Soil-Structure-Interaction Study of a Nuclear Structure Supported on Alluvium Soil and Estimation of Stiffness of Its Foundation System

One of the proposed site in Northern part of India for twin unit 700MWe Nuclear Power Plant (NPP) (based on Indian PHWR) is founded on alluvial soil. Considering the founding medium of alluvial soil, the effect of the deformation of foundation, both total and differential, is required to be accounted for in the analysis/design of the structural system. Since the deformation of such founding media has direct bearing on the structural response, detailed analysis and design considering soil structure interaction effects for safety related structures supported on raft foundation system shall be carried out. In the analysis of raft foundation, the most important parameter is determination of exact contact pressure distribution underneath the raft, which is a complex function of rigidity of the superstructure, raft itself and the supporting soil. In this paper, detailed soil structure interaction analysis of typical safety related structure of NPP, modeled as 3D finite elements along with soil has been carried out using Abaqus software [1] for both vertical sustained and lateral loads (seismic loads with pseudo-static approach) using site-specific geotechnical parameters. Elasto-plastic behavior of the raft-soil interface is simulated using three parameter model i.e. elastic slip, maximum shear strength and frictional coefficient. Mohr–coulomb plasticity model is used for modeling continuum soil medium. This paper presents the methodology for estimation of stiffness's of raft foundation system under static condition and compares the same with conventional formulations for static springs as per Vesic's approach using modulus of subgrade reaction.