Abstract: The blasting excavation of tunnels crossing underneath urban areas has disadvantages such as large blasting disturbance, slow construction speed, and high risk. Therefore, a case study is conducted on Nanhu station-Wanshou Road station and a tunnel section of Phase Ⅱ of Chongqing metro line 10. Construction and blasting methods are compared and optimized. The blasting construction is mainly conducted according to the order of "scheme design, blasting construction, vibration monitoring, information feedback, optimization, and adjustment". Specific countermeasures are proposed for difficulties encountered during blasting excavation process. The research results indicate the following: (1) The blasting cross-section is divided into upper and lower parts, the lower part is set as cutting zone, and the cutting hole structure and blasting sequence are optimized. Under the premise of blasting vibration speed of no less than 2 cm/s, this blasting scheme increases the maximum amount of single stage detonating charge and cyclic advance compared to conventional blasting methods. (2) The blasting sequence of upper cross-section first and then lower cross-section rapidly connects each construction process, and it is more beneficial to large-scale mechanical operations and achieve high efficiency than bench-excavation method. (3) The blasting scheme of lower cross-section first and then upper cross-section provides condition for one-time shotcrete supporting, avoiding subsequent arch feet suspension, arch frame damage, and untimely support, which is more conducive to construction quality and safety.

Key words: underpass urban tunnel, blasting first in lower cross-section and then upper cross-section, controlled blasting, rapid construction