Slope stability in the Omotu slip

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Mansergh, G.D. 1977 Slope stability in the Omotu slip. Lower Hutt: New Zealand Geological Survey. New Zealand Geological Survey report EG 291 26 p.

Abstract: The Omotu Slip is recognised as a multi-component landslide in which large rafts of rock and sheets of colluvium have been moved downslope to the north-east at times disrupting state highway 7 and the Midland railway line. The slip is described as a geological unit of unknown thickness which rests on mudstone of the Kaiata Formation. The geometry of the slip is not determined but it is thought that its base dips to the north-east as an irregular surface. Rotational slide failures are recognised within the slide as a whole. The results of previous investigations on the stability of the slip are presented and discussed, but much of the data was found to be conflicting and thus unreliable. Drill logs for the current investigations are presented and discussed and two surface geological maps, one of the Omotumotu Half Bridge area and one of the western half of the Omotu Slip, are included together with a geological cross-section at the half bridge site. Data on the engineering parameter of the mudstone and slip material is presented, but because of the limited data available, these figures cannot be taken as statistically representative and hence may be used only as a guide in design. Three sites, the Omotumotu Half Bridge, McKendry’s Hill and the Brunner Lime Quarries Ltd quarry are discussed. Mudstone is found to extend across the valley at the Omotumotu Half Bridge site and should provide an adequate foundation if proper drainage is installed, but the deviation is subject to risk from the rafted block perched above the roadway. Excavation at McKendry’s hill will effectively lower the height of the bank by the road and thus reducing the risk of failure within that bank. The risk of failure of the ancient landslide at McKendry’s Hill by unloading is not thought to be high., particularly is adequate drainage is maintained. It is recommended that quarrying of limestone on railway land at present being carried out by the Brunner Lime Quarries Ltd continue as this will unload the landslide in that area reducing the stress on the lower portions, and thus reduce the risk of disruption to the road and rail link. (auth)