The influence mechanism of liquid sedimentary layers in urban underground spaces on the characteristics of natural gas explosions and damage risk

Gas explosions are a frequent hazard in underground confined spaces in the process of urban development. Liquid sedimentary layers, commonly present in these environments, have not been sufficiently studied in terms of their impact on explosion dynamics. This study aims to investigate how gas-liquid two-phase environments in confined underground spaces affect the explosion characteristics of natural gas. To achieve this, experiments are conducted to examine the propagation of natural gas explosions in water and diesel layers, focusing on the influence of liquid properties and the liquid fullness degree (L_x) on explosion behavior. The results indicate that the presence of a liquid layer after the initial ignition stage significantly attenuates both the peak overpressure and the rise speed of pressure, in comparison to the natural gas conditions. During the subsequent explosive reaction, the evaporation and combustion of the diesel surface resulted in a distinct double-peak pressure rise profile in the diesel layer, with the second peak notably exceeding the first peak. Under conditions with a liquid sedimentary layer, the flame propagation velocities range from 6.53 to 34.1 m/s, while the overpressure peaks vary between 0.157 and 0.255 MPa. The explosion duration in both the water and diesel layer environments is approximately twice as long as that of the natural gas explosion, although the underlying mechanisms differ. In the diesel layer, the prolonged explosion time is attributed to the evaporation and combustion of the diesel, while in the water layer, the flame propagation velocity is significantly reduced. Under the experimental conditions, the maximum explosion energy reached 7.15 × 10⁶ J, corresponding to a TNT equivalent of 1.7. The peak overpressure surpassed the threshold for human fatality as defined by overpressure standards, posing a potential risk of damage to large steel-frame structures. The explosion shockwave in diesel layer conditions (L_d = 0%, 5%, 7.5%, 12.5%) and water layer (L_w = 12.5%) conditions is observed to be sufficient to damage earthquake-resistant reinforced concrete. This study investigates the impact of sediment layer thickness and composition on gas explosions, and evaluates the associated explosion energy to assess human injuries and structural damage in underground environments. The findings of this study provide a scientific reference for urban underground safety.