Slope stability of western hills, Hutt Valley : an aid to town planning

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Riddolls, B.W. 1977 Slope stability of western hills, Hutt Valley : an aid to town planning. Lower Hutt: New Zealand Geological Survey. New Zealand Geological Survey report EG 271 24 p.

Abstract: The Western Hills of the Hutt Valley are formed predominantly by greywacke and superficial deposits of loess and colluvium. A field and air photograph study of natural slopes has indicated that instability is relatively minor, and restricted mainly to slopes over 25 degrees. Areas preferred for development should be the subject of further engineering geological investigations, especially if cut slopes are required. This will enable the stability characteristics of the different materials to be evaluated more fully. Assessment of stable cut slope angles is always difficult, but observations from existing cuts provides a useful guide for comparison. Until such work has been undertaken, it may be desirable to restrict development to existing slopes less than 15 degrees, which would also help minimise the needs for cuts. Subdivision plans should not be finalized before any earthworks are undertaken so that modifications can be made if adverse conditions arise unexpectedly. Houses should be sited far enough from cut slopes to minimise the risk of damage in the event of slope failure, and also to ease clearing-up or remedial work be providing sufficient access. Unstable slopes can also contribute sediment to streams in times of flood, increasing the likelihood of damage to property and services. Planning should exclude development from areas where this could occur. (auth)