SOFT GROUND - BTS LECTURE | TECHNICAL
Crossing the shafts The plan for South Ventilation Shaft was changed from a rectangular shaft to two 60m-deep circular shafts, to be constructed with diameters of 33m and 26m, respectively, and 1.5m-thick D-Walls. Located next to each other, and slightly offset, they had various constraints on their construction, affected by when and how the TBMs could pass. For the larger shaft, there was enough time to
excavate and build its internal structures before the TBM (S-882) had to cross. From being parked in a plug nearby, the TBM would cross the shaft via a ‘concrete bell’ - basically an area filled with concrete and sand through which the TBM would drive. The ‘bell’ was not watertight and could not sustain the confinement pressure, and so the shaft was flooded, as done at the North Ventilation Shaft. Construction of the adjacent, smaller diameter shaft
could only be partly achieved before the other TBM (S- 881) needed to cross. There was no plug; the TBM would have to be parked within the shaft zone. This required extra strengthening measures for the shaft and also ground treatment, to prevent excessive deformation and loss of the hoop force due to two big openings in the shaft. After the TBM had passed, shaft construction
continued with break-in to its lining, in mid-2018. Beyond the tunnel shaft, the consolidation had been
difficult to achieve. The JV contractor placed added more band drains to those installed previously, by others. While settlement then restarted, checks revealed far less than the necessary OCR of 1.3 was achieved, at less than 1.0, and even 0.75. More ground treatment was needed before the TBMs could come through this last section. A grid of unreinforced barrettes were placed at
8m intervals, extending from the shafts to the Cut & Cover area. They would provide lateral restraint for the TBM drives and then ensure the tunnel lining did not squat too much in the permanent condition. Other types of ground treatment were used, too, such as: jet grouting near the Cut & Cover wall, to create a plug before break-out; and, beforehand, at the shafts, for the ‘berthing’ zones where the floating tunnels and shafts meet, as those are transition zones for settlement. Ground improvement techniques performed by VSL also included cutter soil mixing.
‘Caterpillar-shaped’ Cofferdam Plans for the Cut & Cover section first considered a straight D-Wall option. However, it would need nine layers of struts at 6m spacing, and also, perhaps more critically, a lot of ground treatment to strengthen below the excavation level. It was discounted for reasons of time and cost. As the ‘Caterpillar-shaped’ cofferdam solution had
proven to be quite successful at the North Launching Shaft, a solution was conceived to extend the technique as a much longer, 15-cell cofferdam. Rather than being a ‘caterpillar’, this reduced the first cofferdam on the project to be more of a ‘peanut’.
Y-panels
Temporary 'back arch' during Stage 1
(excavation of cells 1-3)
TBMs
The multi-cell cofferdam solution was almost
500m-long, 43m deep at maximum, and had 432No D-wall panels. Designers involved were Atkins, IHC, Meinhardt, and Golder. Plenty of design models with different software were used for cross-checks. Design challenges included the ‘longitudinal effect’,
with distortions should one cell be excavated but not the adjacent one. A major difficulty, though, was the Y-panel, construction which was particularly difficult due to the size and ground conditions. Talks with the operational team from VSL, doing the D-Walls, were held on the best layout. Various geometries were investigated, eventually resulting in a selection that could have an entire rebar cage of up to 130 tonnes for a Y-panel. Sometimes in-situ installation of the bars above the trench were required. For the TBMs to finish, though, only a few cofferdam
cells were needed. The solution was to add an extra ‘back arch’ to isolate the first three cells. The ‘back arch’ was later removed and the full cofferdam completed. The ‘Caterpillar’ was only a temporary structure. A permanent box structure would be built inside, casting of which would include chamfers to reduce the span of the Y-panel. Travelling formwork was used to complete the concrete casting in strut-free space. The TMCLK road link opened to traffic at the end of
2020. November 2025 | 17
Top:
TBMs breakthrough at Southern Landfall to complete drives on TMCLK
Above:
Drawing of the 3-cell short “Caterpillar-shaped’ cofferdam arrangement used to enable TBM breakthroughs, prior to construction of the majority of the cells
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