TECHNICAL | SOFT GROUND - BTS LECTURE
reaching a total of 12.5m in height instead of 4m as had been planned; and, the surcharge was locally increased further, as required. Extensive settlement monitoring was undertaken
to check the consolidation, and also regular CPT campaigns. Prior to the TBM launches there was another campaign of CPT, at 10m spacing along each tube alignment. Consolidation went quite well in most areas though in spots where not there was jet grouting performed locally. The long and wide North Reclamation area provided
surface space for more than the launching and ventilation shaft structures. There were areas for storage of precast concrete segments and also a very large slurry treatment plant - the biggest for a tunnel project at that time. As the North Reclamation was beside the sea, the precast elements were delivered by barge.
TBMs Two slurry Mixshield TBMs were supplied by Herrenknecht. A 17.63m-diameter TBM (S-880) and 14m-diameter shield (S-882) were used for driving tunnels in the reclamation zone. The larger TBM was needed only for the relatively
short, wider tunnel. The shield was designed with nominal torque of 28 MNm and maximum power at the cutterhead of 5600kW. For the subsea section it would be changed over to another, reduced diameter shield (becoming S-881) that matched the other TBM. Together, the two 14m-diameter machines (S-
881, S-882) bored the twin tube subsea tunnel. Each was designed for pressures up to 7 bar, had nominal torque of 23 MNm, and maximum cutterhead power of 4900kW.
North Launching Shaft - ‘Caterpillar’ Structure The size of the TBMs, and to ensure quick and safe assembly, needed a large strut-free structure for the launching shaft. The plan was for straight D-walls, but propping was difficult for large-spans and an innovative solution was developed based on arched walls: a ‘Caterpillar-shaped’ cofferdam. The arrangement of cells was self-stable. This was the first use of the innovative ‘Caterpillar’
Top:
Aerial view of North Launching Shaft
Above:
Multiple activities in North Ventilation Shaft for TBMs to pass after initial drives
Typically, consolidation of the ground can take approx.
six months and the aim to achieve an Over Consolidation Ratio (OCR) of 1.2 (i.e., temporary load that is 20% more than from the reclamation zone load over the long- term). Driving a TBM inside such a reclamation zone, however, adds complications with ground stresses changing. This results in overload in the ground around the TBM on both sides. Consequently, a higher OCR margin, of 1.3, is sought. The consolidation plan was adjusted to achieve the
higher OCR. Extra band drains were added at reduced spacing (1.2m instead of 1.5m). Surcharge was increased,
12 | November 2025
concept that would be employed in another part of TMCLK, but more extensively, and also since on other projects in Hong Kong and elsewhere. The ‘Caterpillar-shaped’ shaft has D-Walls working
in compression, requiring less rebar and then providing large strut-free areas. The only strut arrangement is at the cross-walls, where some concrete struts are located and also the D-wall, below the excavation level. The difficult part of ‘Caterpillar-shaped’ cofferdam is designing and building the Y-panel, which connects the arches together and to the cross-wall.
North Launching Shaft - TBM Assembly The North Launching Shaft had dimensions of 85m x 44m x 22m (L x W x D). TBM assembly was prepared
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