Study On The Seismic Response Of Nuclear Power Stations With Connected And Disconnected Pile Raft Foundation

Nuclear power is one of the clean and low-carbon energies which has been utilized in China to realize the Zero-carbon target and it has a bright future in the energy market.At present,all the nuclear power stations in China were located in the coastal rock area using raft foundations.However,with the power demand and the limitation of suitable rock areas,it is urgent to research on the applicable foundation for nuclear power stations built in non-rock areas.The traditional connected pile raft foundation(CPRF)has been successfully employed in soft areas projects.For the last two decades,a new disconnected pile raft foundation(DPRF)which consisted of piles,gravel cushion and raft,has gained many researchers attention for the gravel cushion can act as an isolation layer.The DPRF has been utilized in buildings and bridge at home and abroad,but the DPRF has not been designed as an isolation foundation in China.Because of the high safety requirement of nuclear power stations,research were carried out using 1 g shaking table tests,centrifuge shaking table tests,numerical simulations and theoretical analysis to study the seismic response of the nuclear power station system with CPRF and DPRF.The contents and conclusions of the dissertation are as follows:(1)Shaking table tests were carried out on the scaled model with raft foundation and CPRF,and dynamic centrifuge tests were carried out on the scaled model with CPRF.The effect of soil structure interaction(SSI)was studied based on the seismic response of scaled model with different foundation types.The results show that considering the SSI,the fundamental frequency of model with CPRF were decreased and the damping ratio was increase compared with raft foundation model.The raft acceleration of CPRF was greater than that of soil surface,and the structure acceleration amplification ratios decreased with the increase of excitation intensity.The response of soil surface accelerations,pile bending moments and structure accelerations were more significant under long-period waves than under short-period waves.The maximum bending moments occurred at the pile head and increased with the increase of excitation intensity.Besides,the excessive water pressure also increased with the acceleration and the greater the acceleration the longer it needed to dissipate the excessive water pressure.(2)For the DPRF,a series of shaking table tests and dynamic centrifuge tests were used to study the seismic response of the structure with DPRF.Three types of gravel cushion were selected to study the effect of cushion types on the behaviour of structures with DPRF.Two cushion thicknesses were designed to evaluate the cushion thickness effect on the seismic response of the structure with DPRF.The results showed that cushion type has a great impact on the isolation effect,and with the decrease of cushion thickness,the seismic response of structure was modified.The soil surface acceleration and response spectrum ratio in the near-pile area were greater than that in the far-pile area,which was more significant with the increase of excitation intensity.(3)Comparison of the seismic response of the structure with CPRF and DPRF was carried out based on the result of the tests,which can give insight into the isolation mechanism of DPRF and learn better about the advantages and disadvantages of DPRF.The results proved that the cushion layer can act as an isolation layer and the acceleration of structure and raft were decreased compared with structure with CPRF;however,the displacement was increased,which should be paid more attention when adopting DPRF.The maximum bending moments of DPRF were located at the middle of piles and were smaller than that of CPRF.The maximum bending moment ratio between the corner pile and the inside pile in the second row for DPRF and CPRF were used to study the shading effect of pile groups.(4)The accelerations and displacements of structure and raft,and the pile bending moments under unidirectional and bidirectional excitation were analyzed based on the numerical simulations.The results show that the maximum acceleration,displacement and bending moment for cases under bidirectional excitation occur earlier than that under unidirectional excitations.Because of the interaction of bidirectional waves,the structure acceleration and displacement may be a bit lower than that under unidirectional excitation.The seismic response may be more significant than that under unidirectional excitation and the amplification ratio was within 30%.(5)Based on equivalent linear analysis in frequency domain,a simplified pseudo-static method was proposed to calculate the pile bending moment of CPRF and DPRF considering group effect.The effect of different combinations of kinematic load and inertia force on the bending moment of piles were studied.The calculated results of pile bending moment using the recommended combination type agree well with the dynamic centrifuge test results.