Finite element analyses of soil squeezing and its application to offshore foundations in multi-layer stratigraphies

For typical soft over strong soil stratigraphies such as soft clay overlying relatively stronger sand layer, the squeezing solution developed by Meyerhof and Chaplin (1953) is generally recommended in industry guidelines. The solution was developed primarily for flat circular foundation resting in clay underlain by an infinitely stiff rigid layer. Its application to offshore jackup foundations operating in multi-layer stratigraphies needs to be examined from two aspects. Firstly, to improve positioning and extraction jackup foundations are typically conical in shape and secondly, soil is of finite stiffness. To investigate the squeezing mechanism in more detail an extensive centrifuge testing program has been conducted utilising the UWA drum centrifuge in clay with interbedded sand. High-resolution digital images are captured in flight and analysed using the PIV (particle image velocimetry) technique providing insight into the soil failure mechanisms. The centrifuge tests indicated that the bearing resistance in the top clay layer is not independent of the underlying sand and clay layers as assumed in the squeezing theory and also observed consistently in field CPT tests. It was consistently shown that the resistance of the top clay layer strongly correlates with the peak bearing capacity (q peak ) in the underlying sand layer. Complementary large deformation finite element (LDFE) analyses are conducted in soft clay overlying a rigid stratum improving on the squeezing factors adopted in Meyerhofs theory. The LDFE analyses revealed that the conical tip of spudcan foundation precludes the generation of higher resistance near the rigid base predicted by the squeezing theory. The performance of the squeezing approach shows poor performance when compared with carefully controlled centrifuge experiments. A new approach is suggested in which the bearing capacity in the top clay layer can be adequately predicted by successfully overcoming the limitations of the squeezing theory. Copyright © 2016 by the International Society of Offshore and Polar Engineers.