Pukerua Bay-Paekakariki coastal stability project. Preliminary engineering geological assessment of the Beanpole Corner landslide

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Hancox, G.T. 1981 Pukerua Bay-Paekakariki coastal stability project. Preliminary engineering geological assessment of the Beanpole Corner landslide. Lower Hutt: New Zealand Geological Survey. New Zealand Geological Survey report EG 359 21 p.

Abstract: The Paekakariki - Pukerua Bay section of the NIMT railway line, located at the foot of a very steep (35 - 45 o) coastal slope, has been prone to instability problems since completion in 1886. Formed by wave action during the last interglacial, and rising some 250-300m above sea level, the grass-covered slope is dominated by extensive old scree deposits and wave-cut rock faces of variably weathered, closely jointed greywacke and argillite. Several localised areas of instability on part of the slope have caused landslide problems over the years, some of which were probably initiated by excavations into the base of the slope during construction of the railway line. The main landslide types include rock falls, shallow rock slides, debris flows, scree slides, and rock debris avalanches. Of these, the most serious and longstanding problems has been the landslide at Beanpole Corner, about 4km north of the Pukerua Bay railway station. The Beanpole Corner Landslide is a shallow rock slide, situated on a steep, broad greywacke-argillite spur, approximately 100 m above the railway line. This landslide has a long history of instability, which started prior to 1940 as a small scar just above the railway line, and gradually progressed up the slope with a series of small failures and major landslides in the early 1960s, one in 1961 resulting in the derailment of a goods train. The most recent failure, which occured at the head of the slide in May 1979, resulted in partical sliding of a 5m thick slab of top soil and superficial, weathered closely jointed bedrock. An estimated 20,000 m 3 of material moved about 4m downslope, sliding on the intact unweathered bedrock surface, and resulted in the formation of a prominent arcuate scarp up to 4m high and about 200m long. There is some evidence to show that the latest failure occurred at a time of intense rainfall, as a result of ground saturation and the development of a perched water table at the top of the slope. Sensitivity analysis, using reasonable assumed values for shear strength, slide surface geometry, and piesometric conditions supports this conclusion. It also indicates that, in its present condition, the slope is probably stable (F = > 1.0) when the slope is dry. For a detailed stability analysis however, further site investigations are necessary to provide accurate input data on soil strengths, slide surface geometry and groundwater conditions, especially during rain storms. Although subjective assessment suggests that the slide is stable at present, the margin of safety is probably small and there is a strong possibility that even small changes in the external acting forces (such as a locally strong earthquake or very heavy rainfall) could easily trigger further movements that could lead to a complete and rapid failure of the potentially unstable mass. At present the railway line at Beanpole Corner is effectively protected against falling boulders and small rock falls by a 4 m high rail and sleeper fence, but there is no protection against large landslides. As there is now an increased possibility of a major slide at Beanpole Corner, additional protective and remedial measures should be implemented to reduce or eliminate the landslide hazard at that locality. Such measures could include removal or stabilisation of the potentially unstable material; reopening and enlargement of the old No. 12 tunnel which runs underneath Beanpole Corner; construction of a rock shed or rock avalanche shoot; and the installation of an electric warning signal system for traffic control in the event of a major failure.A combination of stablisation measures, mainly drainage, plus a comprehensive electric wire warning system fastened to the existing protective fence, is seen as the best short term solution for both the railway line and SH1. For long term protection of the railway line the reopening of the old No. 12 tunnel has considerable merit, but further investigations are necessary to show that it is an economic and technically viable alternative.