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Precast concrete segmental linings have been produced traditionally in both reinforced concrete and plain (unreinforced) concrete. The use of unreinforced concrete, and the beneficial cost savings in omitting the reinforcement, generally results in other compromises being made in the design. Commonly, this includes the need for more segments in a ring to limit flexural stresses for both temporary conditions (transportation and handling loads) and for permanent loads. Unreinforced concrete also has severe limitations where indirect tensile stresses may develop in a structure that is predominantly influenced by compression loads. This includes the performance of the lining at the longitudinal joints for transfer of the load between segments, and at the circumferential joints where tunnel boring machine (TBM) propulsion ram pressures act on the lining, both cases resulting in bursting stresses.
The use of steel-fibre-reinforced concrete in segmentally-lined tunnels has resulted in designs with fewer segments in a ring than for an unreinforced concrete solution. Steel fibre reinforcement provides a ductility and robustness in the material that is suited to the manufacturing process, the forces imparted during construction, and the long-term loading experienced by a tunnel lining, but which is lacking in unreinforced concrete. There are no internationally recognised design codes for tunnel linings, whether traditionally-reinforced or steel-fibre-reinforced.
Guidance on appropriate design methods may be found in Concrete Society Technical Report 63, Guidance for the design of steel-fibre-reinforced concrete.
Acknowledgement: The Concrete Society
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