Halliday, C.S.; Beetham, R.D. 1985 Kawarau River power investigations : engineering geological feasibility investigations for the Mt Difficulty tunnel, sections KW26 to KW12 and KW23 to KW12. Lower Hutt: New Zealand Geological Survey. New Zealand Geological Survey report EG 386 111 p.
Abstract: The Mt Difficulty tunnel is an important part of three alternative Kawarau River hydro development proposals. It would have a length of 5.25 - 6.7 km depending on the chosen scheme option, would be approximately 9.5 m in excavated diameter, and carry a flow of 200 m3/sec. The proposed tunnel line has been comprehensively investigated using field mapping, cored drillholes, and extensive laboratory and in situ geomechanics testing. Detailed testing was required to determine whether a tunnel boring machine (TBM) would be a viable alternative to more conventional excavation methods. Unresolved geotechnical conditions, such as a possible high level of in situ tectonic stress, appear at this stage to preclude the use of a large diameter TBM. Smaller twin bore TBM's, or use of a tunnel reaming machine utilising a pilot drive, may be viable alternatives to New Austrian Tunnelling Methods (NATM) excavation, although further investigations would be required to support their use.