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Monitoring can help tunnellers if used 'with care'

TUNNELS ARE fully designed in Britain and tunnellers here want instrumentation to allow them to check that all is going as anticipated, and to provide data for future designs. Instruments should be appropriate for their purpose, effective, ideally remote, and able to disseminate data rapidly.

This was the message conveyed by Professor Chris Clayton of Southampton University while addressing a recent British Tunnelling Society seminar at the ICE. Subject was 'Lessons to be learned from recent sprayed concrete tunnelling in London Clay' and Clayton spoke on monitoring and instrumentation, with the emphasis on modern electronic devices.

Before describing use of these instruments and others in sprayed concrete tunnelling work at Heathrow's Terminal 4, he asked: 'Do pressure cells work and are they worth putting in?'

He described how pressure cells functioned and could be used to measure stress in a highly effective manner, but warned that many factors could affect the results obtained.

These ranged from the quality of the installation of the cells, to temperature change through heat of hydration or season variation to shrinkage of the concrete lining.

'There can be an awful lot of movement not attributable to load, ' he said. A 20degreesC change in temperature could equal a 3MPa change in recorded pressure. As concrete shrank around a cell, this would increase pressure too.

Effective employment of pressure cells was a matter of calibration, he added, 'of getting the numbers you see on site'.

Clayton went on to describe radial pressure cells and chain deflectometers and how these had been put to good use at T4.

Chain deflectometers can offer an effective way of observing ground movements around tunnels, he said.

Clayton argued the case for a range of instruments to be used on tunnelling projects and implied that the need to obtain data, process it and to communicate it to all parties rapidly and with minimum health and safety risk should best be met with sophisticated electronic sensors.

'You must certainly assess both need and performance before choosing your instruments, ' he concluded. 'The use of stress cells, for instance, does need improvement but it appears that good and effective data can be obtained, with care.'

Other speakers at the seminar talked of the Heathrow Express collapse and aspects of the Jubilee Line Extension and the Heathrow baggage tunnel.

However, at discussion time Sir Alan Muir Wood returned to the general topic of instrumentation.

Instrumentation should be kept as simple as possible, he said. 'The last thing you want are instruments that give inaccurate readings. You've got to be able to have confidence in your instruments.'

In particular, Muir Wod was 'very sceptical indeed' about the practical, quantative use of stress cells. 'What we want to know is, is it going to stand up or fall down, and to keep things simple.' Calibration could be little more than 'a fudge, to make them (the instruments) fit the data', he added.

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