To study the spudcan punch through in a kind of multilayered clay which is composed of thick soft-thin stiff-thick soft layers, centrifuge model tests were conducted. The results indicate that there are two typical resistance profiles determined by the strength ratios of the soft to stiff layers. When the ratio is 0.23–0.26, the resistance profile has just one peak point in the thin stiff layer. Others, if the ratio is 0.41–0.52, there would be one more peak point that appears in the top thick soft layer, implying that the punch through might happen before spudcan reaches the stiff layer.
The jack-up drilling rig is widely used in offshore drilling from shallow to moderate water depths because of its mobility and cost-effectiveness. It is typical supported by three independent truss legs, each resting on a spudcan with diameter from 10 to 20m. Before operations, the spudcans should be penetrated to some depth below the seabed surface under incremental preloading (Endley et al. 1981). However, spudcan installation in layered soils could lead to catastrophic failure called ‘punch through’ (Craig and Chua, 1990). Punch through refers to that the applied load exceeds the maximum bearing capacity of the stronger layer and hence causing the spudcan to penetrate rapidly into the lower softer layer which has a much lower bearing capacity (Teh et al., 2009). It may lead to the leg buckling or even toppling of the rig.
To study the punch through mechanism in the location where a stronger layer overlays a weaker layer, many researches have been done using centrifuge model tests (Hossain and Randolph, 2005a; Hossain and Randolph, 2010; Teh et al., 2008; Teh et al., 2009; Teh et al., 2010; Tjahyono et al., 2008) and large deformation finite element analysis techniques (Hossain and Randolph, 2005b; Hu et al., 2014; Hu et al., 2015; Lee et al., 2013; Qiu and Henke, 2011; Tho et al., 2012; Ullah and Hu, 2017). Actually, the profile of submarine soil is much more complex than double layered soil. In order to assess the potential of spudcan punch through in stratified sediments with more than two layers, Hossain (2014) conducted centrifuge model tests for spudcan penetrating through multi-layer deposits with interbedded stronger layers of various consolidation characteristics and mineralogy. The results indicate that the downward soil deformation could extend into one more underlying layers. Zheng et al (2015a) carried out research on spudcan penetrating through nonuniform clay with an interbedded stiff layer whose thickness is larger or as same as the top layer by finite element analysis, and put forward that the bearing response in top layer is almost dominated by squeezing but not all, since some of the trapped material was forced into the stronger underlying layer. Based on the results, they described a mechanism-based design approach for assessing spudcan penetration in soft nonuniform clay with an interbedded stiff layer (Zheng et al., 2015b).
