City Rail Link with Andrew Swan and Chris Bird

This will be a shorter post, as I am busy with study at the moment! Today we visited the City Rail Link works, looking at the cut-and-cover tunnel operation on Albert St and the ongoing work inside the Britomart Central Post Office. Our guides were Andrew Swan and Chris Bird from the City Rail Link team, who told us a little about the heritage aspects of the CRL project. We focused on the building monitoring on Albert St and the engineering work which has been done to support the CPO building while the rail tunnels are extended underneath it.

We started with a briefing from Andrew Swan, but I’m going to assume that you’re more or less familiar with the City Rail Link project, which extends the rail network up from the bottom of Queen St to Karangahape Road and then to Mt Eden. (A number of the site visits we’ve done in the past have explicitly addressed the CRL.) If you don’t know much about it, there’s a ton of info at the CRL site, and even for the well-informed, I’d highly recommend a skim through the construction blog which has loads of pictures and is updated frequently.

One facet of the heritage side that had escaped my attention until now is that the Central Post Office building is protected not just by its Category 1 status by by a Deed of Heritage Covenant, which makes it an offence to modify the building without consent. All the work that is happening at the CPO building—and there’s a lot—has been negotiated with Heritage NZ and explicitly permitted.

City Rail Link, Albert St. A monitoring total station swings around, ranging the prisms in its zone.

Hold it right there

Briefed, we headed out to site. My group began on Albert St, where a network of monitoring total stations is taking readings from a myriad of prisms every fifteen minutes.  The prisms, or reflectors, are sited along the route of the tunnel, on pavements, building facades, walls, and so on. As the work goes on, each prism is allowed to move a certain amount. If it moves any more than its pre-assigned tolerance, an alert is sounded, and the CRL team decide what needs to be done—which in extreme circumstances might include stopping the works.

City Rail Link, Albert St. A reflecting prism in place outside Link House, under the watchful eye of the total station.

I asked how the allowable deflections were set, and Chris Bird explained that this was done by consultants through geotechnical modelling and an assessment of the probable effects of the excavation on the surrounding buildings. It’s a case-by-case process, which depends upon the soil conditions at each site, building foundation type, the building’s structure, and so on. There are a range of sensitive buildings along the route, including (next to the CPO building) the Endeans Building, which still is founded on its original kauri piles.

City Rail Link, Albert St. Looking up at Link House, directly above the prism in the previous picture.

Movement monitoring continues inside the buildings themselves, which were surveyed before the excavation began. If significant pre-existing cracks were found, these were fitted with a gauge, allowing a determination of whether the works are causing any further damage. So far, deflections all along the tunnel path are well within the permitted limits.

City Rail Link, Britomart/Central Post Office. A view along one line of underpinning beams transferring load from columns. Image copyright the author. This image may not be reproduced or shared without permission from the City Rail Link.

How to lift a building by one millimetre

And so to the Central Post Office building. As you know, the major work here is to extend the train tunnels to allow trains to run both ways through Britomart Station. The problem is, the tunnels go right under a good many of the columns that hold up the building. To allow the tunnels to be dug, the loads that are coming down the columns need to be transferred out to either side of the hole, and from there down into something nice and solid.

When we visited the project last, in October 2017, the team were working on creating diaphragm walls, using a special drilling rig affectionately known as Sandrine. These walls are sturdy concrete structures, extending along the edges of the Central Post Office building, and inside the building along the edges of where the tunnels will be dug. With the diaphragm walls in place, the CRL team have put in large steel members to serve as underpinning beams. These underpinning beams span across the tops of the diaphragm walls.

The underpinning beams are there to take the weight of the columns of the CPO building. The load has to be carefully transferred from what’s supporting the columns now (the existing foundations) to the underpinning beams. This process is carried out as follows.

City Rail Link, Britomart/Central Post Office. Detail of the load transfer system. The upper red steel box is the collar. Beneath the collar are the four flat-jacks. Short members span the paired underpinning beams. Image copyright the author. This image may not be reproduced or shared without permission from the City Rail Link.

First, the concrete is chipped off from the columns, exposing the original steelwork. A collar is then clamped around the column, and the collar sits across a pair of underpinning beams. The beams aren’t taking any load yet. To transfer the load, four tiny flat-jacks are placed beneath the collar. Using hydraulic fluid pumped to 290 bar inside copper coils, the jacks lift the collars—and the columns—ever so slightly. Half a millimeter—at most a whole millimetre—but no more. They lift until they reach a given displacement or a given force, equivalent to the calculated load in the column. With the jacks in place,  lifted and shimmed, the load from the column is now being taken by the underpinning beams, and from there across and down into the diaphragm walls and into the bedrock. Then the column base, which is no longer bearing the load, can be cut away. (In this animation of the process, you can see the diaphragm walls in light grey and the underpinning beams and collars in red.)

City Rail Link, Britomart/Central Post Office. The CPO building was seismically strengthened in 2002. A recent review of the 2002 work has found that it is still adequate in a post-Christchurch era. In the blue box, one of the shear walls (?) inserted into the structure in the 2002 retrofit. Image copyright the author. This image may not be reproduced or shared without permission from the City Rail Link.

The other major part of the structures that needs support while the tunnels are being dug are the walls, in particular the East and West walls. These are the walls which lie above the tunnels.  The West wall is the Queen Street side, the grand facade of the building. To allow it to span the tunnels without cracking, the team have created two immense post-tensioned concrete beams, one inside and one outside the wall. The beams are tied together with cross members, which run underneath the wall itself. The beams have been cast and tensioned in situ, and Andrew recounted that they were complex to construct and design. On the day that we visited, some work was happening to set up the steel reinforcement for one end of the beams which will do the same work on the Eastern wall.

Parish news

A final note. Andrew mentioned the recent news stories about decision-makers looking to expand the capacity of the new stations on the CRL line, Aotea and Karangahape. I asked about the impact of this on the Pitt St Methodist Church, the Mercury Theatre, and maybe on our friends at Hopetoun Alpha. Andrew’s take was that the plans for bigger stations, if adopted, would be good for the Pitt St Church, since there would be no need for the large ventilation structure which is proposed to be placed just outside the Church. Instead, the second entrance in Beresford Square would provide ventilation for the station. Interesting to see how this all develops.

Thanks!

Warm and very sincere thanks to Andrew Swan, Chris Bird, Sonya Leahy, and Berenize Peita for their time and their willingness to share knowledge and answer questions. With special thanks also to Clare Farrant for organising the tour, and to the indefatigable Jenny Chu.

Britomart CPO and the City Rail Link with Jenny Chu, October 2017

Regular readers of this irregular journal may remember that sometimes your correspondent organises tours but can’t attend ’em. Unfortunately, I missed the Britomart visit in favour of a crook two-year-old—although luckily she was well enough to come down to town with me and hand over the hard hats to the site visitors.

Thanks to the great kindness of Matt Goodall, I have a few pictures from the tour to share with you all. My apologies for the delay in getting these on the web.

This brief post will (hopefully) serve as something of a placeholder. It has been suggested that we might be in with a shot of going back in 2018—which will no doubt be welcome news to the sixty-odd people who signed up but didn’t get a spot.

Those of you who are interested in following the project may also enjoy seeing an animation of the proposed work and the well-illustrated work-in-progress blog.

Many thanks to Jenny Chu and John Fellows of the City Rail Link for their kind hosting and for giving generously of their time.

CPO, general view of the interior. Sandrine le Drilling Machine lurking in the background. Note wrapped columns and protected ceiling. Image courtesy of Matt Goodall, all rights reserved.
Detail of column base. Image courtesy of Matt Goodall, all rights reserved.
Drilling operations. Image courtesy of Matt Goodall, all rights reserved.
Backfilling trench. Image courtesy of Matt Goodall, all rights reserved.