SPRAYED CONCRETE LININGS

Sprayed concrete, or shotcrete as it is commonly referred to, is not new. It was invented in the 1920’s for preserving dinosaur skeletons and was used underground initially in coalmines for the preservation and fire proofing of timber supports. Its use as a support lining in rock tunnelling was developed largely in parallel in Austria and Scandinavia in the 1950s and ‘60s with notable work also being carried out in Australia on the Snowy Mountains Scheme and later in the UK’s experimental Keilder Tunnel.

Its association with the New Austrian Tunnelling Method (NATM) dates from this time, when the name was coined to distinguish this new method of active tunnel support using sprayed concrete instead of the old passive support method of conventional ribs and lagging for the major Alpine crossing tunnels then being constructed.

The adoption of the NATM system in soft ground tunnelling occurred in the 1970s for the development of the Frankfurt and other metro systems in Germany. At that time the dry mix process (water added to the mix at the nozzle) was in vogue, although some wet mix (water added at the mixer) applications were taking place, particularly in metal mining. Both methods produce similar results that are ideally suited to tunnelling in one crucial aspect – it is the only method of tunnel lining support that forms an immediate and intimate bond with the strata that interacts actively with the excavated ground surface instead of passively requiring the ground to move before it begins to resist that movement. In addition, the lining can easily follow complex profiles, in-fill over-break, and forms a strong, durable and reasonably watertight shell. Careful and skilled mix design and application of the sprayed concrete can produce high quality finishes comparable to shuttered concrete.

Nowadays there is a strong move towards the wet mix process that delivers high, consistent production and is suited to robotic systems. However, the dry mix system is still popular for low volume work and does not have the problems associated with transporting and storing a wet concrete paste before application. The current innovations are in the use of robotics, non-caustic accelerators and special additives, and the incorporation of steel and plastic fibres in the mix. Consequently, the finished sprayed concrete lining product is equivalent in strength, impermeability and durability to conventional cast-in-place concrete, and is often better in terms of compaction.

Most, if not all, the old problems or myths regarding sprayed concrete; ‘it’s poor concrete’; ‘it won’t stick to the surface because its wet/clayey/chalky/smooth etc’; ‘it’s a health risk’ and so on have happily been laid to rest and the method looks to an increasingly innovative future as it is finding application as a permanent works lining support.

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