TECHNICAL | SOFT GROUND - BTS LECTURE


QUESTIONS AND ANSWERS The presentation on TMCLK was followed by a Q&A session. Questions and answers, and comment, have been edited and condensed for reasons of space.


Q: Why were there so many cross passage? How was it agreed? You had some lovely ventilation systems but to put cross passages in is always a problem. A: The 100m spacing of the cross passages was a contract requirement from the start. In Hong Kong, all the tunnels have such relatively shorter spacing for cross passages. It’s governed by the Fire Safety Department. The cross passages are only for evacuation to reach the other main tunnel. We looked at increasing the spacing a little but it was not possible.


Comment: Congratulations were given on the achievements with regard to hyperbaric interventions on the TMCLK project in Hong Kong. It was noted that the team worked to French regulations


and used mathematical modelling, monitoring, and there were many submissions that had to go through various stages of checking. It was further commented that this was seen as probably the most sophisticated hyperbaric work ever done in tunnelling - to run two crews from the one habitat, at different pressures, to two machines. And, that it was done extremely successfully. Many lessons have been learned from the project, it was


noted, and those are being published both by the BTS, in the guide to high pressure compressed air work, and by the European standards committee, in terms of the airlock standards.


Q: Could you discuss more about the cutterhead interventions and replacement of tools, and if there is a preference? A: It depends on the situation and the ground conditions. How many interventions you will have to do on a tunnel depends on the ground conditions. I would say that the accessible cutterhead that we have used on Trunk Road T2 is, at the moment, a very good solution - it avoids the need for hyperbaric intervention unless there are some unforeseen conditions. Actually, on T2 we made some progress on this aspect.


When we started with the Accessible Cutterhead the time required for changing the disc cutters was quite long, for two reasons: the team needed to get used to it; and, we needed to develop specific tools inside the cutterhead to lift the cylinders which are quite heavy. We have improved the equipment quite a lot and, by the end, the time for intervention was reduced a lot, and in a safe manner, without need for hyperbaric intervention. With regard to the robot, it needs developed more to


make it really efficient. We also need a big tunnel to have sufficient space for the robot, and to have an arm that possibly can change all the disc cutters - which is a bit of a challenge for the moment. Presently, only the external


Above: Long and full ‘Caterpillar-shaped’ cofferdam built on the Southern Landfall


cylinders can be changed, but our R&D department is working on it. Depending on future projects, the robot can be used again, but for the moment I can say that the Accessible Cutterhead may be the best solution.


Q: Your ‘Caterpillar’ looked larger at the end. Was it so? A: Width of the cells? It was purely due to the road alignment when we needed to get wider approaching the portal. This was part of our modelling. We wanted to be as symmetric as possible and avoid changes in geometry but this was required in order to fit the permanent structure. So, either we could have had the ‘Caterpillar’ wider all the way long, but then we would have some lost space at the deep end, or we could create the widening near the shallow end. The models proved that it was acceptable. The spacing between the cross-walls was also increased, progressively. This was possible as we were going shallower, the loads in the arches getting smaller, and so we could have longer arches and fewer cross-walls. So this was a bit of fine- tuning of the solution toward the end.


18 | November 2025


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