Engineering geological investigations of the Ophir Gorge headworks, Manuherikia irrigation scheme

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Paterson, B.R. 1986 Engineering geological investigations of the Ophir Gorge headworks, Manuherikia irrigation scheme. Lower Hutt: New Zealand Geological Survey. New Zealand Geological Survey report EG 393 163 p.

Abstract: The water rise in the Ophir Gorge which forms part of the headworks of the Manuherikia irrigation scheme has been subject to damage from slope failures during its operational life and is currently under investigation by the Ministry of Works and Development. The water race between the intake and by-pass tunnels crosses the toe of an ancient landslide or rock avalanche deposit which has been steeply eroded by the Manuherikia River. A geotechnical investigation was carried out by the Ministry of Works and Development and New Zealand Geological Survey to determine the nature and extent of the debris above the water race, and to assess engineering proposals to improve or replace the existing aqueduct. The first stage of the investigation identified the main areas of concern, the most important being the debris along the right bank above the water race. The second stage involved subsurface investigation of the right bank deposits using core drilling, down-hole geophysical logging and seismic refraction techniques. Piezometers were installed in the drillholes and a network of survey stations was established to monitor slope stability. The results of these investigations were used to assess the stability of the exisiting route and to compare it with several alternative proposals. The latter included pipelines and tunnels on both sides of the river and a weir at the downstream end of the gorge. In addition, alternative access routes into the gorge were investigated. The survey network established that a section of the debris above the water race, where tension cracks are visible, is creeping downslope at a rate of about 50mm per year. There are several active debris slides along the steep slope above the water race, across the entire landslide or rock avalanche deposit, and limiting equilibrium analyses indicate a low factor of safety for circular failures in this material. As a result there is a high probability of damage to either a surface or buried aqueduct along the right bank where the existing water race is located. An alternative pipeline along the left bank is not feasible because of difficulties in construction and the probability of damage from unsuitable scree material. The tunnel proposals are less prone to disruption from slope failure, and one of the alignments may not necessitate the upgrading of access into the gorge. Although the schist should provide good tunnelling conditions the cost of construction will be a major deciding factor. The geotechnical feasibility of a weir has not been proven but it is probable that a satisfactory site could be located. One of the most important factors to be considered in the selection of engineering options for further study is access into the gorge. Of the three routes investigated, routes A and B, which approach the gorge from downstream would be difficult to construct, particularly where they traverse high bluffs. Route C which descends into the gorge from the top of the ridge is geotechnically the most favourable. However, effective control of run-off water will be essential in the top section because of the susceptibility of the material to slope failure, and the bottom section should be re-routed along an existing track. The latter is also prone to damage from slope failure.