SOFT GROUND - BTS LECTURE | TECHNICAL
with Herrenknecht and undertaken with supported by VSL, a sister company of Bouygues, using a big gantry crane over the shaft. The main drive of the biggest TBM was 480 tonne, the cutterhead 560 tonne. As assembly proceeded, the TBM sections were added and everything pushed forward, allowing installation of the backup gantries. The process took about two months. Given its success as an innovative solution, the
‘Caterpillar-shaped’ cofferdam was adopted for the launch shaft of the Trunk Road T2 project (see box panel). The shaft had similar constraints but was deeper, at 40m, and it was close to a large existing building - only 4m away during excavation works (in 2020-21). Settlement limits were stringent, at 20mm maximum, plus there vibration constraints were set. Trunk Road T2 project saw its TBM assembly
performed in late 2021, again with VSL’s gantry straddling the shaft.
North Approach Tunnels At TMCLK, the North Launching Shaft location there was little cover, only 4m above the larger of the two TBMs. The North Approach Tunnels are space 8m apart and run on a 5% gradient down to the North Ventilation Shaft, near the sea end of the Northern Landfall. Reaching the shaft, the cover is 35m.
North Ventilation Shaft - TBM Crossing The circular North Ventilation Shaft is 50m-diameter, a little more than 40m in depth and presented the TBMs with different hurdles and constraints. The bigger TBM was first to break into the large
shaft. Several options to do so were to use: 1) a big plug outside the shaft, but its size would be expensive; 2) a ‘steel bell’ inside the shaft; or, 3) flood the shaft - a simpler solution to balance pressure inside and outside of the shaft, then dewater and retrieve the machine. Option (3) was chosen. Preparations to changeover the TBM to a smaller
diameter shield were developed well in advance with Herrenknecht. Many pieces of the TBM would be reused in the smaller shield that would be assembled directly in front. While new sections were placed for the new TBM the assembly process also awaited the reuse parts
- such as the main drive - to be brought forward by the gantry crane from the larger shield. The backup gantries were adapted for reuse. The other TBM (S-882) had to cross the shaft space
with minimal disturbance to the activities underway. A ‘steel tunnel’ was built between the shaft walls, its diameter was only slightly larger than the TBM. It was filled with sand and concrete. The TBM bored through the mix and transited the shaft, effectively without knowing it had done so. The transit took one week, including a cutterhead intervention, compared to the normal operation of shaft breakout, transit and then break-in that can require about two months.
SUBSEA TUNNEL - TBM INTERVENTION STRATEGIES
Cutterhead Monitoring The intervention strategies for the TBMs in the subsea section were among the key drivers of success in the tunnelling operations. The challenge arose from the geology and
consequent adverse ground conditions for tunnel boring, with a lot of rock and CDG in the first part of the drive. The tunnel alignment was more than 50m below the sea level, adding the need for daily hyperbaric interventions to change the disc cutters. With the TBMs, the slurry was in front of the bulkhead
for the excavation, and behind was atmospheric pressure. For maintenance intervention, there is a pressure balancing operation to enable access behind the cutterhead. Compressed air is introduced at slightly greater than hyperbaric pressure for the relevant location, and slurry is removed. For TMCLK, the pressure was 5 bar - 6 bar, depending on depth. Qualified and trained divers entered the compressed
air environment through airlocks and worked to change the disc cutters. Knowing which disc cutters were damaged was key. As the disc cutters had sensors, condition monitoring was performed using Bouygues’ Mobydic system. It also helped to have geological face mapping and data on pressure applied to the disc cutters, enabling expected versus actual performances to be compared, TBM driving parameters adapted, and intervention work planned for the cutterhead.
TRUNK ROAD T2 The Trunk Road T2 project, in Hong Kong, is also a subsea tunnel that has been constructed
with two TBMs, each 14m-diameter slurry machines. The road tunnel is 3.5km in length, in total, and each of its twin tubes has two traffic lanes. Different key sections of the project are:
● Subsea tunnel - 2.5km-long, bored by TBMs; ● Cha Kwo Ling Tunnel - 400m-long, excavated by Drill & Blast/Break technique; ● Cut & Cover Tunnels and Approach Ramp - 500m (incl 420m of existing structure); ● Cross Passages - 33No, two-thirds of the total constructed by mini-TBM pipejacking.
The project also has two ventilation buildings and plenty of infrastructure and M&E works.
November 2025
| 13
Page 1 |
Page 2 |
Page 3 |
Page 4 |
Page 5 |
Page 6 |
Page 7 |
Page 8 |
Page 9 |
Page 10 |
Page 11 |
Page 12 |
Page 13 |
Page 14 |
Page 15 |
Page 16 |
Page 17 |
Page 18 |
Page 19 |
Page 20 |
Page 21 |
Page 22 |
Page 23 |
Page 24 |
Page 25 |
Page 26 |
Page 27 |
Page 28 |
Page 29 |
Page 30 |
Page 31 |
Page 32 |
Page 33 |
Page 34 |
Page 35 |
Page 36 |
Page 37 |
Page 38 |
Page 39 |
Page 40 |
Page 41 |
Page 42 |
Page 43 |
Page 44 |
Page 45
