The effect of back-fill grout characteristics on long-term ground settlement induced by shield tunneling

ABSTRACT: A 10 km long tunnel of Tehran Metro Line 7 was excavated by a 9.164 m diameter EPB-TBM through an old congested part of the city where ground settlement is one of the main construction challenges. Among different sources of settlement in shield tunneling, the settlement caused by the closure of the annulus gap behind the shield tail was discussed in this paper. This settlement usually takes place over a relatively short period of time from few hours to few days. To minimize this settlement, this gap should be properly filled with grout. In this project, a two-component grout was considered to fill the gap and the tunnel route was properly instrumented to monitor ground movements and assess the efficiency of grouting on the ground control. The grout mixture was studied by conducting over 40 laboratory tests. Based on the quality control recordings, where the optimized mixture was not properly made during the construction, ground settlement was analyzed that showed relation between the grout characteristics and the ground settlement.