Riddolls, B.W. 1975 Results of subsurface investigations of unstable slopes between Waimiha and Poro-o-tarao, NIMTR. Lower Hutt: New Zealand Geological Survey. New Zealand Geological Survey report EG 237 15 p.
Abstract: The nature of unstable slopes affecting the North Island Main Trunk Railway has been investigated by drilling, which showed that they are underlain by colluvium to depths of several meters, beneath which are rocks of the Mahoenui Group. The colluvium consists of grey silty clay with rock fragments, apparently derived from the Mahoenui Group. It is generally firm to stiff, with medium plasticity, but locally may be very soft and highly plastic, owing mainly to a lack of rock fragments. Surveying and slope inclinometers have been used to monitor surface and subsurface movements respectively. Recent movements of one landslide deformed an inclinometer tube sufficiently to indicate that sliding occurs there at the colluvium/bedrock interface. Elsewhere, highly plastic material within the colluvium may deform compressibility, therefore causing downslope movement. Slope stability analyses and observations of ground water levels in drill holes suggest that stability is affected by relatively small changes in these levels. Lowering of these levels by the construction of subsurface drainage and therefore seems to be the most effective way of achieving long term stability. The planting of trees may assist de-watering through the process of evapotranspiration. (auth)