Geological factors affecting foundation stability along the Ohau B Canal line east of canal metreage 1800

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Macfarlane, D.F. 1977 Geological factors affecting foundation stability along the Ohau B Canal line east of canal metreage 1800. Lower Hutt: New Zealand Geological Survey. New Zealand Geological Survey report EG 278 36 p.

Abstract: The Ohau B Canal line follows the northern edge of the Benmore Range. This report examines foundation conditions between canal metreage 1800 and the penstock intake at 2840m. In this section the canal crossed outwash gravel, fan alluvium and greywacke with associated slopewash deposits. Investigations were concentrated on the fan area where it is anticipated that significant settlement under load will take place due to the precence of perched groundwater and lenses of (often saturated) silt/clay within the fans. Both the Younger and Older Fan Alluviums of Read (1976) are present. The thickness of fan material below the canal line is estimated at 12-15m. Significant differential settlement is expected across the greywacke/fan alluvium contact as a result of settlement and consolidation of the fan materials. This cannot be prevented but drainage and loading of the fan at an earlier stage in the construction program can be used to encourage consolidation and minimise differential settlement. A blanket is thought to be the most feasable form of drainage. Discharge capability is difficult to asses as the groundwater regime is not known in detail. Settlement is not anticipated between canal metreages 1800 and 2200 where the canal is in cut and the foundation materials may be reguarded as effectively pre-loaded. Only minimal settlement is expected for the outwash gravels east of the fan. Differential settlement is again anticipated unless consolidation of the fan is encouraged prior to the structure being completed across the contact. The rates and amounts of expected settlement have not been calculated as there is insufficient test data available. The additional data required for such calculationsis detailed as Appendix 2. Shaft logs are included as Appendix 1. (auth)