Instrumentation of cut slope and embankment constructed in Tertiary age soft rock terrain on State Highway 4 at Hirotis Bluff

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Millar, P.J.; White, T.; Read, S.A.L. 1987 Instrumentation of cut slope and embankment constructed in Tertiary age soft rock terrain on State Highway 4 at Hirotis Bluff. Lower Hutt: New Zealand Geological Survey. New Zealand Geological Survey report EG 417

Abstract: As a result of the realignment of State Highway 4 between Wanganui and Raetihi at a locality known as Hirotis Bluff, the opportunity was provided to monitor the performance of a cut slope and an embankment constructed in soft rock terrain. In the 25 m high cut slope, 1 m long extensometers monitored movements of up to 10mm that occurred near local slabbing failures. In the 12 m high embankment, settlements up to 90mm were recorded over the construction period and the following winter. During the winter a general rise in the watertable of 0.5 m was also monitored. Much of the settlement is thought likely to have occurred in colluvium not removed from the embankment foundations. The stability of the cut slope and embankment was calculated using strength parameters determined in the laboratory. On the cut slope, local slabbing failures are more likely to influence stability in the homogenous siltstone lithology. On the embankment surface rilling may occur, but there is an adequate factor of safety for general stability and little likelihood of failure is expected. From the results obtained during this study, recommendations on construction practices in soft rock materials are made.