Study on Dynamic Response and Safety Assessment of Adjacent Underpass Tunnels Influenced by Collapse Impact of Building Demolised by Blasting

The impact vibration caused by blasting demolition of tall buildings (structures) may affect the service status of adjacent subway tunnels. In order to demolish a 24 story frame structure building only 6.5 meters away from the subway tunnel, on-site tests and analysis of blasting vibrations and dynamic strains were first conducted. And then, a three-dimensional finite element calculation model was established for analyzing the dynamic response of the subway tunnel structure by ANSYS/LS-DYNA, which can reasonably describe the impact of the collapsed part on the ground during the blasting demolition of the building. Finally, the vibration response characteristics and dynamic stress changes of the tunnel structure under the impact of building collapse were simulated and compared with the field measured data. The research results show that the peak particle vibration velocity (8.61 mm/s) in the subway tunnel caused by the drilling and blasting of the load-bearing columns is greater than the peak particle vibration velocity (4.95 mm/s) caused by the impact of the building on the ground. The main frequency of the blasting vibration and the collapse impact vibration are about 100 Hz and 2 Hz, respectively. The vibration velocity of the subway tunnel under the action of the collapse impact load is equivalent to the structural vibration caused by an earthquake with an intensity of Level Ⅲ (3.82~8.19 mm/s). The low-frequency collapse impact vibration can cause relatively obvious additional dynamic stress to the subway tunnel. The dynamic compressive stress generated in the circumferential direction of the tunnel is about 4 MPa, and the dynamic tensile stress generated in the axial and tangential directions is about 0.4 MPa. The existing cracks may expand, or delamination may occur when the internal damages or construction defects are generated in the tunnel structure under the impact of dynamic tension and compression cycles. The safety allowable particle vibration velocity value of 10~12 cm/s when f<10 Hz required in the current blasting safety regulation (GB6722—2014) is dangerously high, and it should be adjusted in combination with the frequency and dynamic strain characteristics.