Study On The Static Optimization Design Of Nuclear Island-Piled Raft Foundation On Layered Soil And Its Influence On Seismic Response

Nuclear power,as a clean energy,is one of the positive energy sources in China in the future.The safety of nuclear power plant is the key to ensure the safe operation of nuclear power project.In recent years,the installed capacity of nuclear power stations in China has increased year by year,and the distribution area extends from coastal areas to inland areas.With the decrease of good foundation,the foundation form of nuclear power plant gradually develops from rigid foundation to flexible foundation.The application of large pile raft foundation in nuclear power plant foundation becomes very important.It is generally considered that the bearing capacity of pile raft foundation and the safety performance of the whole structure can be improved by increasing the amount of piles and pilesoil replacement rate for important buildings,but the actual situation is that the pile-soil interaction and excessive differential settlement will increase the secondary stress of raft and superstructure.Secondary stress is easy to cause deformation,cracks and even affect the safety of the structure.Therefore,it is very important to find the optimal design method of piled raft foundation with wide application range and high optimization efficiency.There are a large number of seismically active zones in China,and nuclear power plants may be affected by earthquakes.Due to the high shear wave velocity of bedrock,nuclear power plants directly built on bedrock are less affected by soil layer.However,for the nuclear power plant built on layered foundation,the seismic wave will be amplified by the low shear wave velocity soil layer when it is transmitted to the upper structure of the nuclear power plant,and its dynamic response will be larger than that of the nuclear power plant built on the bedrock,and the optimization design of pile raft foundation will further affect the dynamic stiffness.Therefore,in order to ensure the safety and stability of nuclear power structure in the whole life cycle,it is necessary to systematically study the static optimization design of pile raft foundation and its influence on seismic dynamic response.In this paper,the variable stiffness optimization design method of pile raft foundation is studied,the influence of optimization design on dynamic stiffness of pile raft foundation and dynamic response of superstructure is studied,and the simplified calculation method of seismic design of pile raft foundation is studied.The main research contents are as follows:(1)Based on the stress distribution at the pile top(BSDPT),an optimization design method based on the stress distribution at the pile top is proposed.In this paper,combined with the finite element analysis method,a three-dimensional finite element model is established,and the Duncan Chang constitutive model is compiled in APDL language to consider the nonlinear characteristics of soil.The pile-soil interaction and the uneven load generated by the superstructure are considered simultaneously in the calculation.Based on the analysis of the stress distribution characteristics of the pile top in the finite element calculation results of the traditional uniform pile layout design scheme,an optimization design method to reduce the differential settlement of the pile raft foundation by changing the pile diameter is proposed.The results show that the differential settlement of piled raft foundation and the overall bending moment of raft are significantly reduced after optimization.(2)Based on the settlement distribution characteristics of raft,an adaptive optimization method(AOM)is proposed to change the pile spacing,and the excess bearing capacity of pile group after optimization design is transformed into the reduction of the number of foundation piles.The adaptive optimization method economic saving(AOM-ES)is further realized.Based on the analysis of the settlement characteristics of the raft in the traditional uniform pile layout design,the settlement surface of the raft is discretized by interpolation method,and the relationship between the raft settlement and the positioning coordinates of the foundation piles is established.By combining the finite element analysis with MATLAB programming,the adaptive optimization design of reducing the differential settlement of pile raft foundation by changing the pile spacing is realized.The optimization effect of differential settlement of pile raft foundation with different load types and raft shapes after adopting AOM optimization design is studied.By further extending the method,the paper studies how to transform the surplus bearing capacity of pile group into the reduction of the number of pile groups while maintaining high optimization efficiency,thus realizing economic optimization.The results show that the differential settlement of pile raft foundation in different cases is significantly reduced after optimization,which indicates that the method has strong adaptability and stability while achieving high optimization efficiency.(3)Based on pile to pile interaction(PPI),a variable pile length optimization design method is proposed.In this paper,the pile pile interaction coefficient is calculated by the combination of single pile load test data and numerical analysis,and the optimization problem of pile group is transformed into single pile problem.The "length-settlement-p(1-s-p)" surface of single pile under the same soil layer is calculated by using the finite element simulation of single pile load test.The pile-pile interaction coefficient is combined with the single pile 1-s-p relationship to describe the settlement of pile groups.By changing the length of pile to solve the problem of differential settlement of pile raft foundation under complex load,the optimization effect of this method on differential settlement of pile raft foundation under different load types and load levels is studied.The research results show that the method can effectively reduce the differential settlement of pile raft foundation according to the different initial design states of pile raft foundation,and will not add too much work to the construction.(4)The influence of horizontal variable stiffness optimization design on dynamic stiffness of pile raft foundation and dynamic response of pile soil superstructure is studied.In this paper,the analytical model of dynamic impedance function of piled raft foundation is established by direct method.The influence of equal stiffness distribution and variable stiffness distribution of pile raft foundation on dynamic impedance function is studied,and the variation characteristics under different ground motion levels are also studied.The results show that the optimal design of pile raft foundation will affect the dynamic stiffness of pile raft foundation,and the influence degree is related to load frequency and ground motion level.Taking a nuclear power plant as the research object,a finite element analysis model including superstructure,pile raft foundation and soil in finite area is established.The influence of infinite foundation is considered by applying viscoelastic boundary at the boundary of the model.By comparing and analyzing the dynamic calculation results of the variable stiffness pile raft foundation optimization design model and the traditional design model,the influence law of the pile raft foundation optimization design on the dynamic response characteristics of the pile raft foundation and the superstructure is revealed.The results show that the optimal design changes the distribution of vertical stiffness of pile raft foundation along the horizontal direction,which makes the resistance center of pile raft foundation coincide with the action center of superstructure.(5)Aiming at the dynamic response of pile raft foundation under seismic wave,a quasistatic calculation method based on bottom shear method is proposed.In this method,the influence of superstructure inertia force and free field deformation on pile internal force is considered separately.Finally,the load effect combination is carried out to obtain the final result.Under the action of multiple seismic waves,the pseudo static analysis method and time history analysis method are used to calculate the pile raft foundation model,and the shear stress envelope diagram of the pile top of the center pile and the angle pile and the shear stress envelope diagram of the pile group top are compared and analyzed.The results show that the proposed method can accurately obtain the peak value of shear stress and its distribution law under horizontal earthquake,and significantly improve the analysis efficiency,and the application of the method is not limited by the scale of pile raft foundation and soil stratification.