Reasearch On Bearing Behavior And Simplified Calculation Methods Of All-vertical-piled Wharf In Offshore Deep Water

With the large development of transport ships,new ports tend to be constructed in offshore and deep water area.Foundation soils of offshore and deep water area are mostly saturated soft soils.All-vertical-piled wharf structure is a new high pile wharf structure type which is proper for soft soil foundation in offshore and deep water area.However,it is different from high pile wharf structure type with inclined piles or brace piles,and its bearing behaviors,failure mode are quite different from those of the traditional ones.The intensity of wave loads is higher.So,soil cyclic softening effect caused by wave loads is remarkable.Piles of all-vertical-piled wharf in offshore deep water are long,and flexibility of structures is relatively great.Fundamental period of structure is close to periods of wave loads and impact loads.So,dynamic response of structures,interaction of storm surge wave and structures are outstanding.Cyclic accumulative deformation characteristics of all-vertical-piled wharf under wave cyclic loads are unclear.Bearing behavior,failure mode,the stability influencing of soil cyclic softening effect,static and dynamic simplified calculation method and cyclic accumulative deformation simplified calculation method of all-vertical-piled wharf in offshore deep water were researched to solve above problems combining with a certain design scheme of offshore deep water port.The main research contents are as follows:1.By the finite element analysis software ABAQUS,the 3D elasto-plastic finite element model for the all-vertical-piled wharf and soil foundation are established.Structure stress,deformation,bending moment along the piles and soil pressure distribution of piles lateral under the wave loads and impact loads were analyzed using loads control mode combining with loads coefficient and the concept of perfect plastic flow.Combining bending moment of pile and soil pressure distribution of pile lateral,the ‘the plastic hinge' failure mode was suggested as the criterion of horizontal ultimate state of all-vertical-piled wharf in offshore deep water.2.Cyclic strength and Mises yield criterion were combined through subroutine of the finite element software.A pseudo-static analysis model considering the cyclic softening effect of soil was established,and then cyclic softening effect of saturated soft clay soil under wave loads was simulated.The influences of cyclic softening effect on the structural stability were analyzed using finite element method(FEM).Finally,the influences of the buried depth and the thickness of piles,different soil conditions,cyclic softening effect and dilatancy angle on horizontal ultimate bearing capacity of the structure were researched.3.The applicability of m method,P-Y curve method and NL method calculating all-vertical-piled wharf in offshore deep water were validated through the FEM.The conclulations show that m method is suitable for small displacement,and P-Y curve method and NL method are in good agreement with the FEM results for small and large displacement.Combining with ‘the plastic hinge' instability criterion standard,a simplified method using the ratio of ultimate bending moment and the maximum bending moment of piles under design loads to calculate horizontal ultimate bearing capacity was proposed based on P-Y curve method and the NL method.4.The natural vibration characteristics of all-vertical-piled wharf in offshore deep water were studied employing modal analysis method.The analysis indicates that fundamental period of structure is longer,and it is close to periods of wave loads and impact loads.So,dynamic response of structure is remarkable.Then the further calculations have shown that the dynamic response of all-vertical-piled wharf can be calculated by the theory formula of the dynamic amplification coefficient of single-degree-of-freedom system.On this basis,the dynamic response simplified calculation method was proposed based on m method,P-Y curves method and NL method.In comparison with the dynamic finite element method,the dynamic simplified calculation method is valid and feasible.5.According to characteristics that the cyclic cumulative deformation of soft clay are similar to the creep under static loads,the number of cycles are seen as time,the model calculating the cyclic cumulative deformation was established based on creep theory.And the pseudo-static elasto-plastic finite element method was suggested to calculate cyclic cumulative deformation of soft soil.To illustrate the feasibility of this method to calculate cyclic cumulative deformation of all-vertical-piled wharf,cyclic triaxial test results were numerically simulated using the subroutine of ABAQUS.The predicted and test result are in good agreement.Finally,the cyclic cumulative deformation characteristics of all-vertical-piled wharf were studied.