Paterson, B.R. 1977 An engineering geological appraisal of remedial measures proposed for Otepopo railway tunnel, main south line. Lower Hutt: New Zealand Geological Survey. New Zealand Geological Survey report EG 293 19 p.
Abstract: In 1969 the original limestone brick lining of the Otepopo railway tunnel was partially removed to increase clearances for container traffic. The exposed rock, the Lower Tertiary sandstone, began to fret and, despite the installation of steel rib and wooden lagging support, has continued to deteriorate necessitating periodic maintenance. Three alternatives proposed by N.Z.R. consist of relining with either steel liner plates or shotcrete, or completely daylighting the tunnel. The geological factors affecting each proposal are discussed. A marked difference in stability is noted in the sandstone exposures in the tunnel and the approach railway cuts which is related to the presence of iron concretions. The tunnel walls and the south portal railway cut consist of concretionary sandstone and are stable; the tunnel roof, the north portal railway cut and possibly most of the material above the tunnel is a non-concretionary sandstone subject to fretting and slaking. Slake durability tests show that the non concretionary sandstone has a very low slake durability index which confirms its behaviour in the north portal batters, and highlights the difficulties that are likely to be encountered if the tunnel is daylighted and extensive batters of this material are exposed to weather. The existing problem in the tunnel is one of relaxation and fretting of non-concretionary sandstone in the roof which could be remedied by installing a continuous lining. There is no evidence to date of rock loading support. The tunnel is almost completely dry and is ideally suited to the use of shotcrete. This method of relining is recommended as the most favourable alternative. (auth)