Construction work has started on the large subterranean concrete box that will eventually house the HS2 platforms at the new Old Oak Common station.
Work to construct the 850m-long station box at HS2’s Old Oak Common station began as the concrete frame and substructure specialist, Expanded, started on site. The station box is a reinforced concrete structure that will form the frame and base for the HS2 station building. When completed, the station box will be 850m long – the length of eight football pitches.
Schematic (c) HS2
The construction method will see the side walls dug down around the box and the columns pushed down into the ground where needed, and then the ground floor will be cast in reinforced steel as a slab on the ground. They will then dig down underneath the concrete slab to excavate the soil down some 20 metres to the bottom, where the base slab will be poured.
This form of “top down” construction allows them to dig the box using the ground floor slab to hold the sides up without needing to use huge props to keep the side walls upright. Apart from that benefit, having the ground floor slab in place at the start of construction makes it possible to start above-ground building works even while the station box is being dug underground.
The muck being dug out of the ground for the station, and the tunnels will be shipped by rail to three locations – to fill in a disused pit near Cambridge for housing, landscaping at Cliffe in Kent, and landscaping a future nature reserve on the edges of Rugby.
The construction of the box at the east of the site will also allow the Tunnel boring machines (TBM) to be launched towards Euston later. The west of the site is where the spray concrete lined tunnel between Old Oak Common and the Victoria Road Crossover box finishes.
Start of construction at Old Oak Common (c) HS2
Once complete, Old Oak Common station will have 14 platforms and allow a mixture of six high-speed platforms in the subterranean box, and eight conventional service platforms for the Elizabeth line and National Rail on the surface.
When it opens, it’s expected that more than 90 million passengers per year using the station, which means this one HS2 railway station alone will handle more passengers than (pre-pandemic) Heathrow Airport.
Though not likely a concern here, top down construction also has the advantages of being able to balance the weight on the soil, if you dig out a big hollow basement, there can actually be upwards pressure from below from water table or just the “suddenly unloaded” soil. With top down you can be adding the weight of the floors above while you dig out the soil below. Other than for the poor sods who have to work in a pitch black basement with mini diggers it works really well.
They had something similar for the Canary Wharf box – had they built it without the construction on top, it would have floated!
The Canary Wharf box was designed for a larger oversite development that never happened, and they had to reinforce the foundations to cope with the changes. Heaving from expanding soil is a known issue and some buildings now include a sacrificial layer of compressible material under the basement to absorb the expanding soil.
The construction contractor published this fly-through video of the upcoming construction the other day https://youtu.be/4C8ohnXf3js
Good to see the development is still going forward. What’s the scheduled opening date for the crossrail and NR platforms?