Effect of fluid pressure gradient on the factor of safety in rock stability analysis

Abstract

Inappropriate assessment of rock stability may result in anthropogenic geohazards during underground space development and energy extraction. To reasonably estimate the factor of safety (FOS) for rock stability, it is critical to address uncertainties involved in the estimation. Fluid pressure gradient is of great concern in the estimation of the FOS. We carried out laboratory experiment and numerical modeling to reproduce fluid pressure gradient on a smooth fracture in granite. We compared the FOS derived from a nearly uniform fluid pressure gradient and that from a significantly non-linear fluid pressure gradient and found that the non-linear gradient of fluid pressure may amplify the FOS value. We also conducted a theoretical analysis to compare the FOS values for uniform, linear, and non-linear fluid pressure gradients. The results revealed that a considerable gradient and a pronounced non-linearity of fluid pressure are likely to cause premature failure of a rock fracture. Moreover, the upper bound of the FOS with a non-linear fluid pressure gradient depends on the initial FOS for a rock fracture without fluid pressurization. These findings are conducive to interpreting rock instability due to fluid pressurization and developing a more robust FOS estimation method.