Poro-o-tarao tunnel duplication: summary of engineering geological investigations

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Kermode, L.O.; Riddolls, B.W. 1975 Poro-o-tarao tunnel duplication: summary of engineering geological investigations. Lower Hutt: New Zealand Geological Survey. New Zealand Geological Survey report EG 224 22 p.

Abstract: Extensive investigations have been carried out for the siting and design of a new railway tunnel at Poro-o-tarao, where the existing one is now too small and in poor condition. The ridge to be tunnelled forms the watershed between the headwaters of the Mokau and Wanganui Rivers. The terrain is formed by soft sedimentary rocks that have been mapped elsewhere in the King Country within the geological unit known as the Mahoenui Group. At the surface at Poro-o-tarao, landslides are indicated by the presence of hummocky ground and numerous small scarps. Although the subsurface investigations do not cover the entire proposed alignment, a drive excavated at the north portal showed that at this locality, gently-dipping massive interbedded weak claystone, siltstone, and sandstone are present. Bedding planes and joints form weaknesses in the rock mass, but breakage occurs mainly through the rock material. Narrow zones of crushed rock occur locally, which cause water to enter the tunnel during construction. These conditions should persist over most of the total length, but uncertainty exists near the south portal. Because of this, drilling ahead of the face should be undertaken if thee is any deterioration in rock condition at the face during construction of the tunnel. Exposures in the proposed drainage drive should be logged to provide further data in the south portal area. The southern approaches to the new tunnel will be excavated largely in landslide debris that consists of silty clay with rock fragments. This material covers much of the area, and is known to be unstable locally. Mainly because core recovery from drilling was poor in places and interpretation often difficult, it has not been possible to determine conclusively whether the bedrock underlying the landslide debris has also been involved in mass movement. The prospect of any major movements of this nature is considered to be unlikely, particularly since the existing tunnel shows no definite evidence of having been affected by such a process. (auth)