The Amethyst hydro scheme aims to address the increasing demand for power in the area. It involves diverting water from the Amethyst Ravine through penstocks in a 1040 m tunnel to a powerhouse on the floodplain of the Wanganui River. The 3.5 x 3.5 m horseshoe shaped tunnel was excavated using drill and blast methods and is the focus of this paper. The tunnel was constructed in Haast Schist, between the Alpine Fault (2km Southeast) and the Amethyst Ravine Fault (intake portal). This regional tectonic regime has had an impact on the orientation of defects within the rock mass. It was found that large shear zones form major structural controls within the rock mass, and schistosity orientation changes either side of these shear zones. Schistosity steepens and dip direction becomes more parallel to the tunnel alignment away from the Alpine Fault. Water is linked to shear location, and a few major incursions of water (up to 205 l/s) were linked to large (1.6 m thick) shear zones. 3DEC numerical modelling shows that the tunnel is likely to deform in the invert, with movement also occurring in the right rib above the springline. This is due to the angle of schistosity and the interaction of joints, which act as cut off planes.
Investigating rock mass conditions and implications for tunnelling and construction of the Amethyst Hydro Project, Harihari