CRL Connection
22 August 2017
Sandrine starts D-wall work on Lower Queen Street This week, Sandrine, the new 90-tonne piling rig, started work on the City Rail Link’s 18-metre-deep diaphragm walls in Lower Queen Street. Also known as D-walls, they will form the structural support for Britomart Station’s Chief Post Office building and the CRL tunnel box. They will also provide ground support to the tunnel excavation and prevent water ingress. The construction process involves Sandrine’s trenchcutter, with its two counter-rotating discs, being lowered through reinforced concrete guide walls to begin excavation. The resulting spoil (excavated soil) is sucked out and replaced with bentonite slurry, which supports the walls of the excavation so they don’t cave in. Once the full depth is reached, the trench cutter is removed and four 4.5 metre-long steel reinforcing cages are coupled together and lowered into the excavation through the slurry. Concrete is then pumped into the bottom of the excavation and slowly displaces the slurry, which is sucked out and returned to the large red holding tanks for filtration and re-use. D-wall construction is expected to take about six months. D-wall construction underway. Sandrine’s two counter-rotating discs allow her to excavate downwards.
Click here for more information and videos on Sandrine and the CRL’s D-wall construction methods.
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