Effect of Undrained Strength Anisotropy on Spudcan Penetration-Paper

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The study examines how undrained shear-strength anisotropy affects predictions of jack-up spudcan penetration in overconsolidated clay offshore northeastern South America. Site investigations at three locations identified a thick, medium-to-hard clay layer beneath variable depths of very soft clay. Laboratory tests showed substantial differences between strengths measured in triaxial compression and direct simple shear, likely reflecting horizontal silt layers and fissures.<br /><br />The authors compared conventional isotropic-strength predictions using the Hossain–Randolph method with finite-element analyses using the NGI-ADP constitutive model, which explicitly represents different strengths for different shearing modes. They also used finite elements with an isotropic Tresca model to distinguish the effect of strength representation from the effect of computational method. Field penetration observations were available for all three sites.<br /><br />Both isotropic and anisotropic predictions bracketed the observed penetrations. However, isotropic predictions produced a wide range of possible penetration depths, reflecting the difficulty of choosing a single representative strength from markedly different test results. The anisotropic NGI-ADP analyses substantially narrowed this range while remaining consistent with field observations. Using averaged strengths in an isotropic model also narrowed predictions, but could underestimate penetration and thus be unsafe.<br /><br />The results suggest that improved prediction depends principally on representing soil strength appropriately, rather than simply using a more elaborate numerical method. Where site characterization indicates anisotropy, the authors recommend laboratory testing that includes direct simple shear and triaxial compression, ideally supplemented by triaxial extension, and using a constitutive model that explicitly captures strength anisotropy. This approach can provide more robust penetration estimates and a clearer assessment of uncertainty.

Keywords

undrained shear-strength anisotropy

jack-up spudcan penetration

overconsolidated clay

finite-element analysis

NGI-ADP model

Hossain-Randolph method

direct simple shear

triaxial compression

soil strength anisotropy

offshore geotechnical engineering