A new nonlinear spring-dashpot model of CPRF of NPP structure based on coupled BEM-FEM approach

In this paper, a BEM-FEM coupled model of combined piled raft foundation (CPRF) is proposed for both the frequency domain and the time domain dynamic analyses of a NPP structure in the linear, equivalent linear and nonlinear soil domain. The piles are modeled using the finite element method as a beam element, while the raft and internal domain of soil are modeled using the eight nodded brick element. It is assumed that the piles are rigidly connected with the raft. The nonlinearity of soil is modeled using the hyperbolic stress-strain relation and Modified Drucker-Prager Model. The boundary of the layered soil domain is modeled using the boundary element method. For simplification, it is assumed that the raft and piles are rigidly connected with the soil that means there are no slip and separation conditions at the interface of pile-soil and raft-soil. In order to validate the proposed approach, numerical results, that is, impedances in the horizontal, vertical and rocking directions, are compared with the published literature. Further, dynamic analyses of a lumped structure founded on CPRF is carried out in the time domain and frequency domain using these impedances and results are compared with the FEM. It is found that the spring-dashpot models give the reasonably good agreement with finite element method for linear, equivalent linear and nonlinear models for both frequency domain and time domain analyses. Moreover, practical application of proposed model is shown for a very large problem of raft-pile-soil-structure interaction such as a NPP structure on the layered soil. It was found that the nonlinearity significantly increases the response of the NPP structure. The proposed model has many advantage over the finite element model, for example, first it is very simple and second it reduced the computation time significantly with the good accuracy.