Size effect and rock mass strength

The size effect associated with rock mass strength is assessed from the perspective of laboratory testing data, hard rock pillar data, excavation back analysis data, and synthetic rock mass (SRM) models. The assessments are completed to investigate whether the size effect is due to increasing rock mass volume or for other reasons. The size effect is well established as one of the reasons field scale rock mass strength is less than that measured in the laboratory. We find that the observation of the size effect for rock masses in the field is due to failure mechanism change instead of size. In addition, we show that SRM specimens, as currently numerically tested and interpreted, do not provide a sound means for estimating rock mass strength unless boundary conditions, structural character, and failure mechanisms are adequately considered.