Effect of Undrained Strength Anisotropy on Spudcan Penetration

A site investigation campaign was carried out to asses jack up leg penetration in several sites of an offshore area in the Northeastern coast of South America. The investigation identified a thick layer of overconsolidated, medium to hard clay, with a thickness between 30 to 40 m, in which the undrained shear strength showed strong dependency on the mode of shearing, i.e. significant differences between results of triaxial tests (UU/CIU) and Ko consolidated simple shear tests. To examine the influence of this feature in the prediction of spudcan penetration, numerical analyses were carried out in which the anisotropy of undrained shear strength was incorporated explicitly within the constitutive model. For the computation of penetration, the clay behaviour was represented using the NGI-ADP constitutive model, a total stress formulation featuring undrained strength anisotropy. Alternative numerical procedures and constitutive models were also employed, to investigate the sensitivity of the results obtained. This work presents the procedures followed for site characterization, describes the numerical analyses and their results and compares those with the observations obtained during jack-up installation.