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The Tuen Mun–Chek Lap Kok tunnel will link the New Territories, north of Hong Kong to Lantau Island, where the international airport is located. Dragages Hong Kong, a Bouygues Construction subsidiary was awarded the project, consisting of the construction of a submersed twin-tube tunnel, each 14m in diameter and carrying two lanes of traffic.
Could you give us a brief overview of the project and the stage that the project is currently at?
SR: The first phase of the project, a 16.5ha sea reclamation, was completed at the northern end of the project in late 2014. In 2015, 0.7km of twin TBM tunnels with a diameter of 14m and sitting 17.6m below the reclamation was completed.
All facilities for the sub-sea TBM tunnel drives have been set up on the reclaimed land. This includes construction of a peanut shaped TBM launch shaft, a 60m diameter 45m deep ventilation shaft, and approach tunnels under the reclamation. The TBM’s drive of 4km for the twin sub-sea tunnel section commenced in December 2015 and April 2016 respectively. The tunnel section in rock has been excavated with most of the remaining drives in soft ground. With the tunnel depth around 50m below sea level at axis, we use the saturation technique for changing the cutting tools of the TBM cutter heads in hyperbaric conditions. So far, ground conditions have generally required daily interventions. We started eight months ago and we’re now at our 15th team of hyperbaric workers in saturation cycle.
Construction of the cross passages commenced at the beginning of 2016. Seven have been completed out of a total of 56 with four completed by the TBM (a total of 44 will be completed using this method). With the majority of cross passages still to be created, there are a number of major technical risks ahead that need to be managed effectively to maintain safe excavation at 5 bar pressure and ensure water tightness.
The project has drawn global attention, given its status as one of the world’s mega-TBM projects – what are the unique conditions or features of working with a TBM of this scale?
SR: The ground conditions in the tunnel alignment are highly variable and the water pressure is high – up to 6 bars. The first half of the tunnel drive is predominantly in rock and mixed ground including reclamation fill, marine deposit, weathered rock and granite. The second half is in soft alluvium.
We’re using mix shield slurry TBMs due to the mixed ground conditions which require regular cutter head maintenance. The cutter heads used have a diameter of 14m with 98 disc cutters each. In completely decomposed granite (CDG) we change 5-10 disc cutters per day to keep the cutter head in good condition. The larger the diameter, the more maintenance is required. The cutter head on the 17.6m diameter TBM used on the northern tunnel last year had 125 disc cutters, requiring even more maintenance per lm of tunnel.
With large diameter TBMs, the ring build is also more difficult as the segments are so large it is hard for the operator to see all contact zones simultaneously during installation. Large TBMs also increase the need for additional care as hard rock torque can increase suddenly and cause severe damage.
What innovative technologies or approaches have been used during the design and construction phases? What advantages have they brought to the project?
SR: The project has already seen a number of innovations created for increased safety and productivity. Three innovations developed by the Bouygues Construction Research & Development Department to reduce manual operations in hyperbaric conditions include:
Mobydic – A sensor system built into the disc cutters allows on-going monitoring of the cutters’ wear and allows real-time geological mapping of the rock face without human intervention under pressure.
Snake – A remotely controlled exploration arm equipped with a high-pressure jet to clean the TBM head. Snake eliminates clogging and provides CCTV images increasing safety, productivity and efficiency.
Telemac – a remotely controlled robotic arm located behind the TBM cutter head that replaces disc cutters and eliminates the need for workers to enter the hazardous excavation chamber. In January 2016, the first fully automated disc cutter change took place. Telemac has since been used on both TBMs in the subsea tunnel with specialised disc cutters developed to further ease the difficulty in changing disc cutters.
Saturation diving has also been introduced on this project, minimising the exposure of frequent pressure change which can cause hyperbaric trauma especially when intervention pressures are over 3.5 bars. It is the first time saturation diving has been used on TBMs in an industrial manner for long periods. Specific diving procedures were imported from the oil and gas industry and adapted for the specifics of this project. Gas mixes, equipment, storage pressures and excursion steps have been designed to mitigate health risks and fit with the working pressures and programmes of the twin tunnel excavation.
Construction of the cross passages by pipe jacking slurry TBM method was developed to mitigate ground conditions and programme risks. The fact that there are 56 cross passages on the project, with 41 in the subsea section, made it imperative to select the most risk adverse technique.
Can you tell us about any unforeseen challenges that the team has overcome to ensure the project is moving forward as planned?
SR: The catalyst to use slurry TBM pipe jacking methods to construct the cross passages came from the borehole and freezing tests completed on samples from the cross passage positions. These revealed high clay content with much lower strength than expected. This meant an increase in the thickness of the ice ring required and thus an increase in risk of the ground freezing methods intended.
We had to find a mechanical solution to construct the cross passages that would be less sensitive to ground conditions and which would provide better certainty in terms of water tightness and programme.
Can you share an interesting fact about the project with us?
SR: In October 2014 we had to decide on a TBM design and have it manufactured within 12 months to be delivered to the reclamation site by December 2015.
The north approach ramp, which constitutes the first 700m of the tunnels, was to be constructed in one kilometre reclamation built at the beginning of the contract. The depth was 20-45m below reclamation level, so we had to look at using traditional cut and cover or bored tunnels using a TBM. Both solutions had pros and cons but we finally chose the bored tunnel option with approval from the project owner. This would require a larger diameter TBM of around 18m which would be the largest diameter ever used. The reclamation had not yet begun and we had no land, only sea water. It was a crazy challenge but it is instances like these that make tunnelling so exciting right?!