Optimizing Sequential Excavation Method for Ground Settlement Control in Tehran Subway Tunnel Line 6

The present study aims to investigate the excavation method, excavation sequence, and optimum trailing distance between excavation stages of the tunnel face to control ground settlement caused by constructing the Tehran Subway Tunnel-Line 6 in the northern part. To this end, 3D numerical modeling is performed, and settlement data validate the obtained results. The results showed that among the sequential excavation methods, the ring-cut (RC) and central diaphragm (CD) have more appropriate control over the surface settlement. The maximum settlement resulting from the RC method and CD is within the permissible settlement range. Considering the better control of the RC method compared to the CD method in limiting the ground settlement, the RC method was chosen to determine the excavation sequence and optimum trailing distance between excavation stages of the tunnel face. Increasing the number of excavation sequences and excavation support core in the final stages of the face can play an effective role in limiting ground settlement. Also, the results showed that when the trailing distance between excavation stages is more than one-time the tunnel diameter, due to the less interference of the stress distribution zones, the settlement of the ground surface declines dramatically and falls within the permissible settlement range.