Lower Waitaki power development: engineering geology of the proposed canal route, Waitaki dam to Black Point, report no. 2

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Macfarlane, D.F. 1977 Lower Waitaki power development: engineering geology of the proposed canal route, Waitaki dam to Black Point, report no. 2. Lower Hutt: New Zealand Geological Survey. New Zealand Geological Survey report EG 295 34 p.

Abstract: The Waitaki River flows in a fault angle depression in which Cretaceous and Tertiary sediments unconformably overlie basement schists in fault contact with greywacke. Northeast trending faults cut the Tertiary sediments. Quaternary gravels up to 15 m thick overlie the sediments and are not faulted. Geological investigations have allowed the delineation of Tertiary and older rock distribution below the gravels. Micropaleontological evidence has resulted in the re-interpretation of limestone distribution near Dunstroon (cf McLean, 1977). In general the units follow the terminology introduced by Gage (1957). Exceptions are in the differentiations of gravels, the restriction of the McDonald Limestone to Earthquakes Marl, and the use of Suggates’ (1961) Haast Schist in place of ‘Kakanui Metamorphic Group’. From an engineering geological point of view it is suggested that variations in material properties within the gage’s units necessitates subdivision of some Tertiary units into subunits (members) of similar engineering characteristics and that definition of these should be an integral part of the detailed site investigations. The geological structure of the valley is not yet understood. It is important to rectify this as the structure controls lateral variations in foundation conditions along the canal line and influences the selection of power station sites. Similarly, effective selection of borrow areas requires understanding of geological structure. Groundwater inflow will be a major construction problem. Most flow is within the gravels but open joints are flow paths within Tertiary sediments and solution channels/cavities may exist in limestones. Artesian conditions have been experienced in at least one borehole. At least two of the proposed power station sites (U4 and U6) are likely to have unsatisfactory foundation materials and alternative sites should be investigated. (auth)