Geological aspects of failures in roads : State Highway 1 (North Island), Mangaweka Deviation

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Read, S.A.L.; Mildenhall, M.; Roach, P. 1995 Geological aspects of failures in roads : State Highway 1 (North Island), Mangaweka Deviation . Lower Hutt: Institute of Geological & Nuclear Sciences Institute of Geological & Nuclear Sciences science report 95/29 47 p.

Abstract: The Mangaweka Deviation on State Highway 1, the principal road link between Auckland and Wellington, is between the townships of Mangaweka and Utiku in the central North Island. Reconstruction over an 8 year period in the 1970s transformed a 13km long winding section of highway into 11km of fast open highway. Feasibility studies included some of the first usage of computer technology to facilitate selection of the route through steep erosion-prone terrain underlain by Tertiary-age sedimentary soft rocks ('papa'). After geological 'walkover' inspections, investigations concentrated on determination of colluvium thickness, and cut slopes in bedrock were designed at 1:1 in mudstones and 1/4:1 in sandstones. Construction, with continual on-site supervision allowing verification of and modification to design as necessary, was completed without major problems. Operation of the highway has been affected by a small number of blockages caused by slope failures (e.g. 1988 and 1994), with other smaller failures with lesser or little impact occurring on a more regular basis. The greatest impact has been from failures of colluvial materials in both cut slopes and natural slopes. From an engineering geological review of the investigations, construction, and operation of the Mangaweka Deviation in this study, three principal geological lessons are apparent. Firstly, the satisfactory performance of cut slopes in colluvium needs careful design, including assessment of their likely performance in cyclic periods of wetter weather during operation. Secondly, the failure of natural slopes above cut slopes presents a hazard to the highway as it can result in the deposition of debris on the highway. Thirdly, lithology has a major influence on the degree of surface degradation (slabbing and slaking) in unweathered 'papa' bedrock and cut slope batter angle selection should acknowledge this. (auth)