A lot is happening inside the Britomart Transport Centre (the old CPO) building where you used to enter to catch the train - and you wouldn't recognise it!
Sandrine – the 90-tonne piling rig constructing new walls to support Britomart’s Chief Post Office (CPO) building and the City Rail Link tunnel box – has now completed her work on Lower Queen Street and is moving inside the CPO.
Over the past few months, contractor Downer Soletanche-Bachy Joint Venture has been carrying out the incredibly sensitive and technical task of demolishing the ground floor of the CPO building to allow the new 15 to 18-metre-deep diaphragm walls (or D-walls) to be constructed.
The interior sections were demolished all the way down to basement level to allow for construction of approximately 164 metres of guide walls that enable accurate construction of the D-walls. The one to two-metre-deep guide walls are an essential part of the D-wall construction, as they ‘guide’ Sandrine as she excavates.
In order for the D-wall construction works to occur inside the CPO, a strong temporary operating platform had to be built for Sandrine and the floor rebuilt to the height of the new guide walls. This was done using a lightweight fill that comprised just over 400 large polyfill blocks stacked three high with a concrete slab over the top. The remaining sections of CPO floor were brought up to the same height.
Once the piling rig Sandrine has completed her D-wall work inside the CPO in early 2018, the weight of the building will then be transferred off its existing foundations and onto the new D-walls. The CRL’s open cut and cover tunnel excavation can then begin.
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.
What is going on inside the CPO?
This Downer JV animation shows the work plan:
Where will the tunnels go once it is extended at Britomart?
The Britomart Station building was Auckland’s Chief Post Office (CPO) before being re-purposed as a train station, and is a Category 1 listed heritage building.
Construction teams are working meticulously, in association with Heritage NZ, to make sure the heritage aspects of the interior are not impacted by the work needed to modify the building from a dead-end to a through station. The original features of the building, such as the pillars, ceiling and skylights were protectively wrapped, and the entire building monitored by the cyclops system to ensure no excessive movement occurs as the building is underpinned and the tunnels built.