Foundation settlement in bridges: case study of Tuninara Bridge in Nepal

A comprehensive study was conducted to evaluate seismic-induced stresses, structural response, and differential settlement between consecutive bridge foundations considering soil–pile interaction. The bridge analysed has mixed foundation types: a mat foundation on hard rock at the end support and a pile foundation in soft soil at the mid support. A three-dimensional finite element model with nonlinear spring elements was developed to simulate pile–soil interaction. Twelve spectrum- matched seismic records to the target maximum-level ground motion were selected, consistent with NBC 105:2020 and normalized to the bridge’s natural period and resulting accelerograms were scaled in 0.025g increments upto 1.5g for incremental dynamic analysis. Key responses, including stresses in piers and piles, vertical and lateral displacements, and curvature, were recorded and compared between two models: rigid pile base (Model A) and flexible base (Model B). Results indicate that flexible foundations reduce stress demand but significantly increase displacement-related vulnerabilities, highlighting a critical design trade-off. The performance of the Tuninara Bridge underscores the importance of detailed geotechnical investigation, accurate soil–structure interaction modelling and foundation solutions. The findings provide practical insights for the design, assessment, and retrofitting of bridges in Nepal’s seismically active and geotechnically complex regions.