Riddolls, B.W. 1974 Slope stability at Waipawa, and its significance in residential development. Lower Hutt: New Zealand Geological Survey. New Zealand Geological Survey report EG 175 7 p.
Abstract: Residential development is being encouraged at Waipawa and because mudflows are known to have occurred there, the factors contributing to slope instability have recently been investigated. Most of the ground at Waipawa is gently sloping, and slopes over 20 degrees are uncommon. The flatter ground at various elevations is attributed to terrace formation by different river levels in the past. The town is underlain be weak mudstone, but this is rarely exposed owing to thin but widely occurring deposits of loess and gravel. The mudstone is overlain in many places by clayey colluvium, which from X-ray diffraction studies is shown to contain montmorillonite, together with kaolinite and illite. Although not a strongly swelling type, the montmorillonite has contributed to the past slope instability, which has occurred mainly in the clayey colluvium, which from X-ray diffraction studies is shown to contain montmorillonite, together with kaolinite and illite. Although not a strongly swelling type, the montmorillonite has contributed to the past slope instability which has occurred mainly in the clayey colluvium. Slopes steeper then 20 degrees in this material commonly show evidence of creep. Although no major damage is known, some existing houses built on sloping ground at Waipawa have in the past shown signs of minor stress. This could be caused by slow slope movement b ut, because they contain montmorillonite, the expansive behaviour of both colluvium and loess is a possible mechanism in some cases. Because it has flowed in the past, construction on clayey colluvium should not be undertaken except where it is sufficiently thin to allow foundations to be located in the undisturbed mudstone beneath. Its sporadic distribution requires inspection of each site individually. Effective drainage is essential wherever construction is undertaken in this area, to minimise the potential hazards associated with soils containing montmorillonite. (auth)