Engineering behaviour and mechanical - empirical relationships for a problematic New Zealand tropical residual soil

Unlike sedimentary clays, many residual soils do not exhibit clear mechanical-empirical relationships to assist in their engineering characterisation. In contrast, this paper discusses one residual clay in which such relationships may be determined, and examines whether the effects of structure in this clay may be assessed using a framework previously developed for sedimentary clays. The Northland Allochthon residual clay of New Zealand is a problematic soil of the fersiallitic type, prone to slope instability. Atterberg limit tests on soils from five field sites in the same geological unit show considerable variation, but that they are mechanically related. Triaxial tests were performed on reconstituted and intact soil specimens from one field site. Normalization of the strength envelope using the equivalent stress on the intrinsic compression line suggests that soil structure, destroyed in reconstituted specimens, plays a role in the shear strength of this soil in its intact state. Overconsolidated behaviour, in the absence of geological preloading, suggests the existence of a pseudo-preconsolidation pressure associated with weathering processes. The results show that the saturated mechanical behaviour of this residual soil is in line with that of sedimentary clays and that mechanical-empirical relationships developed for such clays may be applied in this case.