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Ohau B power project : Ruataniwha dam spillway, preliminary report on foundation geology

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    Macfarlane, D.F. 1978 Ohau B power project : Ruataniwha dam spillway, preliminary report on foundation geology. Lower Hutt: New Zealand Geological Survey. New Zealand Geological Survey report EG 308 31 p.

    Abstract: The Ruataniwha Dam spillway is a concrete gravity structure located in a rock cut on the true right of the earth dam site. The combined spillway/diversion structures is founded entirely on Torlesse Group greywacke of Mesozoic age. Detailed geological logging of foundations surface has shown that the foundation rock is a steeply dipping sequence of interbedded hard sandstone and mudstone. The rock mass is closely fractured, with twelve joint sets recognised on the basis of stereographic projection, and extensively sheared as a result of tectonic activity. In addition to numerous small shears developed in weaker mudstone beds, the rock mass contains eight major zones of weakness at foundation level as well as several small shears of limited extent cross-cutting bedding. The foundation rock is generally unweathered or slightly weathered. The major foundation weakness is the steeply dipping crush zone designated Zone A which runs the full length of the structure, crossing the centre line at a low angle. It is widest under the LHS of the gate structure area where a shatter zone 3 m wide includes up to 400 mm of gravely clay gouge. Zone A is a potential leakage path and a provisional for a cut-off shaft has been made. Permeability and dispersion testing of the gouge material is in progress. Two other weakness zones (B and C) are present under the gate structure but form lesser leakage threats. The other crush features (D to H) do not appear to be potential leakage paths but crushed/shattered rock adjacent to Zone H forms a weak area under the downstream edge of the Left Abutment (Block A2). Joint orientations and characteristics are generally very favourable to foundation stability. Sliding failures of the foundation rock is unlikely unless undetected low angle features are present within the rock mass or tension-induced fractures develop in the plane of the silting basin floor anchor roots. Foundation improvement measures utilised during construction have included localised treatment of wet or weak areas, drainage, ground anchoring and both curtain and consolidation grouting. Curtain grouting, ground anchoring and drainage programs are still in progress. The permeability of the rock mass is not fully known. Testing in consolidation grout holes indicated values in the range of 10-3 to 10-5 cm/sec but at greater depths (where stress relief will not have occurred) it is expected that values in the order of 10-5 to 10-6 cm/sec will be more typical. Curtain grout and rock anchor holes will check this. A separate report assessing foundation permeability, grout takes etc will be issued at a future debate. (auth)

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