Shotcrete for Stabilisation and Lining

 2. Lining using shotcrete

The final lining of a tunnel is the permanently visible visiting card of the tunneling contractor. The exception is a final lining with paneling. Inner lining concrete (shell concrete) and shotcrete are both used for a durable final lining. The higher the specifications for the evenness of the concrete finish, the more likely it is that a lining of structural concrete with interior ring forms will be used. Formed interior finishes are also considered to be aesthetically superior. Although new and additional installations are necessary on a large scale for this lining, the cost can be offset by the economics of the interior ring concrete, depending on the length of the project.

This work demands massive inner ring moulds and the machine technology for concrete delivery, compaction and moving the forms. Conventionally produced concrete requires considerable compaction work because inner lining concrete generally has a substantial wall thickness. Accessibility is usually difficult, which means that so-called form vibrators are used, although they have a limited depth effect and are therefore very labour-intensive and subject to wear, which also results in significant additional noise pollution. An important innovation may be the use of self-compacting concrete (SCC) which replaces the whole mechanical compaction process and has a free-flowing consistency which enables to fill these forms completely.

Without the maximum evenness specifications, shotcrete is also suitable for the final lining. Before installation of the waterproofing membrane, the shotcrete surface is often leveled as smoothly as possible with a finer gunite, which greatly improves the conditions for laying the waterproofing membranes without wrinkles.