SLOPE INSTABILITY ALONG STATE HIGHWAY-73 THROUGH ARTHURS PASS, SOUTH ISLAND, NEW-ZEALAND

Within Arthur's Pass National Park, debris flows, rockfall, and a larg e rock avalanche on State Highway 73 are responsible for major highway reconstruction required at the Zig Zag and in Otira Gorge. The high r ate of erosion is due to combined effects of high altitude, frequent h eavy rain, ongoing uplift of the Southern Alps, and highly fractured b edrock. Major active faults occur within 20 km of Arthur's Pass, and e arthquakes of magnitude M7 have occurred nearby in historical times, i nitiating landslides which damaged the highway. At the Zig Zag, the hi ghway crosses the large Otira rock avalanche deposit [(43 +/- 2) x 10( 6) m(3)], which is undercut by the Otira River, causing repeated colla pse of the highway. Future highway realignment higher up the slope cou ld destabilise scree slopes and take the highway into regions of more frequent rockfall and snow avalanches. The Zig Zag is to be replaced b y 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 t he highway from rockfall, analyses show that another rock avalanche in itiated from the ridge above the Zig Zag could damage or bury the viad uct, depending on the magnitude of slope failure. Such an event could be triggered by a major earthquake with seismic shaking intensities gr eater than or equal to MMVIII at Arthur's Pass, estimated to have a re turn period of greater than or equal to 100 years.