Undrained Capacity Of Offshore Shallow Foundations Under Combined Loading

Offshore shallow foundations are generally subjected to combined vertical,horizontal,moment and torsional loading due to the complex environmental and operational conditions.The failure mechanism of shallow foundations includes vertical overall shearing failure,sliding failure and overturning failure.The load-carrying capacity for shallow foundations under combined loading in traditional bearing capacity theory is obtained by modification of vertical capacity of surface strip foundations resting on semi-infinite homogeneous Tresca soil.The failure envelopes approach developed in recent years overcomes shortages of traditional method and can explicitly consider effects of load inclination,load eccentricity,foundation shape and embedment,and degree of soil strength heterogeneity on load-carrying capacity for shallow foundations.Although the failure envelopes theory has been widely employed on design of offshore shallow foundations,the specific shape of a failure envelope depends on a variety of factors,which motivates more investigation for the theory.In this thesis,the application of failure envelopes approach on the engineering practice is firstly discussed based on an engineering project,and then failure envelopes equations for shallow foundations under a range of boundary conditions are proposed through the calculation of finite element model.The aim of this thesis is to perfect failure envelopes theory for shallow foundations and provide theoretical guidance for design and optimization of offshore shallow foundations.The main contents and conclusions in this thesis are summarized as follows:(1)Based on failure envelopes theory,the calculation method for multi-footings system of four-leg platform on sand is established,which is validated through an engineering project.The application of failure envelopes theory on the calculation of load safety factor is advised.The design of foundation by combination of failure envelopes approach and partial factor method is presented.(2)The numerical simulation method for zero-tension soil-foundation interface is proposed.Agreement with published results and experimental data validates that a zero-tension soil-foundation interface can be simulated by Coulomb friction with the angle of friction approaching 90°.The undrained load-carrying capacity of surface strip and circular foundations with zero-tension interface under planar V-H-M loading is investigated by the numerical simulation method.(3)The undrained load-carrying capacity of surface circular foundations and surface rectangular mudmats with different width-to-length aspect ratios for unlimited-tension interface and extremely complex loading conditions is explored in the form of failure envelopes.Effect of interface condition on the undrained capacity of subsea mudmats under fully three-dimensional loading is investigated.(4)Generally,the traditional method solutions under-predict the finite element(FE)results obtained under zero-tension interface(ZTI)condition.The failure envelopes for the case of ZTI fall inside of those for the case of unlimited-tension interface(UTI)under combined loading that including moment.(5)The H-M failure envelopes contract inwards proportionally with the increasing torsion,implying the torsion can effectively reduce the size of failure envelopes but has insignificant effect on the shape.Therefore,to consider the effect of torsion,the size of H-M failure envelopes can be reduced by respectively reducing the maximum horizontal load and moment used for normalization according to interaction relationships of H-T and M-T.(6)A series of expressions are proposed to describe failure envelopes presented in this thesis and how the proposed expressions used in geotechnical design are explained through example applications.The proposed expressions can be conveniently implemented into an automated calculation tool to essentially instantaneously generate failure envelopes and aid the design and optimization of shallow foundations in port and ocean engineering.