COMPENSATION GROUTING is not the panacea for all tunnelling ills, members of the British Tunnelling Society decided last week. Nor is it 'Viagra for sagging erections'.
Even the proposers of the motion that 'Compensation grouting is the panaacea for all tunelling ills' were unable to give it unqualified support at the BTS's Great George Street debate. They maintained instead that the technique was better described as the best way of counteracting the subsidence caused by tunnelling operations. In the event, however, it was the original motion that was put to the packed audience - and overwhelmingly rejected.
Opening the light-hearted debate, Amec Grouting's Abed Haimoni said the use of compensation grouting 'meant more tunnels, tunnels in the right place, and reduced risks.' He added: 'You can't tell me you could tunnel under Big Ben or build the complex of new Jubilee Line tunnels under Waterloo Station without compensation grouting.'
It was Haimoni who introduced the Viagra image to the debate, saying that his first thought after agreeing to propose the motion was that it was an impossible task. 'But it would not be the first time a tunneller had asked a grouter to do the impossible,' he added with a smile.
Opposing the motion, Peter South of Amec Civil Engineering warned that engineers should always remember that for every action there is an equal and opposite reaction. 'Compensation grouting puts loads on the tunnel itself, causing deformation and higher face loads,' he said. 'And it puts up the costs significantly.'
The technique should be seen as a last resort, South argued. Instead, tunnels should be routed away from sensitive buildings - or minor settlement accepted.
Seconding Haimoni's arguments, Bachy Soletanche's Phil Hines said there were only two ways of dealing with significant building settlements: compensation grouting or underpinning and jacking. Compensation grouting was said to be much less disruptive, much more popular with infrastructure owners, and much cheaper.
Opposing the motion, Helen Nattrass from Haswell criticised the amount of 'unwarranted' monitoring associated with the technique, and the very pessimistic predictions of settlement used to justify its adoption. She warned that the costs involved could kill many tunnel projects, especially those outside London.
Others present, however, pointed out that the Crossrail project would be politically unacceptable without compensation grouting, and that tunnelling projects using the technique could now obtain insurance cover against settlement - for the first time ever.
One speaker from the floor even suggested the term 'compensation' grouting itself had unfortunate financial overtones, and that the American 'mudjacking' name might be better.
Summing up, Haimoni pointed to the success of the technique on the JLE project. Around 35,000m2 of compensation grouting had cut settlement from 'hundreds of millimetres to tens of millimetres', he said.
South disagreed saying 'The blanket use of compensation grouting would push costs up and lose work. He added: 'I'm not calling for the abandonment of this technique, just its proper use.'