Vibration response and safety control for blasting vibration of the existing tunnel with defects

Current studies on blasting construction of small clear-distance tunnels have not considered the impact of existing tunnel lining defects when establishing safety controls. This paper offers a series of study results based on the blasting project of a new tunnel adjacent to the existing defect Xinling tunnel to thoroughly examine the dynamic response, safety control standards, and measures of the existing defect tunnel. First, structural models were developed to investigate the influence of the presence or absence of specific defects (like lining cracks and cavities behind the lining) on the dynamic response of the current tunnel lining to identify the most unfavorable defect distribution. Then, establish safety control standards for intact linings and those with the most unfavorable defects. Eventually, two types of control measures, single safe charge and reasonable delay time, were studied based on the established safety control standards. In particular, the most adverse position of cracks was the wall facing the explosion, the rise in depth was more unfavorable for vibration response, and the impact of the longitudinal crack was restricted to the vicinity of the crack. While the vault was the most adverse cavity position, the rise in cavity area was more damaging, and the influence range varied with longitudinal cavity length. Moreover, the impact of cracks was mainly evident in the amplification effect of stress at the crack region. In contrast, cavities had varied degrees of amplification effects on the vibration velocity and stress response and a relatively extensive influence range. Safety control research was conducted, when the tunnel was intact, with a right wall crack, a vault cavity, and both vault cavity and crack for this project, the peak particle velocity (PPV) of the safety control standard for vibration velocity was 13, 10, 13, and 8 cm/s, respectively, and the respective single safe charge could be adjusted at 64, 53, 37, and 25 kg. However, the presence of different defects had a relatively negligible effect on the reasonable delay time; 25 ms was recommended for existing tunnel lining with and without the defect.