Effect of CPRF on nonlinear seismic response of an NPP structure considering raft-pile-soil-structure-interaction

Abstract

A number of Nuclear Power Plants (NPPs) in India are coming up in soft soil condition particularly on alluvium in active seismic zone on Combined Piled-Raft Foundation (CPRF). So far, no study has been reported considering nonlinearity of soil for the NPP structures founded on CPRF considering earthquake loads.In the present study, the dynamic responses of raft, pile group and CPRF of a NPP are investigated by simulating the nonlinear finite element model. The effects of nonreflecting boundary condition, pile-soil-interaction by the embedded constraint, raft-pile interaction by the tie constraint and raft-soil-interaction by the penalty constraint, were considered. The material nonlinearity of soil was taken into account using an advanced constitutive model Drucker-Prager with cap plasticity. The geometrical nonlinearity is also considered using the concept of the contact surface. Thus, a complete 3D finite element numerical model of a nuclear structure considering raft-pile-soil-structure interaction (RPSSI) for nonlinear seismic analysis is developed. The validation is carried out by comparing the results from the present study with experimental and numerical results available in the literature. Nonlinear analyses of raft, pile group and CPRF in the time domain finite element formulation carried out and results are compared with that using the linear analysis.A comparison of two analyses shows that linear analysis underestimates the response of soil, piles and structure. The effect of type of boundary used in the analyses is significant. It was also observed that the response of NPP structure is significantly affected by the type of foundation used under the superstructure. The effect of SSI is examined with two major novelty i.e. (a) Effect of CPRF (b) Consideration of Nonlinearity of Soil. Effect of frequency of excitation and PGA of earthquake motion is examined. This study indicates the importance of CPRF over the raft foundation and pile group foundation for the design of nuclear power plants. Also, it examines the effects of nonlinearity on the response.