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Big CRL rig ends her construction journey

Published: 5 February 2018
The giant piling rig working inside the former Chief Post Office (Britomart Transport Centre).

The bright red 90 tonne piling rig named Sandrine that has been working around the historic Chief Post Office (Britomart Transport Centre) building has just finished her journey. That also means the distinctive silos outside the CPO have finished their work.

The rig has played a major part in preparing for the tunnel construction in the building.

With Sandrine's help, 64 diaphragm walls (or D-walls) have been built by the Downer Soletanche JV CRL construction workers. The wall panels form the structural support for the tunnels to be built in the building and also prevent water ingress.

A D-wall is a continuous wall constructed in the ground, typically to form an underground barrier or structure. They are constructed by an engineered fluid, typically a sodium bentonite mud, which is later replaced by permanent concrete.

Sodium bentonite is an absorbent clay that expands when wet. Bentonite slurry (a mix of bentonite powder and water) was pumped into the trenches being excavated for the diaphragm walls to ensure the sides don't cave in and ground water levels remain constant.

Now that they are completed, the walls will be part of the permanent structure being built by supporting the building and the CRL tunnel box.

Sandrine and the silos

The first public sign of this stage of the construction was last year with the emergence of large distinctive red silos outside the CPO in Lower Queen Street.

The silos were part of a specialised bentonite plant, shipped over from France by CRL contractor Downer Soletanche-Bachy Joint Venture, to help with the construction of those diaphragm walls.

Sandrine and the associated silos and plant will now be decommissioned and removed from the construction area.

The CPO is a heritage building and no damage was caused during the work even though the enormous machine had to manoeuvre around a small and difficult interior area.

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