Engineering geology of Utiku-Mangaweka rail and road deviation

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Oborn, L.E. 1969 Engineering geology of Utiku-Mangaweka rail and road deviation. [s.l.]: [s.n.]. New Zealand Geological Survey report EG 100 19 p.

Abstract: This report describes the geological setting of the proposed rail and road routes between Toi Toi and Mangaweka, and comments on relevant engineering geological factors that should be taken into account during the planning and design of the North Rangitikei and Kawhatau (rail and road) bridges. Comment is made on the proposed embankment across Blind Gully, and the rail and road alignments. The only bridge site (South Rangitikei) that has been investigated by test drilling and laboratory testing has already been reported to the Ministry of Works and is only briefly discussed. Three massive siltstones and two interbedded nodular concretionary sandstones, all upper Tertiary in age, occur between Utiku and Mangaweka striking ca 140 degrees and dipping southwest at about 7 degrees. The Rangitikei and Kawhatau Rivers have planed extensive flat terraces on the siltstones, and cut deep gorges into the sandstones. The routes are through a region in which the terrain is notoriously unstable. Instability is known to occur in old slide debris and at and near the contact between siltstone and overlying sandstone. Spalling occurs parallel to exposed surfaces, especially in siltstone, as a result of shrinkage of drying steep slopes. Fall and slide debris softens with excess moisture and can flow when saturated. The removal of lateral support, by river (or man) also includes a state of instability. The route and the structure sites reported on should present few geological problems, although care will have to be taken not to disturb the ground any mare than is necessary during excavation. Unnecessary blasting or excessively heavy ripping could loosen incipient joint planes near bluffs and gorges thereby reducing cohesion across joint surfaces, and allowing infiltration of water which could increase pore water pressure. Good drainage will be essential, especially in siltstone, to ensure that excess water is removed. Ideally these rocks should be kept at about their natural in situ moisture contents. The rocks on which the bridge abutments and supports are likely to be found are competent to stand the comprehensive loads likely to be applied to them. All such components should be taken sufficiently far into the rocks to be beyond the influence of surface weathering, fretting, spalling, and cutting by rivers. River erosion is active in the Kawhatau River both upstream of the road bridge and upstream of the railway bridge, but this is not considered to pose a threat to the future stability of the proposed bridges. Not standards can be postulated by which to judge the amount of disturbance that these rocks can withstand before their strength and the site stability will be impaired. This will vary from place to place, and will be more serious in siltstone that in sandstone. The stability of the road south of the east end of Blind Gully would be enhanced if the alignment were to be deviated slightly away from Blind Gully. This would have the additio nal merit of depriving excavations of their inward facing component of dip in this small but potentially unstable zone. Awanui Bluff will be excavated in sandstone and should present a stable slope. The cutting southwest from the western end of Blind Gully will also be in sandstone but the upper part of this could be less well cemented than other concretionary sandstones in the area. (auth)