Engineering characteristics of soils prone to rainfall-induced slope failure in Viti Levu, Fiji

Abstract

Fiji's infrastructure is regularly affected by rainfall-induced slope failures, but the engineering properties of failed soils are rarely described. We report mineralogical, geotechnical and index properties of soils from headscarp exposures of 18 slope failures from tropical residual soils of differing parent rocks in Viti Levu, Fiji. Scanning electron microscopy and X-ray diffraction revealed that kaolinite and smectite are the dominant clay minerals in the soils. Index properties included in situ moisture content (28–114%), dry bulk density (0.7–1.5 g cm −3 ), Atterberg limits (25–56% plastic limit; 38–79% liquid limit), effective particle size (0.4–12.6 μm) and clay fraction (0.6–19%). Geotechnical measurements included field compressive strength (127–461 kPa), hydraulic conductivity ( c. 10 −7 m s −1 ), shear vane (16–128 kPa), ring shear (9.3–17.4°) and Emerson dispersion. Collectively, results indicated that most of the soils were cohesive, stiff, sensitive and in a plastic state in the field. Soils plotted below the A-line on the plasticity chart as fine silts of intermediate to high plasticity, and can theoretically sustain >50° slopes. Failure of these soils following high rainfall events is influenced by low permeability and the presence of expanding clays (e.g. smectite), causing temporary porewater pressure increases. No explicit relationships between soil properties and parent lithology were evident.