It’s hard to miss these big 10m-high red silos that have gone up in Lower Queen Street outside the Britomart Transport Centre in the former Chief Post Office, or CPO. But what are they?
It turns out they will have an important role in preparing the building for City Rail Link (CRL) tunnel excavation.
Shipped in from France
The silos are part of a specialised bentonite plant, shipped over from France by CRL contractor Downer Soletanche-Bachy Joint Venture.
Bentonite will help with the construction of diaphragm walls, a special type of wall that will support the CPO building foundations and the CRL tunnel box.
The plant consists of six of these silos, which will remain on site until early 2018.
What’s bentonite?
Sodium bentonite is 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.
Why are we doing it this way?
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.
You can keep up to date with what's happening on the CRL at their website or Facebook page.
