An onsite fabrication facility was required to complete complex diaphragm wall works at High Speed 2’s North Acton site in London.
High Speed 2 (HS2) main works contractor Skanska Costain Strabag (SCS) Joint Venture is building the Victoria Road crossover box (VRCB) in North Acton, west London. It is west of the future HS2 superhub at Old Oak Common station. Design House, comprising Arup, Typsa and Strabag, is the design engineer for this part of the project.
The crossover box will be used as the launch pit for the two Northolt tunnel boring machines (TBMs). The machines will set off from the Victoria Road site towards Green Park Way in Ealing and dig for 5.5km to create the eastern section of HS2’s 13.5km Northolt Tunnel.
The TBMs are due to arrive in early 2023 and will begin the 12 month tunnelling programme later that year.
The site is also connected to the logistics hub at the Willesden Euroterminal by a conveyor system that will remove spoil excavated from the tunnels.
When construction is complete, the VRCB structure will have a permanent function, housing a crossover which will allow trains to switch between tracks when going in and out of Old Oak Common station.
It will also have a ventilation duct and will allow the heat to escape from braking for trains as they slow down on the approach to Old Oak Common.
The crossover box will be 130m long and 24m deep, and will be housed within a 1.5m thick diaphragm wall (D-wall). The base slab will be supported by 77 piles, installed at a 20m depth.
SCS contracted Züblin’s UK branch to construct the perimeter D-wall.
The final element of enabling works for the crossover box site was completed in early 2021. SCS oversaw the installation of 200m of sheet piling to prepare for the construction of the box.
Permanent works then kicked off in February 2021, and work on the D-wall began in spring of last year.
The arced D-wall will retain a cellular caterpillar-shaped shaft that will make up the VRCB. The wall panels are being constructed using a Bauer MC-96 hydraulic grab and a Liebherr HS 8130 rope grab. As the ground is made up of London Clay, it is relatively easy to dig.
Due to the shape of the caterpillar shaft, the D-wall incorporates buttress panels at the interface between the cells. These will take on the tension from the arced wall panels and have a more complex design, making the construction of the steel reinforcement cages more difficult.
There are eight buttress panels, each 36m deep, 1.5m wide and 4.5m long and material is excavated in one and a half “bites” of the grabs. The buttress panel design features wing walls incorporated into the cages, forming a cruciform shape, due to their position at the interface. While the panels are large, excavating them is not the main engineering issue – it is the shape of the reinforcement.
Züblin senior construction manager Darren Kavanagh says: “[The box] is only the second or third one done in this shape, and as far as I know, with five cells, it’s the longest one that’s been carried out in the UK, with some of the widest [buttress] panels at 1.5m.”
This width is needed to allow for the circular shape of the shaft cells to be excavated without propping. This is because the tension on a circular wall is transferred around it – a mechanic called hoop stress – putting all the panels into compression.
Fabrication and lift
Due to the complexity of the buttress panels, their reinforcement cages were fabricated on site, across the road from where the box is being built.
A full length cage could be transported from the fabrication yard on a self-propelled modular transporter (SPMT) and across Victoria Road onto the main site.
The cage was then lifted using a 650t crane at one end and a 600t crane at the other. Once the cage was vertical, the temporary works support – a steel truss to give the cage stiffness as it is lifted – was removed. The cage was then lowered into the hole and concreted with approximately 550m³ per buttress. SCS also wants to reduce the carbon footprint of the works, so 50% of ground granulated blast furnace slag has been added to the concrete mix, replacing cement.
The decision to move and install the cages in one piece was made when it became apparent that on site splicing – the standard way of joining cages – would be too complicated, and there were safety concerns about connecting the cages.
Züblin lifting and logistics manager Stephen Williams says: “Normally we would join cages together in sections vertically over the hole, and it became apparent we couldn’t do that, for one, because the quantity of steel needed was too great to allow the concrete to flow freely and comply with the specification.”
The splicing is done by Ancon couplers in the cage, which gives very little room for displacement.
Williams continues: “Once we were aware of the engineering issue with the spacing of the concrete – it would have been outside of the specification – we couldn’t guarantee the integrity of the panel. So, once we got to that point, we had to build the cage away from the site, because it had to go in in one piece [fully welded]. So, it was a question of finding where to do it.
“So just by chance, one of the early contractors, CSJV, Costain Skanksa JV, was just about to finish, and one of their yards was located across the road.”
Kavanagh adds: “[The fabrication set up] was never expected. We knew the buttress was going to be a bespoke cage. But until the design was confirmed, we didn’t realise fully what we were up against. We knew it would be circa 70t to 80t, which was manageable. But because of this design and the width of the panels, it just became a monster.”
The heaviest panel includes around 80t of rebar, plus temporary works and a Megashor truss that adds 30t on top of the panel, giving an overall weight of 110t. The Megashor is attached to the top of the cage to stop deflection during lifting from the SPMT transport to the vertical for placement in the hole.
Kavanagh adds: “The problem with the cages is that practically each cage is bespoke because of the superelevation on the [base] slab. As they are the same cage types, same references, you’d imagine they’d be identical, but they’re not because of the couplers. Every cage is designed separately.”
The weight and size of the rebar also dictated the way the panels had to be installed.
“They’re 50mm bars, and we cannot bend those bars. Let’s say there was a problem with transport, something bent, the cage would never go together.
“So, we were really worried about work done at height and the weights above the workers’ heads. If the splice failed, how would we rejoin and couple them? What was the consequence of them not coming together?” Kavanagh asks.
“It was about health and safety. We risk assessed splicing the cages. We looked at tilting beds – building it on the lifting bed and lifting it off and in.”
However, the structure would have been too big for a tilting bed.
Byrne Looby was engaged to do the temporary works design to verify the easiest way of tilting the cages and lifting them up.
The cages themselves, at one point, were expected to weigh about 120t. Kavanagh explains that the weight was brought down because the manufacturing could be done in “one hit”.
Züblin had finished installing the buttresses by end of August. Once it has completed its work on site, it will hand over to SCS to monitor the structures as excavation proceeds.
“The standard of the cages built on site has been fantastic – better than anything that came out of a factory. We’re really lucky to have had top-class welders and steel fixers involved. The cages came together like a dream,” Kavanagh concludes.