Passive earth pressure of narrow backfill considering seismic-unsaturated seepage multi-field coupling effect

• A novel calculation method is proposed for seismic passive earth pressure in unsaturated narrow backfill, integrating inclined thin-layer units, interlayer shear stress, arching effect, and multi-field coupling. • Increasing the aspect ratio, seismic acceleration coefficient, or unsaturation parameter α significantly reduces the earthquake passive ground pressure of narrow backfill. • Decreasing the effective internal friction angle, soil cohesion, wall-earth friction angle, or vertical discharge also reduces the passive pressure. • Parametric sensitivity analysis identifies key factors controlling the passive thrust action point, aiding seismic design of retaining structures near existing buildings.