Between July and October 1998 three separate landslides affected SH3, one of New Zealand's strategic highway routes. At the July slip the highway dropped approximately 5m as 320,000 m3 to 600,000m3 of material adjacent to, and including, the highway slipped affecting around 8ha of land. The landscape of the area is one of rugged high hill slopes of steep rolling country, with limited vegetation cover and highly erodable soils. Native bush was cleared many decades ago to form this essentially sheep farming area. The topography is steep with gully development formed by transitional slides and debris flows within the overlying colluvium. As a major strategic highway in New Zealand's North Island, the reopening of the road and its reinstatement were paramount and urgently needed. Options for the permanent reinstatement needed to take account of the hostile climatic conditions, short construction period and general difficulty in treating local materials for earthworks use. The selected options for the slips included two mechanically stabilised earth (MSE) walls. This paper considers the geological context of the project, the failure mechanism and remedial options, together with a detailed assessment of the adopted MSE walls which were completed and opened to traffic after a three month construction period.
Stockman's Hill is located on State Highway 3 (SH3), approximately 12km south-west of Piopio and some 100km north east of New Plymouth (Figure 1). SH3 is part of New Zealand's main transport corridor providing access from the Auckland, Hamilton and the Bay of Plenty to the Taranaki Region in the south-west of the North Island. The initial slip (Stockman's I) occurred on 8 July 1998 when a 120m– 150m section of SH3 dropped 5m as adjacent land on the eastern side of the road slipped downslope immediately closing the road to traffic.