A review of the geotechnical characteristics of loess and loess-derived soils from Canterbury, South Island, New Zealand

Loess and loess-derived soils cover much of Canterbury, from the foothills of the Southern Alps to the Pacific Coast. These deposits are of variable thickness, from a few metres up to 40 m at the base of slopes on Banks Peninsula. In many areas the primary, air-fall loess has been reworked by slope processes to form a loess colluvium. Although primarily silty, these soils contain up to 45% clay, giving rise to low plasticity clay behaviour. These deposits are relatively dense, with dry densities of 1.6 t/m3 and 1.8 t/m3, hence they do not display collapse behaviour common to other loess deposits around the World. Loess and loess-derived soils across Canterbury have high dry strength but weaken rapidly with small increases in moisture content. In the wetter parts of the region, on Banks Peninsula in particular, periodic wetting leads to a variety of slope failures related to internal erosion (tunnel gullying) and rapid loss of shear strength (debris flows, soil slides and rotational failure). Elsewhere in the drier parts of South Canterbury loess deposits commonly form vertical exposures and evidence of slope instability is comparatively rare. The current understanding of the geotechnical characteristics of these deposits is based on a limited number of studies with restricted geographic extent. Thus a full understanding of regional and stratigraphic variability is yet to be established. Macroscale heterogeneities, including fissuring, development of fragipan cemented horizons and stratification, indicates that, as well as the general behaviour presented in this paper, there may also be significant local variation. In addition, the effects of soil microstructure, including bonding and particle shape and orientation, on overall soil behaviour are recognised, but have yet to be investigated in detail.