Slope instability along State Highway 73 through Arthur's Pass, South Island, New Zealand

Abstract

Within Arthur's Pass National Park, debris flows, rockfall, and a large rock avalanche on State Highway 73 are responsible for major highway reconstruction required at the Zig Zag and in Otira Gorge. The high rate of erosion is due to combined effects of high altitude, frequent heavy rain, ongoing uplift of the Southern Alps, and highly fractured bedrock. Major active faults occur within 20 km of Arthur's Pass, and earthquakes of magnitude M7 have occurred nearby in historical times, initiating landslides which damaged the highway. At the Zig Zag, the highway crosses the large Otira rock avalanche deposit [(43 ± 2) × 106 m3], which is undercut by the Otira River, causing repeated collapse of the highway. Future highway realignment higher up the slope could destabilise scree slopes and take the highway into regions of more frequent rockfall and snow avalanches. The Zig Zag is to be replaced by a curved viaduct, c. 450 m long and 30 m high, constructed along the river course at the base of the slope. Although this should protect the highway from rockfall, analyses show that another rock avalanche initiated from the ridge above the Zig Zag could damage or bury the viaduct, depending on the magnitude of slope failure. Such an event could be triggered by a major earthquake with seismic shaking intensities ≥MMVIII at Arthur's Pass, estimated to have a return period of ≥100 years. © 1996 Taylor & Francis Group, LLC.