Jack Knight (Haswell) referred to the speaker’s article in Tunnels & Tunnelling and asked if he had made any modifications to his ideas since then. He also asked about the practicality of obtaining horizontal borehole information. Barton replied that the ideas in the article were relatively unchanged but had been developed considerably over the last two years culminating in the publication of his recent book ³. Directional drilling or horizontal drilling from a rock/tunnel face could be carried out. He had seen good quality horizontal core that gave a more representative RQD value and a better estimate of joint frequency. He reiterated his observation that different rates of penetration observed with percussion drilling would reflect the different advance rates for a TBM.
Barry New (Geotechnical Consulting Group) referred to his experience on the Kielder tunnels where seismic velocity had been used to predict and evaluate tunnelling machine performance in relation to various measures of rock quality. They had found that shear wave velocity gave a better correlation than compressive wave velocity because at depth overburden loads tended to render rock joints "acoustically closed" to P-waves. Barton agreed that P-wave correlated well with weathering at the surface where stresses are minor. Shear wave velocity gave a better correlation deeper where there were higher stresses. New continued that at another site in north-west England he had observed a strong correlation between TBM advance rate and home Manchester United football games!
Jack Knight asked if the speaker had taken the use of foams and slurries into account when considering cutter wear in slurry TBMs in rock. Barton replied that he had not. He had been considering open-face rock TBMs.
Mike Francis (Brown & Root) asked about the use of measuring seismic velocity at the tunnel face. Barton replied that he knew of Japanese data from carrying out seismic refraction along a tunnel wall. This produced a good correlation with rock quality. However reflection seismic for looking ahead of the tunnel face only told half the story. It would detect a major reflector ahead, which could be interpreted as a major change in tunnelling conditions. Without a velocity measurement this did not say enough about the actual rock quality. Even cross-hole tomography was no good at 600-700m depth. New added that he had developed geophysical/tomographic rock quality methods for the nuclear waste disposal programme. He found that when using seismic techniques to evaluate rock support and excavation requirements it was vital to look at wave transmission velocity (or attenuation) through the rock mass as a whole. Sampling just a short path length at the tunnel face (or in a borehole) would usually only be indicative of intact rock properties.
Richard Moore (Brown & Root) asked about the development of borehole penetration rate for estimating TBM advance rate. Barton replied that this could be done, but would require a core for fine-tuning. The speed of drilling rounds had been used as a measure of rock quality on one project he had seen.
Roy Slocombe (Herrenknecht International) commented that the techniques described were useful for contractors at tender stage. Unfortunately many of the parameters discussed were not normally included in site investigations. Perhaps consultants should be encouraged to include these in future where TBMs are planned in rock tunnels.
Dr Eda de Quadros e Barton (Instituto de Pesquias Tecnológicas, Sao Paulo, Brazil) asked about the detection of weak and weathered zones by geophysical techniques. Barton replied that weathering profiles in faults near the surface would be picked up by seismic refraction, P-wave velocity. This technique would probably not show a deep weathering zone, particularly where this zone was "hidden" by an overhang of competent rock. In this case cross-hole seismic would probably give a better picture.
Jim Buchanan (retired) commented on the deterioration of output with time. He said that historical TBM advance rates had made the prospects of the Channel Tunnel look dim. Yet once started, the TBMs went well and towards the end, broke records for production. He proposed that management expertise was a big factor in TBM advance rates. Barton assumed that all tunnelling teams would aim to optimise their output. Longer tunnels’ production rates fell more because they had logistical problems to solve as well as all the other variables. He summed up by saying that rock quality varies over several degrees of magnitude, which is much more than the variation experienced in good or bad management. He believes that rock quality exerts a more profound influence on production rates than the management skills of contractors.
