Donald Lamont of HSE asked Rob James whether, in the light of the quoted figures of 40% to 50% trigger times, he was aware that the new Physical Agents Directive coming into force soon in Europe would reduce the exposure level to 2.5m/s².
James said that machine design would have to keep up and hopefully the lowering of the limit would not have too much effect on the handwork periods.
Jack Knight, Haswell, commented that one of the stumbling blocks that had faced the BTS HAVS subcommittee was the intermittency of use. Here this problem did not seem to exist.
James confirmed that the HSE had in fact closely monitored all the operations involving handwork at Harrods and had been party to all the results.
Colin McKenzie asked Phil Ball about the face losses indicated after the compensation grouting had taken place. These were nothing like those experienced on the JLE.
Ball replied that there had been a great deal of co-ordination and co-operation between the grouting teams and the tunnelling teams to restrict volume loss to a minimum. The figures illustrated of 1.5% could be regarded as significantly less than the JLE where handwork or NATM was involved. Figures of 0.5% had been registered in Amsterdam and there were even cases where the ground had been pushed up with no effective ground loss.
David Wallis asked if there was an effect to be gained from normal annulus grouting to limit the settlements where compensation grouting was not available to be used.
Ball replied that this was indeed an effect that always helped the settlements. The quicker you filled the annulus with grout under pressure, the less were the effects of settlements.
Myles O’Reilly, in referring to McKenzie question said that the ratio of face loss in hand-mining to mechanised tunnelling was not surprising and it was clearly possible that significant reductions had been made in these works.
Paul Haines, Hochtief, asked if the exclusion zones were effective and what were the recorded movements in the lining?
Ball said that the exclusion zones were very effective but it could be awkward working round them all the time. The trick was to get the grout to flow into the exclusion zones from the grouting taking place outside. It was difficult to put an exact figure on the movement of the lining because of the different shapes and sizes but none exceeded the specification for the rings.
