Kaiser 2014). Currently different methods exist to determine CI and CD from compression tests. The aim of this paper is to compare these methods and to observe stresses as well as axial and lateral strains at CI and CD. Mainly triaxial tests were conducted under low confinement as this is the region where spalling occurs. At higher confinement the stress envelope makes a transition to the CD defined envelope (Diederichs & Martin 2010). Few test data in this range are available. Another objective is to detect the effect of low confinement on CI.
Five different sedimentary rocks were selected with differences in their petrologic properties (sequence from high to low average compressive strength).