Effect of Backfill Stiffness on Minimising Settlement in Rail Bridge Approaches

Abstract

Non-uniform settlement at the interface between rigid bridge structures and adjacent embankments is a persistent problem in rail bridge approaches, degrading ride comfort, increasing maintenance costs, and, in severe cases, threatening operational safety. While backfill compressibility is widely recognised as a key contributor to this problem, quantitative guidance on how the elastic modulus of backfill materials governs the magnitude of differential settlement and displacement remains limited. This study addresses this gap using three-dimensional finite element (FE) analysis in Midas GTS NX to systematically investigate the influence of backfill elastic modulus, ranging from 25 MPa to 28,000 MPa, on the settlement and displacement behaviour of a jacked-frame-bridge rail approach. The soil was represented using a modified Mohr–Coulomb constitutive model calibrated with parameters derived from a geotechnical investigation report for a jacked frame bridge project in Shandong, China, while the backfill, subgrade, track and bridge structures were modelled as elastic materials. Static construction-stage analyses were performed to obtain the maximum vertical and horizontal displacements, together with the differential settlement and differential horizontal displacement across four monitoring sections spanning the bridge–embankment interface. The results show that increasing the backfill elastic modulus substantially reduces both differential settlement and maximum displacement, with the greatest improvement occurring within the 0–10,000 MPa range; beyond approximately 15,000 MPa, further increases yield only marginal benefit. The maximum vertical settlement decreased by 10%, the maximum ground heave by approximately 81%, and the differential horizontal displacement across the transition zone approached zero at high modulus values. These findings provide engineers with a quantitative basis for selecting backfill materials that balance mechanical performance against cost, and for identifying the elastic modulus threshold beyond which further material upgrades bring diminishing structural benefit.