Constraining the far-field in situ stress state near a deep South African gold mine

Abstract

We present and test a new technique for determining the far-field virgin state of stress near the TauTona gold mine. The technique we used to constrain the far-field stress state follows an iterative forward modelling approach that combines observations of drilling-induced borehole failures in borehole images, boundary element modelling of the mining-induced stress perturbations, and forward modelling of borehole failures based on the results of the boundary element modelling. Using this approach, we constrained a range of principal stress orientations and magnitudes that are consistent with all the observed failures and other stress indicators. We found that the state of stress is a normal faulting regime ( S v * ⩾ S Hmax * ⩾ S hmin * ) with principal stress orientations that are slightly deviated from vertical and horizontal and, therefore, denoted with a (*). The maximum principal stress, S v * , is deviated ∼10° from vertical plunging towards the NNW with a magnitude gradient of ∼27 MPa/km. The intermediate principal stress, S Hmax * , is inclined ∼10° from horizontal plunging towards an azimuth of ∼156° and has a magnitude gradient of ∼24 MPa/km. The least principal stress, S hmin * , is inclined ∼5° from horizontal plunging towards an azimuth of 247° and has a magnitude gradient of ∼14 MPa/km. This stress state indicates that the crust is in a state of frictional faulting equilibrium, such that normal faulting is likely to occur on cohesionless pre-existing fault planes that are optimally oriented to the stress field. Modelling of breakout rotations and gaps in breakout occurrence associated with recent fault slip on critically stressed faults located >100 m from the mine further confirmed this stress state.