Spudcan penetration analysis for three case histories in sand overlying clay

The presence of problematic seabed profiles presents challenges for the stability of jackup rigs during spudcan installation. An example is a soil profile that consists of a relatively thin layer of sand, which temporarily supports the spudcan, underlain by a weak clay layer. Such soil stratigraphy is one of the main causes of spudcan punch-through, which in turn may lead to substantial damage to the leg structures. The International Standards Organisation (ISO) recommends to use either a load spread method or the punching shear method to calculate the peak resistance for a spudcan on sand overlying clay, for which the failure mechanisms were based on small scale experiments. The formation and subsequent progression of the sand plug trapped below a penetrating spudcan is not considered when calculating the penetration resistance in the underlying clay layer. Therefore, the accuracy of the prediction is limited not only by the reliability of the interpreted soil strength parameters, but also by the applicability of the design method currently in use. In this paper, comparisons between the methods recommended in the ISO industry guideline and a recently published method of the full load-displacement profile (called the Hu et al. method here) were made. By back-analysing three field cases, the Hu et al. method shows a significant improvement in predicting the spudcan bearing capacity-depth profile, which could not be achieved by existing methods based on a wished-in-place shallow foundation assuming a single failure mechanism applied repeatedly at different penetration depths. The Hu et al. method captures the essential characteristics of spudcan penetration behavior in sand overlying clay, which in turn enables the potential punch-through failure and the corresponding punch-through distance to be better assessed.