Selected Physical and Geotechnical Properties of Loess in Wairarapa Valley, New Zealand

Abstract

Loess up to 10 m thick covers marine terraces, old river aggradation terraces, and gentle hill country slopes in Wairarapa, New Zealand. In the drier eastern and southern parts of Wairarapa, the loess is subject to dry summer and autumn and wet winter and spring in most years. Under these conditions, and in association with strongly seasonal evapotranspiration, the loess has formed Pallic Soils. The objective of this study is to define physical and geotechnical properties to explain the field behaviour of the loess in order to support applications in agriculture and engineering works. From the outset, Wairarapa loess may have been already more rich in clay than many other deposits around the world, because of the Mesozoic greywacke–argillite parent rock from which it was derived. Wairarapa loess is characterized by being very compacted, with subsoil fragipans with densities up to 1.8 Mg m −3 . In winter, water perches on the impermeable fragipan, saturating the soil above, often to the surface. Wetting and drying in this zone promotes weathering, clay formation and translocation. Ohakean and Ratan loess deposits are more silty than the underlying clay‐rich Porewan loess. Whereas the former loesses blew from extensive low angle aggrading fans, at least some of the Porewan loess was derived from lacustrine deposits on the floor of Wairarapa Valley. Textures of all Wairarapa loess, and particularly the Porewan loess, are clay‐rich. The weathering regime and textures result in extremely low hydraulic conductivity for the loess. Atterberg limit tests indicate that the loesses, particularly the Porewan loess and underlying lacustrine clays, are poor to very poor construction materials. Atterberg limits also help to understand the behaviour of wet Pallic Soils under stocking and traffic. Pallic Soils compact readily when moist and are very weak when saturated, acting as liquids when disturbed, resulting in extreme pugging, dispersion, and soil loss. Pallic Soils in Wairarapa are prone to shallow landslides, even on gentle slopes because of their loss of strength when saturated.

The authors' abstract, as published at the source. New Zealand Journal of Geology and Geophysics, 2026 · DOI ↗