Investigating the dynamic response of a double-box utility tunnel buried in calcareous sand against ground surface explosion

Abstract

Ground surface explosion experiments were conducted on a double-box utility tunnel buried in calcareous sand to study the structural dynamic responses and damage characteristics. The measured data, including shock wave propagation, structural deflection and cracking were presented and analysed. The shock wave attenuation was found to be faster in calcareous sand than in loess or dense sand, and the attenuation law can be described as a power function of the scaled distance, as described in TM5-855–1. The pressure reflection coefficient of the tunnel was obtained as 1.41. There was no residual deflection at the roof of the large compartment when the scaled distance R* exceeded 0.97 m/kg1/3. According to the support rotation of the roof, the tunnel was slightly damaged when R* > 0.49 m/kg1/3. When R* = 0.36 m/kg1/3, the roof support rotation exceeded 2°, indicating that the tunnel roof experienced medium damage. No cracks on the tunnel roof slab were visible when R* > 0.54 m/kg1/3. When the scaled distance dropped below 0.54 m/kg1/3, a longitudinal crack appeared and widened. With the test results, numerical models were established and validated, and then the effect of surface charge location, reinforcement arrangement of the roof slab, and the span ratio (ratio of the clear roof span of the small compartment to that of the large compartment) on the double-box tunnel structural response under the ground surface explosion was studied. The deformation mode of the roof changed when the charge location moved from the large span to the small one. The roof of the small compartment exhibited upward bending deformation when the detonation occurred above the large compartment midspan. The edge of the roof adjacent to the middle wall was the most vulnerable location to structural damage. Increasing the span ratio and the ratio of primary reinforcement reduced roof deflection significantly.