Hegan, B.D. 1988 Mohaka River power investigations : report on engineering geological investigations : M55 headrace tunnel options. [Lower Hutt]: New Zealand Geological Survey. New Zealand Geological Survey report EG 401 28 p.
Abstract: Engineering geological investigations at M42 (Willowflat) identified a deep-seated translational landslide at the proposed powerhouse site which lead to the site being abandoned. With the lack of suitable foundations for the M42 powerhouse a series of alternative headrace tunnels options from the M55 reservoir to a powerhouse near to the M15 reservoir level are considered. Three alignments ranging in length from 3.5 km to 11.2 km and passing through easterly dipping generally soft, weak sediments are identified. Tunnelling conditions are generally suitable for TBM excavation although cemented beds may slow excavation rates. A low pressure headrace tunnel (high level tunnel) is recommended to minimise expected high groundwater pressures conditions combined with the effects of in situ stress. Further cost benefit analyses are required to rank each of the proposed alignments before future surface and subsurface investigations are considered. (auth)