A case study is presented to show how stress field information can be used to assess the long-term rheological behaviour of an equivalent gao-material. The example concerns a granitic rock mass at the km3 scale where an underground hydropower scheme including a new 10 km long power conduit and a powerhouse complex located about halfway in the conduit and 500 m below ground level will be constructed. For design of the underground cavern and hydraulic pressure tunnel, several in situ stress measurements were carried out, using hydraulic, overcoring and flat jack techniques. Integration of the several in situ test results through a continuum mechanics model shows that the long-term behavior of this granite rock mass differs markedly from the short-term behaviour as defined by laboratory measurements on cores. The large scale stress field is found to depend mostly on the softer material that fills up the faults and hence results from the shear stress relaxation over a large number of pre-existing fractures and faults. The overall granite rock mass may be viewed as a combination of stiff elastic blocks separated by soft low strength material, leading to a fairly large scale homogeneous axisymmetrical stress field with vertical axis, with local strong anomalies that correspond to either high or low stress magnitudes.
In situ measurements are essential for characterising the natural stresses that exist in a rock mass and such a characterisation is required for designing deep underground structures, such as caverns or tunnels. Depending on the domain of application, the most commonly used stress determination methods involve stress relief techniques, hydraulic tests in boreholes, analysis of borehole breakouts and drilling induced fractures, or inversion of seismic focal mechanisms. More techniques exist, and comprehensive reviews of stress determination methods may be found in Amadei & Stephansson (1997), Ljunggren et al. (2003) or Zang & Stephansson (2010).