Rock bolting criticised

The use of rock bolts has been criticised in a new report into the Bilsthorpe Colliery mining disaster which killed three men last year.

But the criticism is tempered by assertions that other support measures would not have prevented the collapse of the mine access tunnel and that rock bolting is no more dangerous than other techniques.

Doubts over the use of the technique have concerned contractors involved in the Jubilee Line and Heathrow Express which are using rock bolts in the New Austrian Tunnelling Method.

Rock bolting entails installing massive screws or bolts into the roof of a tunnel and stressing them up so the rock is compacted and then held together. A coating of sprayed concrete completes the support technique.

Sir Bernard Crossland's report to the Health and Safety Commission of his Public Hearing said that rock bolting in an abandoned working next to the tunnel 'might have contributed significantly to the accident.'

He is recommending that a national research programme into rock bolting technology should be agreed between the industry and those universities with a rock mechanics capability.

The increasingly popular New Austrian Tunnelling Method uses rock bolts and consequently this approach has become an important tunnelling technique in the UK construction industry.

But Dave Hindle, specialist tunnelling consultant with the Dr Sauer Company, claims that the system is fine - but it is the way this technique is used that matters.

He said: 'Rock bolting is a lot safer than other methods because you don't rely on artificial structures. You make the ground support itself.

'The problem with the Coal Board is that it has forgotten why it is using rock bolts: the ground has to transfer its load to somewhere. Rock bolts must be designed in.'

Sir Bernard's report held that 'conventional passive supports would not have prevented the roof collapse or significantly slowed it down.'

Sir Bernard pointed out that in the mining method known as skin-to-skin working (mining next to an abandoned area) which was employed at Bilsthorpe, Nottinghamshire, support of the end of the cantilever depended on the confining stress generated by compaction of waste in the neighbouring abandoned workings.

In this particular case, Sir Bernard said: 'The typically high proportion of sandstone in the roof may have generated a blocky waste which did not fragment and compact adequately to generate a sufficiently high confining stress.'

Accident standards quoted by Sir Bernard indicate a 'significant improvement in safety with the introduction of rock bolting'.

The Department of Transport favours rock bolting and reinforced shotcrete as a method of tunnelling.

The first such DoT tunnel, on the A55 at Penmaenbach, west of Conwy, North Wales, was soon followed by the Saltash Tunnel, Cornwall.