In situ linings
Tunnels cut through hard rock may be lined with concrete cast in situ. Generally a long length (or the entire length) of the tunnel is excavated before lining, with some temporary roof support provided by rock bolting. For tunnels in soft ground, precast linings will usually be installed as excavation progresses, though sprayed concrete may be used to give a temporary support before an in situ lining is cast.
Formwork will be in the form of a series of rings, designed so that, once the concrete has achieved sufficient strength, the final ring can be collapsed and passed through the remaining rings to the front for the next pour. Compaction is by external vibrators attached to the formwork or internal vibrators inserted through suitable ‘doors’.
Because of the tendency for the concrete to settle, there is likely to be an unfilled void over the crown of the tunnel, between the cast lining and the rock. It is necessary to fill this with grout at a later stage.