Geological aspects of cut slope failures in the SH 2 Mount Bruce realignment

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Perrin, N.D. 1995 Geological aspects of cut slope failures in the SH 2 Mount Bruce realignment . Lower Hutt: Institute of Geological & Nuclear Sciences Institute of Geological & Nuclear Sciences science report 95/22 29 p.

Abstract: The SH 2 realignment of Mount Bruce, about 25 km north of Masterton, was carried out between January 1984 and December 1985. Batter failures occurred during excavation in the first two 28 m-high cuts in greywacke, necessitating redesign and recutting of the batters from 76 degrees with 3 m-wide benches every 10 m, to 62 degree batters with similar benches, and a wider clearance between the toe and the carriageway. The third cut of 19 m height (Cut No 3) in greywacke was excavated at the new design batters from the outset, but two intersecting, adversely-oriented crush zones of clayey breccia formed slide planes which initially resulted in the loss of the centre of the east side cut, and later resulted in a major deep-seated movement of a large proportion of the hillside above the cut. Displacement of the slide mass (in two separate phases) however, was less than 3 m. For the southernmost cut (No 4) in the realignment, a 50 degree stepped cut up to 23 m high was designed for the Tertiary age ''papa'' rocks there, similar to the stepped cuts used in the SH 1 Mangaweka-Utiku deviation. But the rock was too weak and too susceptible to wedge failures for the 1 m vertical batters and 840 mm wide benches to survive more than a few months. Repeated cleaning up while allowing the batter to fail back to a more stable angle revealed part of the cause of the problems: steeply dipping faulted contact between greywacke and ''papa'' was present just behind the design batter. Pre-design site investigations did not fully reveal the complexity of the geology in the area, but it is doubtful that a much expanded programme would have enabled prediction of the faulted contact at the back of cut No 4. The investigation drillhole missed it by only a few metres (drilling a few metres to the west would have resulted in Cut No 4 being designed for greywacke), and the two crush zones affecting Cut No 3 would be almost impossible to delineate by drilling, because of the difficulty in recovering core in such materials. The cutting and remedial measures undertaken have been a success, with no further problems and only minimal maintenance required since 1989. (auth/JIH)