A new Severn tunnel - or other crossing?

[whole diary]

[stuving]

Maybe the Swiss could help they seem able to bore very long tunnels (Gotthard base tunnel 35.5 miles) through all sorts of rock, or as it's under water the Finns and the Estonians with their propose Helsinki to Tallinn tunnel.

If a true submersible TBM is needed, I reckon the man to talk to is Martin Herrenknecht - I gather he's usually up for a challenge. His TBMs dug the Gotthard base tunnel, Crossrail, and the last (2002) bore of the new Elbtunnel (which goes under the ...), among others. But of course the trick with tunneling under a river is to do it deep enough and in the right spot so there's not a lot of water to deal with. Then the river is essentially irrelevant.

[johnneyw]

I wonder if any existing geological surveys of the Severn Tunnel area have suggested a possible depth/route for a 2nd tunnel which minimizes flooding.
This might be something that has already occurred to someone somewhere if any concerns have been raised yet about the present tunnel's capacity being exceeded in the foreseeable future.

[Robin Summerhill]

I recalled watching a documentary some years ago about the railway tunnel under the Bosphorus in Istanbul. Looking it up on Wiki tells me that the underwater section is essentially a submersed tube. More details under the subheading "Engineering" on this page:

https://en.wikipedia.org/wiki/Marmaray_Tunnel

Would a scheme like this work for the Severn?

[ellendune]

I recalled watching a documentary some years ago about the railway tunnel under the Bosphorus in Istanbul. Looking it up on Wiki tells me that the underwater section is essentially a submersed tube. More details under the subheading "Engineering" on this page:

https://en.wikipedia.org/wiki/Marmaray_Tunnel

Would a scheme like this work for the Severn?

The A55 Conwy Road Tunnel  is a submersed tube.

[eightonedee]

Developing my fantasy national infrastructure theme (see thread re Hanson freight trains and-

Perhaps it is time for someone now to contemplate HS4, a high speed passenger line following approximately the line of the M4 to north Bristol and the M5 south to Exeter?

)

I should of course have added a spur to Cardiff from the new Almondsbury/Aztec West curve using a Norman Foster designed new rail Severn Crossing!

[Noggin]

I think that because of the lay of the land it would have to be a bored tunnel rather than submerged tubes. That's not neccesarily a problem, but ideally you want some reasonably favourable geology to drill through, and even modern concrete linings will leak if there's enough water pressure behind them.

It's very fortunate that the approach on both sides is reasonably straight and not built up, so would presumably be simple enough to dig a new alignment somewhere between the M4 and M48 bridges 

[stuving]

The question was raised a while back of modern safety standards. My understanding is that a single bore would never be approved now without direct access to the surface every 1 km at most. There must be multiple bores with cross-passages every 500 m or closer. The choice is thus between the following:

• Twin bores, each single track.
• One twin-track railway tunnel plus a service/escape bore.
• Twin single-track bores plus a service/escape bore, as in the channel tunnel.

One of the big advantages of a dedicated escape passage is that is can be kept at positive air pressure, which allows much better smoke control.

[Bmblbzzz]

I think an additional bridge from approximately Pilning to Chepstow is far more practical.

[broadgage]

The question was raised a while back of modern safety standards. My understanding is that a single bore would never be approved now without direct access to the surface every 1 km at most. There must be multiple bores with cross-passages every 500 m or closer. The choice is thus between the following:

• Twin bores, each single track.
• One twin-track railway tunnel plus a service/escape bore.
• Twin single-track bores plus a service/escape bore, as in the channel tunnel.

One of the big advantages of a dedicated escape passage is that is can be kept at positive air pressure, which allows much better smoke control.

Yes, but the positive air pressure has an ongoing energy cost, and needs a backup generator at appreciable capital cost and ongoing maintenance cost and fuel for testing.
Lighting is also required together with emergency lighting, more ongoing energy use and maintenance costs.
And a fire alarm system.

These costs tip the balance towards a bridge.

[JayMac]

Two tidal barrages with railway atop. Railway on rising inclines from each shore, with a bridge in the centre of the channel.

[eightonedee]

Sorry BNM - your idea does not get my vote - too much risk for internationally important wetland wildlife habitat in the estuary........   

[martyjon]

Sorry BNM - your idea does not get my vote - too much risk for internationally important wetland wildlife habitat in the estuary........

Sorry, I don't share your view either. So long as the water level in the areas above the barrage are kept at a mean level I don't see any reason why there shouldn't be a barrage after all, wildlife establishing a residence at a low tide sees it flooded at the next high tide.

[MVR S&T]

Mother nature always finds a balance, some species move or die out, while others pick up on the new habitat, and of course she will bite back if not happy.
Better to get our energy from the tides than burning Russian gas...

[CyclingSid]

You probably wouldn't get the vote of those surfboard users who want ride the Severn Bore.

[martyjon]

You probably wouldn't get the vote of those surfboard users who want ride the Severn Bore.

They can always go to Bristols' new surfing lake.