City Rail Link visit – behind the covers and under the streets

On Friday, Patrick and I were treated to a tour of the work sites of the City Rail Link. Thanks to Scott, Dale and Glen for showing us around.

Britomart

For most of us, our experience of the works at Britomart the large blue walls at the top of the main stairs, the temporary station building, or the hoardings that surround Lower Queen St. But behind all those walls and hoardings an impressive feat of engineering is being undertaken to build the tunnels while preserving the historic Chief Post Office.

If you follow the City Rail Link website or social media pages, you’ll likely have seen various images of the works underway at the station. I’ve always found them a bit difficult to follow as it’s hard to get perspective of what you’re looking at in the confined spaces of the former Chief Post Office (CPO). Looking at the works in person gave me a much greater understanding of the scale and complexity of what’s going on.

We started our tour from the front of the CPO where workers were busy working on the piling that will support the tunnel between the station and the Commercial Bay site. From there we headed upstairs to the offices above the ground floor. Once the home to various companies, they’re now used by the project team to manage the works as well as provide facilities such as a lunch room for the workers. We then moved on to where the physical works were happening via some temporary stairs at the beck of the building. That gave us a good chance to see an overview of the works

Essentially, the contractors are having to rebuild parts of the station three times. It started with them having to cut out and lower the floor, without disturbing the existing foundations. This was needed so that there was enough room to get the piling rig inside the building. Moving large pieces of machinery around in such a tight space must have created more than a few nervous moments but crews were successful and piling was completed a few months ago.

The next step was to shift the existing pillars onto those new foundations. To do this they had to cut back to the original steel in the pillars and attach brackets. Powerful jacks lifted the pillars by just 1mm, just enough to take the pressure off the old foundations and slide some steel beams under them. Throughout the entire project they’ve installed hundreds of prisms around the building to measure movement to ensure the building isn’t moving in a way they didn’t intend. So far they say, things are looking good.

In the image below you can see the steel beams that have been installed. This is looking out towards Lower Queen St and the tunnel that connects to Platform 1 will essentially pass straight down the middle of these pillars. You can also see how much the floor has been lowered if you look to the right of the image. That thick concrete line and the door are where the old floor used to be.

This process has been repeated for all of the pillars within the station but the dealing with the facade is a bit different because it’s much heavier and more difficult. In that case, large, heavily engineered concrete beams are being built both inside and outside of the CPO. They will be joined together and clamp the facade in a vice like grip so it can be lifted and separated from its original foundations.

Below is some of the original steel framework that has been exposed along with reinforcing steel for one of the concrete beams being built to hold up the facade. You can also see at the back left of the shot the stairs that the image above was taken from. you can also see the care that’s being taken to protect the decorative elements on the ceiling.

This video shows the various processes that they are undertaking

Apart from building the tunnels, one of the big benefits of all of this work is when complete, the previously raised section of floor in the middle of the CPO will be removed, apparently not an easy feat. Removing that was essential to help improve pedestrian flow as the station gets even busier. The station gates will also be moved up to the ground floor level.

Commercial Bay

We didn’t go onto the Commercial Bay site but did get a close look at it from the side of Albert St. Workers were busy pouring the concrete roof on the section of the tunnels where they’ll join Albert St.

One interesting thing I learned about the development is that it doesn’t actually touch the tunnels. So while it looks like the walls in the image above are connected to the pillars, there’s actually an 80mm gap which is so that any vibration from the trains isn’t transferred into the structure above.

Albert St

Watching the trench under Albert St take shape from the surface has been fun to watch, but not as interesting as getting inside it and seeing it up close. One thing we discovered is that inside the trench is surprisingly quiet and peaceful. You don’t hear any of the noise or distraction from the street above the trench.

The whole process of building the tunnels here was described as being a bit like a moving factory. At one end, workers are still busy digging out the trench and stabilising the walls.

They then line the walls with waterproof membranes and build the proper floor slab.

A bit further down the trench, large formworks are used to build the side walls of the tunnel. The formwork is on wheels so slides down the tunnel. You can also see the completed walls.

At the very southern end of the trench, workers are now preparing to pour the first section of tunnel roof which is due to be poured in a few weeks. This process will snake its way north along Albert St until it reaches the intersection of Customs St where a different process will be used to connect to the tunnels under Commercial Bay.

While most of the tunnels under Albert St are being built inside of a trench, and then filled in later, because of the fear of hurting vehicle traffic, to get under Customs St they will actually mine under the intersection. Apparently a much trickier and more dangerous proposition. As such, it is expected to take five months to build that 30m section. A good example of trying to maintain traffic flow at all costs.

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