As ground conditions deteriorate, the type number increases, the number of supporting processes (such as rock bolts, steel plates, and forepoling) increases, and the length of excavation per cycle decreases from 3.5 metres to 1.0 metres.
3) Thirteen operations: surveying and marking, explosive drilling, charging and blasting, ventilating, scaling, loading and hauling, muck cleaning, steel rib installing, cast lining, floor cleaning, rockbolt drilling, rockbolt installing, and forepoling;
In this study, the tunnel was assumed to be excavated in green field condition (ground surface above tunnel with no existence of buildings), and auxiliary methods such as sleeve-pipe grouting, forepoling pipe grouting, and feet-lock bolt were not considered.
Therefore, other risk mitigation measures, such as the sleeve-valve-pipe grouting technique, double-layer pipe grouting, and forepoling pipe grouting, are adopted to ensure the safety of tunnel construction and surrounding environment.
The forepoling pipe was installed prior to the excavation to improve the stability of the excavation face and the soil properties in the front of the tunnel face.