Tunnel lining without reinforcement

The discussion at hand is restricted to circular tunnels with high overburden, however the basic results can be applied to other tunnel sections to a certain degree. Amongst tunnelling experts deviating positions are taken in the question whether the concrete lining of these tunnels must be reinforced or not. The tunnel static considerations are based on different models and consequently lead to diverging results. Extreme positions assess at one hand that reinforcement is required to cover moments in the lining and at the other that reinforcement has negative rather than positive effects. As long as the lining is seen as a structure loaded by the rockmass assumptions on loadings, moments and tensile stresses are projected to the lining and must be covered by reinforcement. Whereas when the lining is considered to be part of a composite system together with the rockmass between the two elements, the model of moments is not relevant and no reinforcement is required. The question whether concrete linings must be reinforced or not has an increasing meaning with mechanical excavation as there reinforcement can be implemented only with considerable difficulties.