| Deep-seated soft soil subgrades are widely distributed in the most economically promoted and socially developed coastal areas of China,such as the coast of Bohai Bay,offshore of Northern Jiangsu Province from Lianyungang to the south,the Yangtze River Estuary and the Hangzhou Bay,the Minjiang Estuary,the Pearl River Estuary and northwest areas of Hainan island etc.,which bring great difficulties to the design of port and coastal engineering structures.It is inevitable to build marine structures on soft soil subgrades.Employing new types of coastal engineering structures suitable for soft soil conditions is an effective way to build breakwaters and wharfs at present.Cellular steel sheet pile structures are formed by arranging straight-web sheet piles in a cylindrical shape and filling the enclosed volume with granular materials,and cellular steel sheet pile structures are suitable for the deep-seated soft soil subgrades.However,the interaction between cellular steel sheet pile structures and soil is complicated comparing with related bucket foundation structures,the bearing mechanism and failure modes of cellular steel sheet pile structures are not clear and the design calculation methods are not trustworthy,which place restrictions on the practical application of cellular steel sheet pile structures.Furthermore,wave cyclic loading is transferred to the intercalated soft layer through the breakwater,rubble bed and backfilled sand layer,which generates additional cyclic stress.The impact of cyclic stress to the undrained strength and deformation characteristics of soft layer remain unclear.The bearing mechanism,failure modes and stability characteristics of cellular steel sheet pile breakwaters on soft foundations under wave cyclic loading still remain to investigate.Therefore,in this paper,a series of research work have been carried out on cellular steel sheet pile breakwaters and wharfs in combination with engineering practices.The main contents and conclusions are as follows:1.Based on the geometric characteristics of cellular steel sheet pile structures,a 3D static finite element model is established for stability and stress analyses of cellular steel sheet pile structures by employing ABAQUS finite element software.Shell element is used to simulate the steel sheet pile,hinge connector is set to imitate the influence of relative rotation of adjacent steel sheet piles.Friction in the hinge connector is considered.Stress-strain relationship of the soil is imitated by the Mohr-Coulomb constitutive model.Sliding,splitting and closing between sheet pile and soil inside and outside cellular are imitated by contact elements.For large quantities of numerical calculation,two finite element models are established respectively to simplify the numerical model,one model takes the cell as a whole,and sheet piles are simulated by shell elements,the other adopts entity elements to simulate the cell.Loading coefficient method for stability analysis of breakwaters and strength reduction method for stability analysis of wharfs are established.Finite element analysis methods of the cell strength and soil shear deformation in the cell are built.The accuracy of the finite element model in calculating the stability and stress of cellular steel sheet pile structures is verified combined with engineering examples.2.Based on a pratical engineering example,the overall displacement and soil shear deformation of structures,earth pressure distribution inside and outside cellular and hoop stress distribution of the cell under static wave load are comprehensively studied.The bearing mechanism and failure modes of cellular steel sheet pile structures are investigated,which lay the foundation for the engineering application,practical design and calculation method of cellular steel sheet pile structures.Comparing with results considering the thin-walled and hinge joint property of steel sheet piles,the thin-walled and hinge joint property of steel sheet piles to the failure mode and soil shear deformation of structures,earth pressure distribution inside and outside cellular and hoop stress distribution of the cell under static wave load are explored,respectively,and simplified modeling approaches are suggested to design practical engineerings.A large number of finite element numerical simulations have been done for cellular steel sheet pile wharfs.Failure modes of cellular steel sheet pile wharfs are obtained,stability of wharfs are investigated.3.3D elastoplastic implicit dynamic analyses are carried out based on modal analysis results.Dynamic responses,such as the overall displacement and hoop stress distribution of the cell under cyclic loading,are investigated.A dynamic stability analysis method is built and put into use.Combined with results of the static model,those dynamic results lay the foundation for the dynamic calculation of cellular steel sheet pile breakwater considering undrained strength degradation of soft foundations.4.Based on the development model of pore pressure and the dynamic triaxial test data,the dynamic calculation model that describes the undrained shear strength as a function of cycle number and stress level is established.Based on the dynamic calculation model and M-C yield criterion,a dynamic finite element method is numerically implemented to predict changes in undrained shear strength of the soft foundation using the general-purpose FEM software ABAQUS,and validity of the dynamic finite element method is proved.5.Based on the dynamic finite element method,3D finite element numerical analyses models of cellular steel sheet pile structures considering undrained strength degradation of soft foundation under wave cyclic loading are built.A stability analysis method is proposed to analyze the stability of structures on soft foundations.The increase of excess pore water pressure,the undrained strength and deformation of soft foundation are analyzed.The bearing mechanism,failure modes and stability of cellular steel sheet pile structures on soft foundations are investigated and compared with results of the static and quasi-static methods.6.For direct engineering applications,a simplified calculation method for stability analyses of the cellular steel sheet pile breakwater is proposed based on finite element results.Results based on no-anchor sheet pile wharf method against overturning are verified with finite elements,which provide basis for the simplified design calculation method of the overturning stability analyses of wharfs.Accounting for the limitation of simplified method,the limit equilibrium method is built considering geometric characteristic,stress mechanism and the position of overturning point of cellular steel sheet pile structures for precise calculation.Combining with the finite element numerical results and the existing design specification,the soil shear deformation calculation method in the cell is suggested and the stress analysis method of the cell is established.The above practical design calculation methods combine advantages of high accuracy,and are more simple and useful compared with the finite element method. |
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