It sounds like a simple bit of engineering. But nothing is simple when it comes to HS2 – as demonstrated by last week’s government ‘reset’ of the project – so, in practice, doing so has involved freezing the ground solid, spraying concrete against it, then threading the works past tunnel-boring machines chewing through 30 metres of London soil a day.
Last month, the Skanska Costain Strabag Joint Venture (SCS JV) confirmed it had completed all 34 cross passages on the 13.44km Northolt Tunnel, the second-longest stretch on the HS2 route, running from North Acton to West Ruislip. The milestone marks the end of the heavy civil engineering phase. Follow-on contractors will now move in to install reinforced concrete secondary collars, fire-rated sliding doors, second-stage invert concrete – and the track itself, alongside the overhead line equipment that will power trains between London and Birmingham.
Designed to allow passengers to safely disembark and walk through to the other tunnel in the event of a breakdown, the cross passages themselves – three-metre-wide tubes connecting the up-line and down-line bores at roughly 350-metre intervals – are an engineering task of their own.
Cross passages have been a vital part of tunnel safety for decades, but making them on this stretch of HS2 has been tricky. The Northolt Tunnel cuts through two different geological areas, explains Ian Heath, asset lead for the Northolt Tunnel at SCS JV. The eastern section, which runs 5.5km from North Acton to Greenford, sits in “98% bone-dry London clay”, which is close to ideal mining material.
The western section, running 8km from Greenford to West Ruislip, is trickier. Heath describes it as “a complete mess” of clays, sands, chalk and significant water pressure. Tunnel depths reach 35 metres in places, and the twin bores sit anywhere between six and 20 metres apart.
All 34 passages were excavated using sprayed concrete lining, where mini-excavators advanced a metre or so into the ground between the bores, immediately spraying quick-hardening concrete on to the exposed earth. That technique worked well on the dry London clay of the eastern section. In the wet, mixed ground of the western arm it was much trickier. On the worst sections, near Greenford, the team turned to a technique normally reserved for the most awkward ground in the world: ground freezing.
A set of 28 steel pipes, each up to 14 metres long, was drilled in a fan pattern around the footprint of the proposed cross passage from inside one of the running tunnels. Chilled brine was then circulated through the pipes – “basically the same as the back of your fridge,” says Heath – and around each pipe a column of ice formed in the soil.
As more columns grew, they overlapped, eventually creating a frozen arch – and in places a near-complete ring – around the void to be excavated. Thermocouples scattered through the ground fed live temperature data back to the surface, letting engineers plot the growth of the ice and decide when it was time to mine.
Once the arch was set, a tap installed at the centre of the passage was opened, the trapped pocket of liquid water drained out, and excavation could begin inside a sealed, dry envelope of frozen earth.
“It is hugely expensive and it takes a huge amount of time to do – many, many, many months of work before you even get around to mining,” says Heath. “But it is extremely impressive when it works.”
It is also tricky. Sprayed concrete is designed to harden quickly, with accelerator injected into the mix at the nozzle so it starts to firm up as soon as it hits the wall. The mixture can struggle when the wall is chilled with ice, however, so SCS JV had to adapt the mix to ensure proper curing while the rest of the project continued around it – the main tunnel boring machines were still advancing at roughly 30 metres a day during much of the works. “Like all things in tunnelling, it was more of a logistics challenge than anything,” says Heath.
Once each cross passage was excavated and waterproofed, a secondary concrete lining was added using a mechanical shutter supplied by Swiss specialist Kern, which expanded inside the passage so concrete could be poured around it to the finished profile. Around the opening, an engineered steel frame was cast into the secondary collar, creating the bracket from which the fire-rated sliding doors will eventually hang.
“Our job is creating the hole in the ground that they can put the train and everything else in, to allow them to have a railway,” says Heath.
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