Shotcrete for Stabilisation and Lining

 

Stabilisation using shotcrete

Shotcrete is the perfect material for excavation stabilization. Its unique flexibility in the choice of application thickness, material formulation (fiber), output capacity, very early strength development (dry and/or wet) and the ability to respray at any time makes shotcrete the complete material for excavation stabilization.

A distinction is made between full excavation and partial excavation according to the load-bearing properties and stability of the substrate. Excavation is by drill and blast or mechanical methods. In line with the old saying about tunneling: “It is dark in front of the pickaxe”, preliminary bores or narrow pilot tunnels often precede the main construction in difficult ground conditions. These exploration tunnels are then incorporated in the excavation of the future tunnel or used as parallel tunnels for many different purposes. In all these applications shotcrete is used for stabilization if the excavated face is not sufficiently stable. A thin base course in the form of a fine skin can be built up very quickly with sprayed concrete. If the load-bearing properties of the shotcrete are not sufficient, it is strengthened with reinforcement (fiber/steel reinforcement). By using steel rings and mesh, shotcrete becomes the lattice material between the beams.

By using bolts, the load-bearing properties of the shotcrete skin can be linked to the increased load-bearing properties of the substrate near the excavation. If there is high water penetration and/or heavy fracturing of the rock, injection and preliminary waterproofing with gunite and drainage channels will create the conditions for applying the shotcrete layer.

Like all construction methods, underground construction has evolved historically on a regional basis. What is different about building underground is the varying geological conditions in the different regions. Because of this and the variety of projects involved (in cross section and length), different methods have developed. In partial excavation, these are basically the new Austrian Tunneling Method (ATM), the German core method and the Belgian underpinning method. The full section is divided into smaller sections which are each temporarily stabilized and are only joined to form the full section at the end. In the full excavation application, partially and fully mechanized tunnel systems have a huge potential for development. In the longer term the constraints on use will be reduced solely to the economics of tunnel boring machines (TBM). Shotcrete application systems will be permanently installed on tunnel boring machines.

 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.