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Getting wet

In the UK, dry process shotcrete has had its day. Such is the view of Miller's chief tunnelling engineer Colin Eddie as he explains the decision to adopt a wet process spraying system on the North Downs Tunnel.

'Strength of the concrete is safety critical. It is difficult to guarantee this with a dry system,' says Eddie. 'A wet process gives you better quality control and more health and safety and environmental control.'

Tunnelling in the past has alternated between the two methods. Work at Heathrow Express used predominantly dry process while contracts 102 at Westminster and 104 at London Bridge on the Jubilee Line Extension opted for wet.

Opinion has been divided over the relative merits of the two systems for many years.

The dry process, where cement aggregate accelerator and water are mixed at the nozzle, is said to be easier to handle on site. It does not carry the risk of setting on the way from pump to nozzle if spraying is held up, and the application of the dry mix material is claimed to be more controllable in the hands of a skilled operator.

Wet process spray concrete is pre-mixed away from the face with only the accelerator added at the nozzle. The dry system does produce up to 50% rebound off the face and hence more dust in the atmosphere and greater quantities of waste aggregate.

But crucially with so much being mixed at the nozzle, it is more difficult to guarantee a consistent concrete strength using the dry mix method, which is also difficult to test before it is insitu. Removing this level of uncertainty now makes the wet process overwhelming favourite.

Modern concrete pumping equipment technology now also reduces logistics problems by allowing concrete to be mixed off site and delivered in a controllable fashion to the tunnel face.

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