A preliminary assessment of geological factors influencing slope stability and landslipping in and around Tauranga City

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Houghton, B.F.; Hegan, B.D. 1980 A preliminary assessment of geological factors influencing slope stability and landslipping in and around Tauranga City. Lower Hutt: New Zealand Geological Survey. New Zealand Geological Survey report EG 348 20 p.

Abstract: Tauranga City occupies parts of three promontories extending into Tauranga Harbour, a drowned Pleistocene river basin. The basement to the basin consists of a number of hard resistant volcanic rock units formed principally in the time interval 1 to 6 million years before the present day. The basin is partially infilled by soft, weakly consolidated fluviatile and estuarine sediments, the Tauranga Formation, overlain by weathered and unweathered volcanic ash. Both deep-seated and shallow mass movements have occurred in the rock units infilling the basin. Deep-seated failures are confined to areas adjacent to the seacliff where the weathered, clay-rich older volcanic ashes are very thick or the Tauranga Formation is unusually clay rich. Superficial failures (1-2 metres deep) are more widespread occurring on sloping ground especially when a thin cover of ash overlies coherent basement rock. Both types of failure are closely related to periods of high rainfall. Factors influencing deep-seated failures are soil conditions, cliff height, rainfall, marine erosion and urban development. Factors influencing shallow failures are soil conditions and structure, slope, rainfall and excavations and road constructions. Storms of the intensity of the March 1979 rainstorm which caused widespread landslipping in the district have a return period of nearly 100 years which may be sufficiently long to exclude them from practical planning considerations. The effects of storms with a shorter return period are insufficiently well known at present. Further work could include lines of investigation to quantify landslip risk and lines of action to minimise risk and damage and to monitor groundwater conditions. It is worth considering delaying any action on the latter unti; the findings of the Abbotsford Commission are released but if Council desires a start could be made to further investigations to clarify landslip risk in the city.