<p>This study investigates the seismic stability of a horseshoe tunnel in cohesive-frictional soils using the node-based smoothed radial point interpolation method (NS-RPIM) within an upper bound limit analysis framework. The soil is modeled as a perfectly plastic material obeying the Mohr–Coulomb yield criterion, with seismic effects approximated via a pseudo-static approach using horizontal acceleration coefficients <i>k</i>_<i>h</i>. The analysis evaluates the influence of key parameters on the seismic stability number of horseshoe tunnels, including the cover depth to width ratio <i>H/B</i>, height to width ratio <i>h/B</i>, friction angle <i>ϕ</i>, soil weight to cohesion ratio <i>γB/c</i>, and acceleration coefficients <i>k</i>_<i>h</i>. Results indicate that the stability number <i>σ</i>_<i>sE</i><i>/c</i> decreases as <i>k</i>_<i>h</i> increases, with greater reductions in shallow tunnels and those with lower <i>ϕ</i> values, while deeper tunnels exhibit improved stability due to enhanced soil confinement. The corrective coefficient <i>e</i>_<i>sE</i>, defined as the ratio of seismic to static surcharge, quantifies the reduction in stability under seismic loading and is influenced by soil properties and tunnel geometry. This study provides detailed design tables and charts as practical tools for engineers in seismically active regions. NS-RPIM effectively captures kinematic behavior and failure mechanisms, outperforming traditional finite element methods by ensuring kinematic admissibility and minimizing numerical errors.</p>