Influence of T-bar test penetration rate on undrained shear strength of a laboratory clayey mixture

Extensive and accurate data from on-site testing is increasingly fundamental in determining design geotechnical parameters of offshore structures. As a novel method for assessing offshore soil, the T-bar penetrometer offers benefits such as high precision, reliability, ample data acquisition and cost-effectiveness, but its application in offshore projects remains limited. In this research, a set of approaches are used to analyze the evaluation of penetration rate effects in a kaolin and bentonite mixture prepared to reproduce Brazilian offshore soil conditions aiming the development of in situ testing technology in the marine engineering area. Miniature T-bar penetrometer tests (10 mm diameter and 50 mm length) at constant driving penetration rate were performed at 0.05; 0.1; 0.25; 0.5; 1.0; 2.5 and 5.0 mm/s. In addition, three variable rate tests (twitch tests) were performed aiming to analyze the penetration rate reductions at each twitch stage. The measured resistances and the drainage conditions were interpreted using the normalized velocity (V), and the results show a strength increase by viscous effects of 10% for each logarithmic cycle of penetration rate increase. The results suggest that the transition between the undrained and partially drained penetration occurs at normalized penetration rates between 20 and 50. These results agree with values previously published in the literature and unequivocally demonstrate how the influence of penetration rate can impact the measured undrained shear strength for T-bar tests. It is concluded that the influence of the test penetration rate on the measured resistance cannot be disregarded in intermediate permeability soils.