HS2’s drilling and tunnelling into the chalk rock below its route has many people worried about drinking water supplies, and chalk streams such as the River Misbourne. This is an engineering challenge, but there isn’t a reason to be alarmed. It’s not a venture into the unknown. A few words on tunnels and piles…
We’ll start with a disclaimer – we’re not water or geology experts, so we’re open to correction if anything we say here turns out to be wrong.
First up: piling in the Colne Valley, north of London. To hold up the viaduct where the railway crosses the valley, HS2 has to drive piles through the upper soils down into the chalk which lies below. This is an idea of what the viaduct will look like when it’s done:
Chalk is a porous rock, and in this case is an aquifer from which quite a lot of London’s drinking water is drawn. The Colne Valley, for all that it looks tranquil enough now, has a busy industrial past behind it.
Along the way a lot of unpleasant muck got dumped, particularly at Newyears Green, a waste facility. Here it is in action:
It’s contaminated land. There’s a rather murky stream, the Newyears Green Bourne, which runs by here and down into the River Colne.
One worry is that contaminated water or topsoil could go down a piling hole, end up in the aquifer and leach into the drinking water. It’s a legitimate concern, and one that needs to be taken seriously. That said, dealing with situations such as this is nothing special.
Any building or viaduct ever built on a brownfield site in or around London has had the same issue. Engineers sink piles through the soil and into the chalk all the time. Here’s the Environment Agency guidance.
With Newyears Green, we’re talking about ‘Pollution Scenario 1’ from that document. To stop water getting in to the pile holes, they drive in a metal casing, then fill the hole up with bentonite clay slurry to keep it under pressure. Then, once the concrete of the pile has set, no water can get down.
So standard stuff, but of course they need to be skilled and do the job properly. We should make sure they do.
Next: tunnelling through the chalk, which HS2 have to do under the Chilterns.
Here, the concern is that the River Misbourne, which the tunnel passes quite close beneath, will be disturbed. The chalk there has a lot of fissures in it, so water could leak away.
The first thing to point out is that tunnelling through chalk is commonplace. The Channel Tunnel is all in chalk. The Jubilee Line extension, Crossrail, the Thames Tideway and the DLR all go through chalk. The techniques for doing it are well known.
Tunnellers always have to deal with water and cracked rock and other geological challenges while working. They understand how to do it. HS2’s particular challenge is to make sure water doesn’t leak into the tunnel through fissures in the chalk.
They do this by pressurising the ground as the tunnel is being dug, using a Mix Shield Tunnel Boring Machine. The pressure inside the TBM balances the water pressure in the ground.
So there would be a genuine risk, if these things are not done right. HS2 certainly should be watched carefully and put on the spot. All the right safeguards and risk assessments need to be done, and should be accessible to the public and to journalists. But there’s no cause for alarm, and no excuse for alarmism.
We think @HS2ltd should be open and up-front about all the engineering they’re doing on the tunnel, and how well-understood the processes are. They should be explaining it to the public in a meaningful and informative way. We don’t think these challenges are a credible reason for anyone to oppose the whole scheme.
Oh: a postscript. Much has been made by some high-profile objectors of the amount of water that the tunnelling process will need – up to TEN MEEEEELION LITRES a day! That sounds like an awful lot – but wait! That’s 10000 cubic metres a day…or, just under 7 cubic metres a minute.
The River Colne discharges about 4 cubic metres per second, so HS2 will be taking less than 1/30th of the river’s flow, and that’s after all the drinking water has been extracted.
By way of context, Affinity Water loses over 20% of the water in its pipes: that’s ONE HUNDRED AND EIGHTY-NINE MEEELION LITRES a day. Nearly 20 times as much as what HS2 needs to use. So yes, HS2 tunnelling uses a fair bit, but it’s not a ridiculous amount, and the tunnels will be far less leaky than Affinity’s pipes.
And another postscript. In an unexpected turn of events, we’ve managed to INSPIRE @markhipwell1990 to do this gorgeous watercolour of how piling works! Thanks!