Abstract
Modern tunnels in hard rock are usually constructed by drill and blast with the rock reinforced by shotcrete (sprayed concrete) in combination with rock bolts. The irregular rock surface and the projection method of shotcrete lead to a tunnel lining of varying thickness with unevenly distributed stresses that affect the risk of cracking during shrinkage of the young and hardening material. Depending on water conditions, shotcrete is sprayed directly either onto the rock surface or over a drainage system, creating a fully restrained or an end-restrained structural system. In this paper, a method for nonlinear numerical simulations has been demonstrated, for the study of differences in stress build-up and cracking behaviour of restrained shotcrete slabs subjected to shrinkage. Special focus was given to the effects of the irregular shape and varying thickness of the shotcrete. The effects of glass fibre reinforcement and bond were implemented in the study by changing the fracture energy in bending and in the interaction between shotcrete and the substrate. The study verifies that an end-restrained shotcrete slab is prone to shrinkage induced cracking and shows the importance of a continuous bond to avoid wide shrinkage cracks when shotcrete is sprayed directly onto the rock.
Highlights
A common construction method of tunnels in hard and good quality rock is by drill and blast
A numerical analysis strategy based on the finite element (FE) method has been presented and demonstrated through examples and comparisons to be suitable for describing restrained shrinkage cracking in shotcrete slabs
The analytical model can describe the difference in structural behaviour between the two structural systems constituted by shotcrete fully bonded to rock and with end restraints only, the latter being represented by shotcrete covered soft drains
Summary
A common construction method of tunnels in hard and good quality rock is by drill and blast. The thickness of the applied shotcrete will mainly depend on rock surface geometry, the shotcreting process, and the skill of the operator. In tunnels and underground caverns, shotcreted linings usually have an outer harmonic shape but, due to the irregular shape of the rock surface, the shotcrete thickness will become highly irregular. This introduces local stress concentrations, that is, possible locations for crack initiation, as well as variations in temperature and shrinkage. Cracks might be initiated in local thin sections [1], while thicker sections lead to an excessive use of material and higher construction costs. Effects of irregular geometry are usually neglected in the design of shotcrete linings, but research regarding stresses in such tunnel linings shows the importance of considering these effects; see, for example, [2,3,4]
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