The impact of confining stress on the rock mass deformation modulus

The paper presents the results of a literature review combined with additional laboratory testing to determine the significance of confining stress on the true stiffness properties of rocks. Classical elasticity theory is introduced to set the scene for what is precisely meant by stiffness increase with confining stress. Previous laboratory-based stiffness testing on both intact and jointed rock specimens is reviewed, and the experimental results and leading equations and theories are summarised. The authors then introduce their own laboratory testing results concerning the same phenomena and present a new expression, which is directly compared with both the previous and the newly conducted test data. In conclusion it is seen that the rock types tested display significant increase in stiffness under confinement (commonly up to 3 times for typical intact rocks and up to 20 times for weak specimens), and that the implications for property definitions for numerical modelling are significant and relevant.