Estimation of intact and remoulded undrained shear strengths from penetration tests in soft clays

Difficulties in obtaining high-quality soil samples from deepwater sites have necessitated increasing reliance on piezocone, T-bar and ball penetration tests to determine soil properties for design purposes. This paper reports the results of an international collaborative project in which a worldwide high-quality database of lightly over-consolidated clays was assembled and used to evaluate resistance factors for the estimation of intact and remoulded undrained shear strength from the penetration resistance of each device. The derived factors were then compared with existing theoretical solutions to evaluate the influence of particular soil characteristics. The overall statistics showed similar levels of variability of the resistance factors, with low coefficients of variation, for all three types of penetrometer. However, correlations of the resistance factors with specific soil characteristics indicated that the resistance factors for the piezocone were more influenced by soil stiffness, or rigidity index, than for the T-bar and ball, while the effect of strength anisotropy was only apparent in respect of resistance factors for the T-bar and ball relative to shear strengths measured in triaxial compression. In the correlation between the remoulded penetration resistance and remoulded strength, the resistance factors for remoulded strength were found to be higher than those for intact strength and with a slight tendency to increase with increasing strength sensitivity but insensitive to soil index properties. Based on an assessment of the influence of various soil characteristics, resistance factors are recommended for the estimation of intact and remoulded undrained shear strength from the penetration resistances of each device for soil with strength sensitivity less than six.