Two large metallic cans are at present being assembled deep below west London to permit HS2’s tunnel boring machines to drill by means of water-saturated floor.
HS2 is utilizing 4 Tunnel Boring Machines (TBMs) to dig two prepare tunnels between West Ruislip and Outdated Oak Frequent. They set off from both finish final 12 months and can meet roughly in the course of the tunnels at Greenford.
To take away the TBMs at Greenford, HS2’s contractors have dug two giant shafts all the way down to the tunnel depths to elevate them out, and the shafts shall be then be reused to supply air flow for the dwell railway. Whereas the undertaking is already difficult, an issue with the soil deep underground makes it extra so.
It’s moist. Very moist.
That makes digging a shaft down a little bit of a ache, however one that’s well-understood and manageable. Nevertheless, these two giant shafts are additionally the place the TBMs will meet and be faraway from the bottom.
Meaning your properly watertight partitions are about to have 4 big holes drilled into the perimeters, so HS2’s contractors, Skanska Costain Strabag three way partnership (SCS), needed to discover a strategy to enable the tunnel boring machines to reach with out flooding the shaft when the waterproofing is breached.
Therefore, an enormous metallic can.
Can design (c) HS2
Earlier than all that occurs although, they wanted to dig down the air flow shafts.
Not like most HS2 air flow shafts, that are one massive shaft dug down between the railway, right here at Greenford, there are two shafts, one per tunnel. They want two as an alternative of 1 due to the requirement to take away the TBMs from the bottom, so every tunnel will get its personal shaft.
They already knew the positioning was waterlogged, with very high-pressure groundwater deep down, so earlier than digging, they needed to “dewater” the positioning, which was executed by pumping the bottom across the shafts with grout to fill in all of the cracks and gaps within the chalk layer, after which drilling a lot of drainage pipes into the bottom to pump out the water — which was pumped again into the bottom close by the place it wasn’t an issue.
They deliberate to dig 32 pipes into the bottom. They wanted 136.
Nevertheless, as soon as it was all executed, they lowered the water move within the tunnel shaft website from 110 litres per second to simply 30 litres per second, which is manageable for building.
Two shafts have been dug down, however one in every of them had added problems as it’s proper subsequent to the Nationwide Rail tracks, so the shaft was dug with a narrower neck on the floor that then widened out underground to the total width. The shafts have been constructed utilizing a mixture of concrete rings on the floor and sprayed concrete as they received deeper underground till they reached the underside and solid a thick slab of concrete to supply the load wanted to carry all the things in place.
To date, building has been difficult however commonplace and makes use of engineering strategies which are effectively understood.
The issue was the right way to enable the 4 TBMs to drill into the perimeters of the shafts with out letting a flood of water in on the similar time.
To place the problem into context, the typical home water strain is about the identical as they’re coping with within the floor. So for those who flip your kitchen faucet on, that’s the water strain they may anticipate within the shafts — however now think about your faucet is now the scale of a railway tunnel.
That’s a whole lot of water.
The TBMs are utilizing pressurised slicing heads and lining their tunnels as they go alongside, so there’s no downside for them, however once they lower into the shafts, there shall be a time period between slicing by means of and including the brand new lining rings — and that’s when the shafts could possibly be flooded.
Therefore, the can.
In what’s considered the primary use of its sort within the UK, the SCS staff is assembling a big metallic can in each shafts that may lie on its facet and create a watertight tunnel for the TBMs to drill by means of and enter. As soon as they’ve sealed the hole between the railway tunnel and the air flow shaft, the can shall be opened, and the TBM lifted as much as the floor.
To see what’s occurring, final week, a gaggle of holiday makers picked up our oxygen rebreathers and have been lowered down the shaft in a manrider — a bucket on a crane — to the underside, the place persons are working to arrange the area for the enormous can to be put in. Two giant metallic rings sit contained in the shaft, prepared to carry the can in place whereas the crimson waterproof lining is slowly being coated up in one other layer of concrete that may full the within of the shaft area.
The metallic rings prepared to carry the enormous metallic can – the small facet tunnels hyperlink to the neighbouring shaft (c) ianVisits
It’s an immense area, and standing on the backside of the shaft surrounded by the orange waterproofing membrane felt virtually like science fiction as if we have been standing inside an area station.
Over the weekend, the primary of the can rings was lowered into the bottom and put in. Over the subsequent few months, the remainder of the can rings shall be lowered down and joined as much as type two large metallic cans contained in the air flow shafts, prepared for the TBMs to reach later this 12 months.
Barely amusingly, the metallic rings are tensioned with wires to maintain them steady whereas they’re being lowered down by cranes, so that they seem like large bicycle wheels.
They did have a look at different concepts, however with the 4 TBMs arriving pretty shortly after one another, the enormous metallic can was the perfect, or no less than, least unhealthy, method of coping with the excessive water strain within the floor.
As soon as the TBMs arrive and are eliminated, the shafts shall be completed to perform as air flow and emergency entry.
On the floor, big air flow followers, sitting on their sides, shall be put in in a headhouse constructing, which shall be assembled above the shaft linking to the London-bound prepare tunnel. The opposite shaft shall be capped off on the floor however could have a facet passage underground to attach it to the opposite shaft in order that each railway tunnels are correctly ventilated.
Shafts and headhouse (c) HS2
The shafts may even comprise entry stairs for the emergency providers to enter the tunnels if wanted. Two giant facet passages hyperlink the 2 railway tunnels, permitting individuals to be evacuated from one tunnel to a prepare ready within the different if that was ever wanted.
Digging the facet passages in water-logged floor would require one other fascinating building approach — they are going to be freezing the bottom by pumping super-cooled brine by means of pipes across the facet passages. Then, the development staff shall be ready dig out the frozen chalk. It’ll take about three months for the bottom to thaw out as soon as they’ve accomplished the tunnels.
One thing else of curiosity is that the prepare tunnels on both facet of the shaft are completely different sizes.
The tunnels between Outdated Oak Frequent and Greenford have an inner diameter of 8.1 metres, which widens to eight.1 metres for the tunnels from Greenford to West Ruislip.
That’s as a result of the trains shall be travelling slower as they get nearer to London, so the strain wave of air in entrance of the trains is smaller and doesn’t want as extensive a tunnel to disipate.
Below the Chilterns, the place the trains are going at full pace, the tunnels should be an enormous 9 metres in diameter.
For reference, the Elizabeth line tunnel diameters are simply 6 metres.
Again to Greenford, in a number of years time, trains shall be dashing previous deep underground, and passengers will don’t know that they’re travelling by means of floor that precipitated a lot bother to dig by means of.
An oxygen rebreather, to be used in emergencies if there’s a fuel pocket or fumes build-up within the shaft
This text was revealed on ianVisits
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