Resistance of full-flow penetrometers in rate-dependent and strain-softening clay

Accurate determination of in situ shear strength using full-flow penetrometers (cylindrical T-bar and spherical ball) requires thorough investigation into soil characteristics such as strain rate dependency and strain softening, which have been shown to have a significant influence on the resistance factors relating penetration resistance to the soil shear strength measured in laboratory tests. In this paper, these effects have been quantified using a large deformation finite element technique, modelling both the strength enhancement owing to high strain rate and gradual degradation of soil during penetration disturbance. A parametric study confirmed that the separate factors quantifying effects of high strain rate and partial softening may be multiplied together to predict the combined effect, to derive a resistance factor for a given soil. Simple expressions are presented allowing the resistance factors for T-bar and ball penetrometers to be expressed as a function of rate and strain-softening parameters. By considering average strength conditions during penetration and extraction of these full-flow penetrometers, an approximate expression is derived that allows estimation of the hypothetical resistance factor with no strain-softening, and hence an initial estimate of the rate-dependency of the soil.