SOFT GROUND - BTS LECTURE | TECHNICAL
Left:
Tuen Mun – Chek Lap Kok Link Geology
also build structures for the Southern Landfall, on an artificial island - the Southern Reclamation - formed by others under a different contract.
TMCLK - PROJECT Geology The alignment of TMCLK is through adverse geology, typical of Hong Kong. Starting at the Northern Landfall, the alignment passes
through the reclamation field, then natural ground of soft clay marine deposits and alluvium, and then it quickly enters some hard layers of rock, including completely decomposed granite (CDG) - which was quite demanding on the TBMs. The subsea tunnel drives had two sections, in effect:
first, mostly in CDG and rock, which would require a lot of maintenance for the disc cutters, and quite slow progress rates; and, then, second, a switch to alluvium (mostly sand and clay) which allowed the TBM to go faster but created some difficulties for intervention to maintain the cutterheads. The Southern Landfall zone presented different
complications again as it comprises very soft ground - clay, alluvium and marine deposits.
Scheme Development Following project award, the JV contractor stepped back a little from the Reference Design and tender. A series of risk workshops were held to develop technical solutions that would improve construction. The Tender Design proposed that the subsea section
be built by 14m-diameter TBMs. At each end of the link, in the reclamation zones,
the plan was for Cut & Cover tunnels that would be quite deep and in difficult ground conditions. So, to improve the start of the job, at the Northern Landfall,
Southern Landfall South Ventilation Shafts
the thought was to maximise the portion to be bored by the TBMs - extending these sections as far back as possible to launch from a shallower location. The result? The proposed Cut & Cover section was replaced with: a shallower launching shaft; and, closer to the sea, a reconfigured ventilation shaft. A challenge persisted, though: one tunnel was to be
wider, for traffic regulations requiring an extra lane on the up-ramp for slow traffic. Three lanes were needed in the landfall area of one tube only and the proposed 14m-diameter TBM was not big enough to create the bigger tunnel required. The solution? To keep with the new plan for a longer TBM tunnels, it was decided to use an even larger diameter TBM, of 17.6m, for that short portion of the tunnel.
NORTHERN LANDFALL Construction One of the big difficulties for driving TBMs in freshly reclaimed land is to ensure that the ground conditions are good enough in two aspects: first, to allow effective tunnelling; and, second, to maintain ring stability over the long-term. To achieve those in the Northern Landfall area required a lot of ground treatment, including: 1) create the ‘break-in’ plug for TBM launches; 2) improve the reclamation field - planned as General Fill but changed to vibro-compacted sand to give better geotechnical characteristics;
3) insert extra diaphragm wall (D-Wall) panels between the adjacent TBM drives, so to avoid the impact of confinement pressure from the 2nd drive onto the 1st tunnel built; and,
4) most importantly in this ground, to ensure proper consolidation of the clay layers below the reclamation depth. Band drains were installed in a grid, and surcharge placed on top.
North Ventilation Shaft Launching Shaft North Left:
Layout of TMCLK project with key structures shown
Subsea Tunnel
November 2025
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