Creep, strength and other characteristics of Hawaiian volcanic soils

Strength, creep and other properties of four volcanic soils from Hawaii were investigated under reconstituted triaxial conditions. The selected soils are of different geological origin and have contrasting index properties. Results from triaxial tests suggest that as clayey volcanic soils weather toward a siltier texture, their response to loading changes accordingly, with dilation at larger strains leading to an increase in strength. The frictional strength of two of these soils, a high-plasticity clay from Manoa Valley in Honolulu and a young ash soil from the Hilo area, does not correlate well with conventional empirical relations. In terms of creep behaviour, the response under constant stress conditions can be described, at least to a first order, with a simple rate process equation. A constant that is a measure of strain rate attenuation with time is observed to be stress level dependent at lower deviatoric stresses, but relatively constant at higher stress levels. This constant may be related to plasticity index, although such a correspondence is tenuous at best. Strength gain is observed in post-creep undrained tests, and is attributed in part to prior volume changes and in part to ageing. In terms of changes to the short-term failure envelope, the effect of a sustained creep phase of 100 days or longer is to shift the failure line upward, albeit at a constant angle.