The influence of weathering on the physical, chemical and geotechnical
characteristics of soils has been examined for both unstable and adja
cent stable ground at a site in North Island New Zealand where earthfl
ows have developed in regolith overlying Tertiary calcareous mudrock o
f marine origin. The same sequence of weathering zones was visually id
entified during sample extrusion for both the unstable and stable grou
nd, although the earthflow sites were characterized by extension of we
athering to greater depths. Both the characteristics of the weathering
zones and the relationships between calcite content and geotechnical
index properties show significant similarities to those reported for t
he Fuller's Earth in the UK. Variations in density and moisture conten
t throughout the profiles are attributed to variations in the extent o
f stress relief fissuring, augmented by a calcite content which varies
with depth and position on the slope. If the variation in dry density
(and moisture content) due to calcite content alone is removed, the d
epth at which stress relief fissuring becomes prominent can be readily
discerned and appears to be associated with the zone of basal shear i
n the earthflows. Physico-chemical swelling appears to be absent, even
though the soil mineralogy is dominated by Ca-smectites. Post-failure
weathering, clay mineral alteration and calcite removal are expected
to provide a mechanism for promoting gradually increasing earthflow in
stability.