Study On Bearing Behavior Of A Pile Under Combined V-T Loads In Composite Soft-stiff Strata

With the increasing demand for marine resources in China,offshore platforms,offshore wind turbines and other structures have been continuously constructed.Pile foundations are widely-used in these offshore projects owing to theirs advantages such as high bearing capacity and flexible installation procedure.Pile foundations located in the marine environment may sustain additional torsional loads(T)arising from eccentric lateral forces,apart from pile-top vertical loads(V).However,the effect of coupling of V and T is less unnoticed in conventional design methods,which may cause the deviation in the evaluation of pile capacity.Moreover,the marine geological environment is complex,and these pile foundations are mostly embedded in the composite strata with alternating soft and stiff soils.Therefore,based on the existing research results,a study was conducted on the bearing behavior of combined V-T loaded piles in composite soft-stiff strata by theoretical analysis,model test and numerical simulation.The main works include:Firstly,the internal force and deformation law of the pile under independent vertical load V and pure torque T was summarized and analyzed.Then,the coupling relationship between V and T was assumed,the bearing mechanism and internal force displacement solution of pile under combined V-T loads in composite soft-stiff strata were further discussed by shear displacement method.Secondly,based on self-designed combined loading system,a series of model pile static loading tests were conducted in four composite strata: composite upper stiff overlying lower soft strata(SC),composite upper soft overlying lower stiff strata(CS),composite stiff-soft-stiff strata(SCR)and composite soft-stiff-soft strata(CSC).The internal force and displacement of the piles,and the failure types of pile-soil system under independent vertical and torsional load,as well as combined V-T loads were obtained.Thus,the influence law of V-T loads magnitude and loading sequence(V→T or T→V)on the bearing behavior of piles in various composite soft-stiff strata was discussed.On this basis,by dimensionless treatment and fitting,the bearing capacity envelopes of combined V-T loaded pile in composite stiff-soft strata were obtained,and corresponding simplified calculation formulas were also given,which may serve as a reference for engineers.Finally,three-dimensional numerical models of the pile under combined V-T loads in composite stiff-soft strata were established based on the ABAQUS,and the reliability of numerical model was validated by comparing calculated results with the indoor model test results and the existing research results.Then,further parameter analyses were carried out to discuss the influence laws of important parameters such as the elastic modulus of pile shaft,the slenderness ratio of pile shaft,the stiffness ratio of soft to stiff stratum,and the soil distribution on the bearing and deformation of combined V-T loaded pile and the stress distribution of surrounding soils.