Numerical investigations into cycling of full-flow penetrometers in soft clay

T-bar and ball full-flow penetrometers have important potential in evaluating the remoulded shear strength in situ, by means of cyclic penetration tests. This paper presents numerical investigations into repeated penetration and extraction of the penetrometers in soft clay, using a large deformation finite element method. This approach was able to uncover remoulding mechanisms and resulting gradual strength degradation of soil from intact to fully remoulded states by modelling the entire process of cyclic penetration and extraction. Analyses showed that a cyclic range of three diameters of the penetrometers was sufficient to avoid overlap of the failure mechanism at the extremes and mid-point of the cyclic range. The resistance degradation curves could be fitted accurately by simple expressions consistent with the strain-softening soil model adopted. The strain-rate dependence was similar in non-softening or post-cyclic soil for a given rate parameter. The resistance factor for the post-cyclic condition is higher than that for the initial conditions, depending to some degree upon soil sensitivity and brittleness parameter.