Numerical Assessment of Dynamic Responses Induced by Underground Explosions in Tunnel Soil Free Field Systems

Underground structures are essential components of modern transportation and infrastructure systems, and evaluating their behavior under extreme dynamic loads such as explosions is critical for urban safety. This study examines the dynamic effects of underground explosions on tunnel–soil–free-field interaction using numerical methods. Finite element-based dynamic analyses are carried out using PLAXIS-2D, in which a single-layer NATM (New Austrian Tunneling Method) tunnel section representing the Eurasia Tunnel is modeled. Blast loads are defined based on closed-space explosion conditions specified in UFC 3-340-02, and force–time histories are generated for different explosion intensities. A parametric study is performed by varying soil type (soft and stiff soil) and tunnel cover depth to investigate wave propagation mechanisms. Response spectra derived from free-field surface acceleration records are compared with the design spectra of the Turkish Building Earthquake Code (TBEC-2018). The results show that increasing explosion intensity significantly amplifies spectral accelerations. Soft soils exhibit longer acceleration wavelengths, whereas stiffer soils result in higher acceleration amplitudes. Shallow explosion depths are found to reduce soil stability and considerably increase surface accelerations. Under unfavorable soil and cover conditions, explosion-induced demands may approach or exceed design-level earthquake spectra.