Scale Effects in a Synthetic Rock Mass

It is generally accepted in rock engineering that the strength of a rock mass decreases as the volume increases. There are two contributors to the rock mass strength: the strength of the intact rock, and the strength of the fractures. When creating a synthetic rock mass, the intact rock is first scaled to the volume of interest using published empirical scaling relationships. These intact-rock scaling laws are reviewed and it is shown that these scale effect are limited to 80% of the standard laboratory uniaxial compressive strength. The effect of fractures on rock mass strength is examined using model studies and numerical discrete element software. It is shown that rock mass strength decreases as the fracture intensity increases. However, the lower limit to this decrease is controlled by the orientation of a single through going fracture relative to the direction of loading, and not the fracture intensity.