The differential control of slope aspect on the stability and management of hill country soils

Abstract

During the winter of 1977 the Wairarapa hill country experienced a series of major mass movement episodes in which there was a pronounced concentration of shallow soil slips on slopes with a sunny aspect. The mass movement episodes occured as a result of medium intensity rainstorms in a winter period which was characterised by high total rainfall of generally low intensity occurring on a large number of wet days. The soils of the sunny aspects have, at high water content, significantly less resistance to forces promoting slip failure than do soils of the shady aspect at the same water content. A number of physical, chemical and mineralogical differences between the soils of opposing aspects may account for the observed differences in soil resistance. In examining the mechanism of slip failure in these conditions, observations confirmed by examination of pre- and post-failure slope forms and supported by determination of soil porosity and strength characteristics, suggest the operation of a flow failure mechanism. Such a flow failure mechanism was first described at Tutira in Hawkes Bay and has since been described and formalised by a number of authors in a variety of locations in New Zealand and California. Further examination of this mechanism is merited and may have implications for change to current soil conservation practices.