Field and laboratory investigation of rainfall-triggered slope failure in unsaturated loess soils, New Zealand

¹ Department of Geological Sciences, University of Canterbury, New Zealand
² Department of Civil and Environmental Engineering, University of New South Wales, Australia

^* Corresponding author: katherine.yates@pg.canterbury.ac.nz

Abstract

Shallow rainfall triggered slope failures occur frequently in loess and loess-derived deposits across the South Island, New Zealand. These failures, which occur in both natural slopes and engineered cuttings, impact road infrastructure, residential housing and rural land use. When dry, the loess can form near vertical cuttings. However, with increase in moisture content loess slopes become susceptible to shallow slope failures. To date, the influence of negative pore-water pressure (suction) on the stability of loess slopes in New Zealand has not been well understood. In this paper, data from long term in situ field monitoring of rainfall, suction and volumetric water content from a loess slope in Banks Peninsula, Canterbury are presented with laboratory triaxial test results undertaken on undisturbed unsaturated loess samples. Field and laboratory soil responses to wetting and drying are compared, and the characteristics of rainfall events which reduce suction in situ and therefore slope stability are discussed.