This study investigates the seismic response of a reinforced concrete (RC) tunnel using two-dimensional plane strain finite element models calibrated and validated against experimental results. A comprehensive parametric study is then conducted to explore the influence of tunnel-soil flexibility ratio, soil relative density, Arias intensity of the input motion, and ground motion components on the seismic soil-structure interaction (SSI). The results demonstrated that the flexibility ratio and racking coefficient increase with overburden height, while soil deformations decrease. Acceleration amplification factors rise from the bottom soil to the ground surface, with dense soil showing higher amplification especially in the regions at and near the tunnel field. The horizontal amplification factor exhibits greater variability with increasing seismic energy intensity, and the effect of the vertical motion becomes more pronounced near the structure. The vertical amplification factor is lowest for the horizontal component, while the vertical and combined components exhibit higher values influenced by the presence of the tunnel with lower earthquake intensity. Soil relative density significantly influences the vertical and lateral pressures on the tunnel, with dense sand causing maximum vertical pressures on the top slab and walls. The vertical earthquake component has a greater impact on the tunnel's top slab pressure distribution than the horizontal component. Seismic bending moments are influenced by earthquake components, with the vertical component leading to the greatest positive bending moment values in the middle of the roof slab. Vertical soil deformation is significantly affected by the horizontal input motion component, whereas the vertical component minimally affects lateral soil deformation. These findings underscore the importance of capturing stress-strain response under cyclic loading, particularly near the tunnel crown, where complex stress interactions lead to increased variability in behavior.

Kathmandu Valley, the capital of Nepal, is located in the seismically active Himalayan belt and has a history of devastating earthquakes causing substantial loss of life and property damage. This study employs Probabilistic Seismic Hazard Analysis (PSHA) using the Foulser-Piggott Attenuation (FPA) model and Travasarou et al. (2003) with R-CRISIS software to calculate Arias intensity in Kathmandu Valley. Historical and recent seismic data within a 500-km radius were analysed, and the earthquake catalogue was declustered and standardized using ZMAP software, a tool developed for the statistical analysis and visualization of earthquake catalogues. Additionally, a Digital Elevation Model (DEM) based topographic analysis was conducted to assess the impact of local topography on seismic site response providing insights into, slope, soil amplification factors, and shear wave velocity across the region. The results reveal Arias intensity values ranging from 0.225 to 0.241 m/s at 2% and 10% probability of exceedance corresponding to 475 and 2475 years, mapped using ArcGIS. The analysis revealed that southwestern Kathmandu and Lalitpur exhibit higher Arias intensity values, while intensity decreases gradually from southwest to northeast. The DEM analysis further revealed that areas with low slopes, particularly in central Kathmandu, have higher soil amplification factors, potentially amplifying seismic waves. The shear wave velocity distribution highlights lower values in sedimentary deposits, indicating increased seismic vulnerability. These findings emphasize the need for effective urban planning and disaster preparedness strategies to mitigate earthquake impacts in Kathmandu Valley.