Another CRL milestone.
Sandrine (the nickname of a bright red 90-tonne compact hydrofraise piling rig) has managed to be moved inside the Britomart building (the former CPO) in Downtown Auckland to enable another major part of the CRL's construction.
The machine is needed to dig the 15 to 20-metre-deep wall panels that will form the structural support for the CRL tunnels inside the building.
Sandrine has already just completed construction of six walls behind the hoardings in Lower Queen Street and now will construct the remaining walls inside. This should be completed early next year.
Once Sandrine has completed her work inside the underpinning works for the building - transferring the weight of the CPO off the existing building foundations and onto the new walls - the open cut and cover tunnel excavation can begin.
Sandrine is being used by construction partners Downer NZ and Soletanche Bachy JV (DSBJV).
What is Sandrine?
Sandrine is the nickname of the bright red machine in the photo above. It's a 90-tonne compact hydrofraise piling rig. Sandrine is helping the construction team prepare for the tunnel excavation so that Britomart becomes a through station instead of all the trains having to make their final stop at Britomart.
What does hydrofraise mean?
It's a drilling machine, a trench-cutter. See the video here.
What is the machine doing?
It's digging the 15 to 20-metre-deep diaphragm wall (or D-wall) panels that will form the structural support for the CRL tunnels at the CPO / Britomart building.
What's a D-wall?
A D-wall is a continuous wall constructed in the ground, typically to form an underground barrier or structure.
The purpose of the diaphragm walls (D-walls) is to allow the rail tunnels to be constructed by supporting the existing foundations of the Chief Post Office (the underpinning structure will be supported from the D-walls), to provide ground retention during excavation for the rail tunnels and to prevent groundwater ingress into the excavations.
How are they constructed?
They are constructed through a narrow excavated trench and supported by an engineered fluid (typically a sodium bentonite mud) which is later replaced by permanent concrete.
The trench-cutter’s two counter-rotating cutting discs turn and then excavate downwards through the ground.
The resultant spoil (excavated soil) is sucked out of the excavation through a large pipe connected to the trench-cutter and is replaced with bentonite slurry which is pumped into the excavation.
The purpose of the bentonite slurry is to support the walls of the excavation, so that the walls do not cave in due to the weight of the soil and pressure from groundwater.
What is bentonite?
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.
Where will the tunnels go once it is extended at Britomart?