Report on engineering geological investigations for the proposed realignment of the southern approach to the Mangatewai-nui bridge, SH2, Dannevirke, southern Hawkes Bay

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Hancox, G.T. 1982 Report on engineering geological investigations for the proposed realignment of the southern approach to the Mangatewai-nui bridge, SH2, Dannevirke, southern Hawkes Bay. Lower Hutt: New Zealand Geological Survey. New Zealand Geological Survey report EG 365 15 p.

Abstract: The geology in the area of the proposed SH2 Mangatewai-nui deviation (about 12km north of Dannevirke), is described and results of engineering geological investigations presented. At the Mangatewai-nui bridge site, the proposed southern approach cut will be excavated through thin, very compact, weathered terrace gravel, which overlies about 10m of very weak, unconsolidated sands, silts, and fine gravels. These sediments are underlain by weathered siltstone and sand which will form the base for the proposed new road. No major construction problems are anticipated. However, some of the materials (fine sands and silts) could prove difficult to work during long periods of wet weather, and minor slumping and sand runs on the 1:1 cut batters are possible during periods of heavy rainfall, especially where groundwater in sand layers is perched on silt lenses. Major batter failures are thought unlikely. The old landslide area, to the east of SH2 south of the bridge, is thought to be inactive and will not adversely affect the proposed realignment. The new road south of the bridge is likely to have a favourable effect on the stability of the old landslide area due to reduced slope surcharge and improved slope drainage. Because of the marginal stability of the slopes on the west side of Mangatewai-nui Stream, the high level terraces to the east of the bridge are suggested as possible sites for the disposal of unwanted material from the cuts.