Engineering geology assessment of alternative highway options at the Zigzag and Otira Gorge, Arthurs Pass

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Paterson, B.R. 1987 Engineering geology assessment of alternative highway options at the Zigzag and Otira Gorge, Arthurs Pass. Lower Hutt: New Zealand Geological Survey. New Zealand Geological Survey report EG 409 31 p.

Abstract: This report is an engineering geological assessment of alternative engineering options proposed by Ministry of Works and Development for two sections of State Highway 73 (Christchurch - Greymouth) between Arthur's Pass and Otira township. These sections, located at the Zigzag and the Otira Gorge, have been identified by MWD as requiring major improvement works. Engineering geological investigation of a study of aerial photographs and existing data plus field inspection of the eastern side of the Otira Valley. The opposite (western) side of the valley was not inspected because of access difficulties and the request (by MWD) for only general information for these high-cost options. At the Zigzag the present highway crosses a large rock slide deposit which partially blocks the valley. Slope instability of this material due to undercutting by the river and surface erosion has resulted in previous re-routing of the highway higher on the slope, close to the toe of a rock scree deposit. Six engineering options have been proposed over this section. Two of these involve alignments on the eastern side of the valley and consist of an embankment alongside the river, and a minor realignment upslope from the existing highway at the top of the Zigzag. Engineering rather than geological factors are likely to have the greatest influence on the feasibility opf both options, although the possible effect of deep excavations at Deaths Corner may require further assessment. The other 4 options involve surface and tunnel routes along the western side of the valley. The tunnel options are considered to be less favourable owing to expected difficult ground conditions (minimal rock cover, poor rock quality and high groundwater seepages). From the ZIgzag, the highway descends steeply via a number of hair-pin bends to the narrow Otira Gorge where it follows along a narrow rock bench across a very steep slope. In the past the highway has been closed by clope movement at Candys Slip and by erosion from Candys Creek. Further north it has also been subject to waterfalls and rockfalls during periods of heavy rainfall. Five engineering options have been proposed for the Otira Gorge. Two of these on thee astern side of the valley, consist of realignments involving half-bridges and rock excavation, for which geological conditions are believed to be generally favourable. The remaining three options, on the western side, consist of a tunnel, an open cut and a viaduct. These appear to be less favourable because of technical difficulties, and high cost.