A comparison of full profile prediction methods for a spudcan penetrating sand overlying clay

Spudcans are the traditional footings used for offshore mobile jack-up rigs. However, the installation of spudcans in sand overlying clay may lead to punch-through failure, which can cause serious damage to the jack-up rig and endanger personnel. This article compares three new methods proposed in the literature and an interpretation of the International Organization for Standardization (ISO) guideline for predicting the full penetration resistance profile. The penetration resistance profile for each of the methods is characterised by two key calculations: the peak resistance in the sand and the bearing capacity within the underlying clay. The punch-through distance - an indicator of the potential for and severity of punch-through failure - is estimated from these calculations. In comparison with 71 geotechnical centrifuge tests, the ISO guideline provides poor predictions, consistently underestimating the peak resistance in the sand and the underlying bearing capacity in the clay. Although all three of the new methods provide a superior response, by assessing the accuracy, scatter and geometric skew of the predictions, two of the methods are shown to be biased in at least one of the key calculations used to define the penetration resistance profile, thus producing bias in the prediction of the punch-through distance. However, one method yields largely unbiased predictions.