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Ideas, anyone?   Options??

All depends how many years-worth of maintenance adds up to more than the cost of a fresh tunnel.

More than 5 & the short-termism of the Government term will see it kickedinto the next Government (& the next, and the next....)

I suspect that a bridge might be preferable.
Long tunnels are somewhat risky in case of fire, and this is reflected in the need for elaborate fire precautions in new tunnels.
If fire breaks in a train on a bridge, being in the open air greatly reduces the risks to life. And if all else fails, rescue by air or by sea is possible. Cant do that with a tunnel!
Also a tunnel has an ongoing energy use, for pumping out water, for ventilating, and for lighting. A bridge has negligible ongoing energy use.

Intermediate supports are possible to avoid building a huge and expensive single span. I suspect that most passengers would prefer a bridge.
A walkway for pedestrians and a cycle track would be most useful additions. This should be wide enough to take a fire engine or ambulance in case of emergency, but not be open to road vehicles normally.

The very substantial supports needed for a bridge could also be combined with wind turbine towers.

It makes me wonder if repairing and upgrading the old Severn Rail Bridge might have been seen as worthwhile now. Mind you, hindsight is a great thing.

Except that it was in the wrong place and IIRC  only single track.

Yes, I'm pretty sure it was single track too, hence my mention of "upgrading" the bridge. It's been done before on rail and road bridges but whether the costs would have been prohibitive or not, I can only guess.

The Severn Rail Bridge would still have been in the wrong place though for services between London and South Wales. Curves would have to be reinstated at Westerleigh (east to north) and Berkeley Heath (south to west). Without those curves its a double reversal, at Westerleigh/Bristol Parkway and Berkeley Road. With the curves Bristol Parkway is bypassed (an important interchange on services between London & South Wales) and the journey time would be increased by around an hour between London and Cardiff.

The old Severn Bridge doesn't take much traffic nowadays. Perhaps it could be repurposed with some rails?

(I'm sure it's completely the wrong sort of structure even if it carried on road traffic at all.)

There was thought given to the old Forth Road bridge for light rail and tram:
https://www.transport.gov.scot/media/24880/j10568b.pdf

Surely the only reason for a new tunnel is to do it with a waterproof lining. Can you get TBMs that operate underwater?

They had them for the channel tunnel

Waterproof relining of the Severn Tunnel? I've got no idea if it's possible, or affordable.

While looking up the history of City Road tube station, I discovered that it closed when the City & South London Line was subjected to a form of angioplasty. It was the original deep tube, and subsequent ones were built with larger-bore tunnels. When it was merged to form (what is now) the Northern Line, its tunnels were rebuilt about 40 cm bigger. Apparently the process was as simple as unbolting the iron lining segments, digging out behind them, and putting them back with small spacers between them.

Presumably much the same could be done to reline modern tunnels, which use precast concrete segments, for whatever reason. It might not work if they are tied or locked together by something hard to remove (such as more concrete). You could use some special-purpose shield structure to make the work easier, especially on large bores.

So on that basis, for a brick-lined tunnel, you would need to cut out some (hopefully not the full depth) of the bricks and insert some lining segments that were capable of being sealed - and resisting the pressure (though that's the case in any tunnel). In the case of the Severn Tunnel, being nothing like round and having that big drain under the floor would make it a wee bit harder, though.

I have worked on tunnels with concrete bolted segmental linings - not much different to their cast iron predecessors. Your would need to put some packers in each joint to sort the angle otherwise some high stresses might build up (the faces are made to quite fine tolerances), but the larger the diameter the less this would be an issue.

I suspect, though, that if you tried that with a brick tunnel lining it was all fall apart and you would just have a heap of bricks! 

We had a film on the building of the Severn Tunnel at SWRS the other week.  It's seems to be rather a miracle that was ever completed once they hit the Great Spring.

Wikipedia has the full story. It took 14 years and Network Rail still pumps 50 million litres a day from the tunnel.

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.

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.

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.

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

)

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!

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 

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.

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

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.

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

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.

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...

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.

Going back to the comment that started it, I do wonder how much "in the know" staff at Bristol Parkway are as to the engineering challenges of the Severn Tunnel and possible alternatives, and as those comments have come third hand via a commuter to a member of the forum, (so we can't know the context in which they were made), I'm not sure I would pay much attention to them.

Just looking up the geology of the Severn Tunnel which I am finding a bit scarce at the moment.  The big problem was/is the Great Spring and the huge volumes of fresh (not estuarine) water that it produces.  Off to the library tomorrow to consult some old tomes but if Stuving et al. has access to the Proceedings of the ICE, Haswell (1973), v. 54 (3), pp. 451-486 - https://doi.org/10.1680/iicep.1973.4855 (https://doi.org/10.1680/iicep.1973.4855) might make an interesting read. A relatively modern HVAC power tunnel that might have been properly mapped. It had water problems too.

While the ICE are helpful only to students, it seems, I've found other things in the cyber-stuffheap. Like a short IEEE piece written in 1970 during the work - which strangely says nothing about springs. It does, however, lament that they (CEGB) were refused an overhead link at Sharpness on amenity grounds - "a somewhat surprising decision decision in view of the fact that a 275kV overhead crossing has existef for some years, and will continue to exist" (which of course it still does). That meant a four-year delay and a cost of £3.5M rather than £1.5M (1970 prices).

The two 400kV circuits in the tunnel were designed for a 2.6GVA rating but to be operated at half power initially. The big issue was cooling - they were to be put in PVC pipes and water-cooled via heat exchangers at the surface. Ironic that they are pumping 63 m³/hr of water out the tunnel, while struggling to get enough water down the pipes to cool the cables - apparently.

National Grid would like a new link along that route anyway, and in planning for Hinckley Point C will need several more new and uprated links locally. A (now rather old) report on that (https://www.google.com/url?sa=t&rct=j&q=&esrc=s&source=web&cd=25&ved=2ahUKEwjF8dragrXkAhXQMMAKHQofArcQFjAYegQIAhAC&url=https%3A%2F%2Finfrastructure.planninginspectorate.gov.uk%2Fwp-content%2Fipc%2Fuploads%2Fprojects%2FEN020001%2FEN020001-000771-5.2.2.1%2520ES%2520Project%2520Need%2520and%2520Alternatives%2520Appendices%25202A%2520to%25202C.pdf&usg=AOvVaw1MnO4XWAV_1NmZxo8zdeaU) says a new cross-Severn link is needed just to reach 2.2GVA per circuit, and if new Hinckley feeds in via Aberthaw (one of the options, though discounted) even more. There are plans for new grid stations at Aust and Iron Acton, and possible Tockngton.

I wonder if they'd get approval for a new high-level Severn crossing if the cables hung off the pylons of a new railway bridge...

Back from a little trip down the library to pull some papers on the geology of the tunnel with a view to looking at a second bore under the estuary. The original report by Charles Richarson is available online for you to read: the University of Michigan’s long out of copyright reproduction of the Proceedings of the Bristol Naturalists’ Society of 1887 (https://archive.org/details/proceedings06socigoog/page/n369) followed by Lloyd Morgan’s description of the geological section: pp. 82ff. Click on the page to turn over.

The coloured sections to which both articles importantly refer are missing from the archive scan, but not from the Bod’s copy from which I have reproduced below for your reference. Richardson’s paper is a fascinating to read. Raise your cap to the courage of diver Lambert who sealed off two devastating breaches – one with an experimental and novel diving suit – nerves of Victorian steel!

From the Haswell 1973 ICE paper on the HVAC tunnel, very similar problems with excessive water intake despite pre-treating the ground ahead of the face: in excess of 1000 gallons per minute continuously over the majority of the excavation (Figs 7 & 12 referenced and reproduced under private research fair use).

The Severn Valley is extensively faulted within variable but often hard Carboniferous and Triassic rocks – a nightmare for tunnelling even with modern methods. Compare that to the clay rich and impervious, but soft, chalk of the Lower Cretaceous in which the Channel Tunnel was bored and the equally workable Eocene London Clay of Crossrail.

All the geological information to hand points to a bridge or barrage if a second rail crossing is required.  Much easier to engineer and much cheaper too if I were to hazard a guess.

A Severn barrage has long been proposed in order to generate electric power from tidal flows. If this could be combined with a new rail crossing, then the idea has considerable merit.

With pedestrian and cycle access it could become an attraction in its own right, for sightseeing, exercise and angling.

A bridge in the centre? So not actually a barrage spanning the whole river? Interesting, would that generate much power and would it avoid the loss of mudflat habitats that a full barrage would cause?

There was always intended to be a navigation channel through the Severn Barrage, IIRC it would have lock gates.  There would need to be a bridge over it.  That said a small gap in the barrage would still allow it to function as there is a very high tidal flow. 

I doubt that a barrage with an opening in the middle would work.
The tidal flow through the opening would be vast and liable to scour away the riverbed under or near the foundations.
An opening closed by lock gates should be fine.

Following discussions on the previous Severn Rail Bridge, I was interested to come across a BBC article at https://www.bbc.co.uk/news/uk-scotland-south-scotland-49675863 describing the "Solway Junction Railway: The ill-fated Scotland to England rail route" which included a long viaduct that proved the line's downfall.

From Gloucestershire Live.  It seems a bit fanciful to me.

https://www.gloucestershirelive.co.uk/news/gloucester-news/another-bridge-over-river-severn-3389047

The article implies this can be looked at as a regional project although it is admitted that no one has any idea where the money would come from.

I again met a Chartered engineer whom has previously spoken in favour of a new combined Severn rail crossing and tidal barrage.

They favour a very substantial structure that is largely a solid causeway with a double track high speed railway atop this. Steadily rising towards the middle which will be a bridge of sufficient height to permit passage of ocean going ships under the bridge and through the locks installed for this purpose.

A hugely expensive project but with two major benefits, a new rail crossing AND a large tidal power generating plant.
A road would provide opportunities for walking, cycling, sightseeing, and angling. Suitable for ambulances and fire engines in case of emergency, but not open to road traffic ordinarily.

This and similar schemes have been previously considered, but are worthy of considering again for several reasons.
Public opinion is moving way from fossil fuels for both electricity generation and road transport.
The current much increased cost of natural gas has tilted the economics towards renewable generation of electric power and away from gas.

A tunnel normally finds favour with our most esteemed leader.

Oh, I thought he preferred bridges.  :)

• Garden bridge over the Thames
• Bridge between Scotland and Northern Ireland

Better yet a bridge between Shirehampton and Pill for the PortZ railway.

Our leader may favour tunnels, but I suspect that both engineers and health and safety consultants would favour a bridge, causeway, viaduct or similar crossing.

Long tunnels are inherently vulnerable to fire.
Tunnels have ongoing energy use.
A bridge or causeway can be combined with tidal or wind power equipment.

Ans finally I suspect that most passengers would prefer a sea view to a tunnel.

And bridges are vulnerable to impact by shipping as in the case of a certain previous Severn railway bridge. 

....but I doubt that anyone would now propose a bridge consisting of a series of relatively short spans between several towers as in that structure.

Yes, but this proposal was for a substantial barrage of stone and concrete that would resist damage from a ship. With lock gates in the middle for ships, and a bridge under which ships could pass.
An out of control ship might damage the lock gates but not the bridge.

The problem with the Severn Estuary was always the issue of silting up and turning the whole expanse into flood plain.  I'm not sure anyone has come up with a practical, economically viable solution to that.  As for the impact on natural habitats....

A causeway would, as pointed out before, be phenomenally expensive. The risks, both economic and environmental, can be modelled, but have a great degree of the unknown about them. The much smaller Rance barrage has caused silting, and seen off a couple of species that previously lived in the estuary. Whether the power it produces swings the balance to positive depends on whether you are talking to an ecologist or a civil engineer, but the effects came as a slight surprise. A Severn Barrage could spring much greater surprises, or it might be perfectly fine, but the uncertainty is probably more than enough to put off private investors.  Government knows that if it promoted the barrage, the battle between green power lovers and nature lovers would go on for years before a decision could be taken, even longer than HS2 or Hinkley C. For these reasons, I doubt it will happen.

The first tunnel seems to have been a mistake that won't be repeated, even if the engineers of the day did get away with it. A new one would, as stated earlier, need a much more sophisticated design of twin bores or escape towers mid-river, and extra Severn bores don't come cheap.

That leaves a bridge as the favourite. I doubt that it could incorporate wind turbines, as they need regular maintenance, and there is a risk of bits dropping off. Not a high risk, but enough to preclude a rail service. I can't see the benefit would justify the cost as things stand, even though the engineering would be much simpler than for a barrage or tunnel.

If we were Danish we would have put rail lines on the second Severn crossing (Prince of Wales Bridge).