Mechanisms of Pore Pressure-stress Coupling which Can Adversely Affect Stress Measurements Conducted in Deep Tunnels

Rock-stress measurements performed in boreholes drilled from a tunnel at a depth of 800 m indicate that the minimum stress has been reduced from pre-excavation levels by drawdown in pore pressure. The disturbance to the ambient stresses extends well beyond two tunnel diameters. A model is developed to explain the inferred strong coupling between the pore pressure and minimum stress in the rock mass around the tunnel. The model includes the effects of poro-elasticity of the intact rock and the stiffness of the fractures. For the field situation under study where the fractures are unusually complaint, both poroelasticity and fracture compliance could be important contributors to the coupling. Changes in pore pressure are shown to produce reductions in minimum stress that are at least 50% of the drop in pore pressure and probably higher. In more general situations, the poro-elastic component is likely to be the more significant of the two mechanisms and alone accounts for a coupling factor of 50%. In this context, the importance of fractures in crystalline conditions probably lies more in their promotion of penetrative drainage than their contribution to coupling coefficient. The results highlight the importance of establishing the extent and distribution of pore pressure drawdown about galleries from which stress measurements are to be conducted.