How can shotcrete stabilize tunnels?
When shotcrete is sprayed on a rough ground surface, it fills small openings and cracks. It serves as initial support and also immediate support after excavation. It decreases the possibility of relative movement of rock bodies or soil particles and, therefore, controls the loosening of the exposed ground surrounding the tunnel.
Shotcrete lining could take up significant loads though it forms a flexible support system. In fact, shotcrete lining is expected to undergo large deformations which enable the intrinsic strength and self-supporting properties of the ground to be mobilized as well to re-distribute stresses between the lining and ground. During the deformation of lining, stresses within the shotcrete lining are relocated to the surrounding ground. As such, this mechanism of load transfer in turn generates subgrade reaction of the ground which gives support to the shotcrete lining.
Friction between shotcrete and the ground also reduces the differential movement of the ground. Even though shotcrete may not form a complete ring, the frictional forces between shotcrete and the ground could provide support to the ground.
This question is taken from book named – A Self Learning Manual – Mastering Different Fields of Civil Engineering Works (VC-Q-A-Method) by Vincent T. H. CHU.