Riddolls, B.W. 1974 Engineering geological appraisal of Line E. Poro-o-tarao tunnel duplication.. Lower Hutt: New Zealand Geological Survey. New Zealand Geological Survey report EG 203 6 p.
Abstract: Recent surface and subsurface investigations have provided a general indication of probable tunnelling conditions along part of Line E. As with the existing tunnel, the new alignment will be in Mahoenui Mudstone, which at the surface has given rise to extensive landslides. The test drive near the north portal exposed well-bedded mudstone, dipping at shallow angles. The average rock strength is low, and breakage occurs principally through the rock material. However bedding planes and some joints form weaknesses in the rock mass. Narrow veins of crushed rock occur locally. Investigations along the entire length of the alignment have not been sufficiently detailed to enable an accurate assessment of rock condition to be made. Although landslides are widespread, it has proved difficult to assess both their extent with depth, and degree of activity. Present knowledge of likely tunnelling conditions is such that it would be prudent to carry out investigations ahead of the face as it advances. (auth)