Settlement of combined piled raft foundation of a nuclear power plant in non-liquefiable and liquefiable soils

Abstract

The settlement analysis of a nuclear power plant (NPP) resting on the combined pile raft foundation (CPRF) under static and seismic loads is very important. However, it has not yet been reported in the literature as most NPPs are founded on raft foundation rather than on CPRF. In this paper, the settlement has been examined in detail. The geometrical nonlinearity at the interface of raft-soil and pile-soil is considered using the coulomb friction. The piles are rigidly connected to the raft. The nonlinear behaviour of the soil in the non-liquefiable soil is modelled using elasto-plastic Drucker Prager cap model and CycLiq model for the liquefiable soil. For this, user defined material (UDM) is implemented in the FLAC3D material library. Thus, a complete 3D finite difference model of an NPP considering raft-pile-soil-structure interaction (RPSSI) for nonlinear settlement analysis is developed. After successful validation of the present CPRF, numerical model with the literature, it has been used for further study under real earthquake acceleration–time history. Next, CPRF with NPP structure is studied under static loading, which is further extended for the nonlinear time domain settlement analysis. The effects of variation in various parameters i.e., spacing between piles, cohesion, friction angle, and Poisson's ratio of soil on the settlement, are examined and found to be significant. The dominant frequency of the earthquake significantly affects the response of the CPRF. It was inferred that the interface nonlinearity has greater significance in the liquefiable soil as compared to the non-liquefiable soil. Higher differential settlement in the liquefiable soil is attributed to the reduction in the stiffness of the soil. The findings of this study could be useful to frame the general guidelines for the economical design of NPP-CPRF systems.