Unlike sedimentary clays, many tropical residual soils do not exhibit clear mechanical–empirical relationships to assist in their engineering characterization. In contrast, this paper discusses one residual clay in which such relationships may be determined, and further examines whether the effects of structure in this clay may be assessed using a framework previously developed for sedimentary clays. The Northland Allochthon tropical 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. Laboratory element 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, which is 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, points to the existence of a pseudo-preconsolidation pressure associated with weathering processes. The results further 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.

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