Interpretation of Interbedded Thin-Soft Layer Properties from T-Bar Penetration Tests

Layered soil conditions are common in marine sediments, and deduction of strength properties for thin interbedded layers is often challenging. Full-flow penetrometers such as T-bar are increasingly being used in offshore site investigations and show high potential for differentiating the shear strength transition between layers. This paper reports the results from large deformation finite-element analysis undertaken to provide insight into the response of a T-bar penetrometer as it continuously penetrates through stiff-soft-stiff clay layers. The numerical results are validated against data from a centrifuge model test. Trapped soil is observed for any nonsmooth interface as the T-bar penetrates from a stiff to a softer soil layer, which affects the measured resistance of the soft layer. The effect of a cavity above the advancing T-bar was also investigated. The presence of a cavity can increase the sharpness of the transition in resistance from one layer to another, which thus affects estimation of the boundary of the soft layer. The evolution of trapped soil from the top layer within the soft layer, and then its erosion in the bottom layer, were explored. The research found that the measured resistance profile in the soft layer is sensitive to the strength ratio between the top layer and soft layer, but the resistance of the bottom layer is more reliable. Based on deduced measurement parameters from the resistance profile, a new interpretation framework is proposed for interpreting the layer boundaries and undrained shear strength of the interbedded soft layer.