Evaluation of installation timing of initial ground support for large-span tunnel in hard rock

In conventional drill and blast tunnelling, initial ground support is placed immediately after the current round is shot before excavation of the next round (i.e. one-round installation method). When tunnelling in hard rock, one round installation of initial ground support conservatively ensures tunnel integrity, but meanwhile brings up other problems such as over-break at tunnel face, slow excavation rate and so forth. In this study, a large-span tunnel in Class III hard rock was monitored by a network of sensors to investigate tunnel internal forces in three construction scenarios where initial ground supports were placed in different timing and sequence: (1) initial ground support installed immediately after current round (2) support installed after two rounds (3) support installed after three consecutive rounds. The collected field measurements together with construction records were evaluated from three aspects: structural stability, constructability and cost-effectiveness. Results show that the installation of initial ground support after two rounds generally led to the most regular and minimum tunnel internal forces of the three construction scenarios, whilst it managed to minimize under & over-break and allow more space for construction convenience. In the meanwhile, this installation sequence significantly accelerated tunnel advance rate at lower material cost.