Undrained Bearing Behavior Of Skirted Spudcan Under Combined Loading In Clay

The research and application of the skirted foundation as a shallow foundation is increasing,especially in the application of wind power foundations of the coastal engineering.Among them,the skirted spudcan foundation is made by fitting with a peripheral skirt,which can be seen as a modified form of the traditional spudcan foundation and can have certain potential advantages in resisting environmental loads.Moreover,it can provide higher safety and better applicability as the foundation of the jack-up drilling platform in the oil and gas industry.Based on the current lack of research on the bearing capacity of skirted spudcan foundations with a certain embedded depth under combined loading,and the industry design specification(i.e.,SNAME)only proposes that skirted spudcan with a certain embedded depth can provide a higher level of resistance,but it does not explain it in detail.In this paper,the 3D small deformation finite element analysis method is used to conduct an in-depth study on the undrained bearing capacity(i.e.,“short term” bearing capacity)of the skirted spudcan foundation buried from shallow to deep in soft clay under combined loading.The failure mechanisms in the different depths and the elastic response of the skirted spudcan foundation under combined loading are investigated.The important factors such as embedment depth,skirt length,soil heterogeneity and foundation-soil contact conditions are discussed.The approximate equation expression that can describe and predict the behavior of the skirted spudcan foundation and the work hardening plastic model are preliminarily proposed.The main research content and the conclusions obtained are as follows:(1)The analysis of the elastic stiffness coefficients for a skirted spudcan foundation in clay was conducted,which considered three main influence factors including the embedment depth,the soil heterogeneity and the skirt length.The elastic stiffness coefficients of the skirted spudcan foundation under different conditions were obtained,and an approximate algebraic expression was proposed by data fitting to predict these elastic stiffness coefficients.(2)The bearing capacity of the deeply embedded skirted spudcan foundation under combined loading was carried out.The research revealed the failure mechanism of skirted spudcan foundation and obtained the analytical expression to describe and predict the behaviour of the deeply embedded skirted spudcan foundation with different skirt lengths under combined loading.The results show that the deeply embedded skirted spudcan can provide higher and more outstanding uniaxial and combined bearing capacity than the conventional spudcan.Moreover,the horizontal and moment bearing capacity are significantly improved.The bearing capacities of the skirted spudcan foundation are affected by the skirt length,but the skirt is not necessarily as long as possible.The results observed from the failure mechanism indicate that the soil backflow still exists around the skirted spudcan foundation.(3)The bearing capacity of the skirted spudcan foundation under combined loading was carried out,which considered the effects of the skirt length and embedment depth.The failure mechanism of skirted spudcan foundation with different skirt lengths at different embedment depths was observed.The failure envelopes of the skirted spudcan foundation in VH,VM and VHM load spaces were obtained.The approximate expression for describing and predicting the skirted spudcan foundation with different skirt lengths at different embedment depths are proposed by data fitting.(4)The bearing capacity of the skirted spudcan foundation under combined loading was carried out,which considered the effects and the combined effects of the embedment depth,soil heterogeneity and footing-soil contact conditions.The corresponding failure mechanisms are developed,and the changes in the bearing capacity and the shape and size of the failure envelopes of the skirted spudcan foundation are explained.An approximate expression that considered those effects of the factors for describing and predicting the behaviour of skirted spudcan foundation under combined loading is proposed through data fitting.(5)A work hardening plastic model for the skirted spudcan foundations with representative skirt length under combined load in soft clay was established by comprehensive analysis.It can provide a frame work and a basis for reference,which will be attempted to use for designing and evaluating the skirted spudcan foundations at certain embedment depth.