1. 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.