It’s hard to miss these big 10m high red silos that have gone up in Lower Queen Street outside the Britomart Transport Centre (the former CPO) and you've been asking us why they are there as part of the CRL construction.
They will fulfil an important task in preparing the building for CRL tunnel excavation.
The silos are part of a specialised bentonite plant, shipped over from France by CRL contractor Downer Soletanche-Bachy Joint Venture, to help with the construction of diaphragm walls that will support the CPO building foundations and the CRL tunnel box.
And now there are six! Three more silos have arrived to complete the set-up of the plant, which will remain on site until early 2018.
Sodium bentonite is an absorbent clay that expands when wet. At the CPO building, bentonite slurry (a mix of bentonite powder and water) will be pumped into the trenches being excavated for the diaphragm walls to ensure the sides don’t cave in and ground water levels remain constant. The slurry keeps surrounding earth stable until steel reinforced concrete can be added in its place.
Once concrete starts to be added from the bottom of the newly-created diaphragm wall, the bentonite is progressively displaced and sucked out of the diaphragm wall by a specialised hydraulic pump and transferred back to the silos. The slurry can then be re-used again and again as required.
When working around sensitive heritage buildings, this method has less impact than traditional piling techniques as it minimises noise, vibration and impact on natural ground water levels.