Fiber-Reinforced Sprayed Concrete as a Permanent Tunnel Lining

Until recently, when sprayed concrete was accelerated by silicates, whether non-reinforced or reinforced by welded mesh, it was logically considered as unsuitable in the long term and only used for temporary support behind a cast-in-place concrete lining. The introduction of polyvinyl chloride (PVC) sheets to waterproof tunnels perpetuated the use of poured concrete because spraying concrete on PVC requires many precautions. However, important changes have occurred over recent years: (a) alkali-free accelerators have replaced silicates, improving long-term performance, personnel safety and environmental protection; (b) the use of fibers has given sprayed concrete ductility and good flexural strength; and (c) an alternative waterproofing method has emerged: membranes are sprayed onto the sprayed concrete and protected by sprayed (or poured) concrete. Thanks to these developments and to improvements in equipment, quality control and design methods, it is now possible to consider sprayed concrete for "permanent" use. Rethinking the design of tunnels provides an opportunity to reduce the concrete thickness and the excavated section, thereby saving time and money. The space saved can also be kept available to facilitate further repair works. This paper highlights past and current practices, and provides several case studies in terms of loads, loads allowability, section optimization, time and cost savings. It provides inputs and perspectives for improving current practices.