Recovery of the flooded caisson involved the installation of two rows of jet grouted piles and a lower plug to stabilise the affected area, followed by ground freezing undertaken by Deilmann Haniel. Groundwater movement here is slow, so it took six months to freeze a 5m wide x 60m long strip of ground in front of the caisson using calcium chloride at -30°C.
However, while ground freezing provided a temporary solution, it was deemed too expensive to maintain over the projected two years of tunnelling. A unique solution to allow tunnelling to begin was therefore developed with Herrenknecht.
With the ground frozen, a reception eye is formed in the 1.5m thick wall using no-vibration high pressure water jet cutters at 5-800 bar. This takes about eight weeks. With the frozen, self supporting face exposed, a steel ring is installed which supports a 3mm thick flexible neoprene membrane reinforced with fibres, made by Contitec of Hannover, a division of Continental Tyres.
This is filled with bentonite and reinforced with steel bands to form a ‘blister’ over the opening, which takes over the job of supporting the face as ground freezing is de-activated.
When the TBM is to be launched, additional steel rings are installed to accommodate the shield, the bentonite is then pumped out and the geotextile seal removed as the machine moves forward (see diagrams). Thus at no time is the face unsupported or is there any danger of flooding to the caisson.
Related Files
Figure 2
Figure 3
Figure 5
Figure 1
Figure 4
