Growing emphasis on sustainable urban development has intensified the use of underground urban space for transport and protection. During war or terrorism, underground tunnels can function as protective structures, so accurately assessing their response to surface blasts is essential. This study presents a three-dimensional numerical investigation of a circular metro tunnel embedded in sandy clay and subjected to surface detonation. The soil is modelled with Mohr-Coulomb plasticity, while the tunnel lining and reinforcement adopt concrete damaged plasticity and Johnson-Cook models. Blast pressures are applied via the CONWEP formulation (TM 5-855-1), and the simulation captures coupled interactions between soil, lining, reinforcement and blast wave. The framework is validated against analytical peak overpressure predictions, empirical crater dimensions and experiments on blast-loaded reinforced concrete slabs, showing acceptable deviations. Crater-size formulas are recalibrated for sandy clay using simulations with 25-1000 kg TNT, improving agreement within this range. An energy-based framework relates TNT weight to kinetic, plastic dissipation and strain energy in soil, providing meaningful insight into blast energy transmission, absorption and dissipation. The effect of charge magnitude on soil stress, liner deformation and tensile damage is assessed for overburden depths of 15, 12 and 9 m; shallow tunnels suffer greater deformation and damage under increasing blast intensity. Performance criteria identify safe and unsafe burial depths for different charges. Regression-based energy and deformation relationships enable quantitative comparisons of tunnel responses within the defined scope and assumptions.
Alsabhan et al. (Mon,) studied this question.