Determination of the stress field in a mountainous granite rock mass

The design of an underground hydroelectric power scheme in northern Portugal has required the characterisation of the local stress field. Nineteen hydraulic tests have been conducted in two, 500 m deep, vertical boreholes. In addition twelve overcoring tests together with twelve flat jack tests have been performed from an existing adit located some 1.7 km away from the location of the hydraulic tests. Results have been integrated into a stress model that takes into account both topography and tectonics effects. Most of the data are consistent with a linearly elastic, gravity loaded model, provided a very soft geomaterial is considered. This implies that the stress field in this granite rock mass is controlled by gravity alone and shear stress relaxation along faults and fractures but is unaffected by present-day tectonics.