The variation of large strain viscous rate effects from undrained triaxial element testing of Kaolin clay

Abstract Viscous (undrained) rate effects lead to an increase in the shear strength of fine-grained soils such as clays when sheared at high strain rates. Correct rate effect parameters are key to accurate predictions of behaviour in a range of geotechnical applications, from subsea cable ploughing and rapid load testing of piles through to numerical modelling of offshore foundation systems. The factors influencing the magnitude of rate effects are, however, still poorly understood. Through a series of 100-mm diameter high-speed undrained triaxial tests (strain rates ranging from 300 to 180,000%/hr and initial effective stresses from 300 to 1350 kPa) on Kaolin clay, this paper investigates the influence of the soil state and stress history on rate effects across a wide strain range. Viscous rate effects ranging from 10 to 16% per log cycle are observed, and the liquidity index is observed to correlate to the magnitude of the rate effects. Viscous rate effects are also shown to reduce with increasing strain level, and a model is proposed which allows the rate effect in Kaolin to be predicted accounting for both of these factors. This model is potentially useful in analytical and numerical studies.