| Summary: | The field area of interest is an intermontane basin lying
100 km northwest of Christchurch on the West Coast Road. Within
this roughly triangular, tectonic depression lies a sequence of
marine and non-marine sediments of late Cretaceous-Tertiary
age. The sediments have been modified by glacial and post-glacial
events to leave steep-sided valleys, flights of terraces and
limestone hills.
Landslides of all types (falls, slides, slumps, flows,
complex movements and mass wasting) are found in the dense,
heavily overconsolidated sands, silts and muds. The different
types of landslides and their, distribution with respect to
lithology and structure is discussed, as is the rates of
degradation and their relative age.
A select area around Broken River - Hogsback Stream is
studied in detail and the stratigraphy, structure and
geohydrology is presented. The sediments from the Enys
Formation were sampled, tested for index and physical properties,
and interpreted in terms of their significance·towards slope
stability. A major slump of this area is back-analysed using
parameters gained directly or inferred from the geotechnical
testing. Several methods are used and a circular arc mode
of failure is rejected in favour of a block slide bounded
by passive and active wedges.
Finally the landslide processes and factors that
have been found operating in Castle Hill Basin are outlined
and it is seen that the most important factors are those relating to the area's intrinsic climatical, geological
and topographical nature. These processes and factors
are summarised in a series of maps, which, when overlain,
produce a landslide susceptibility map showing predicted
areas of likely movement.
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