Hypothetical Severn Tunnel Replacement

[S-Bahn]

Assuming there was a major structural disaster with the Severn Tunnel that was beyond economical repair, which replacement option would be the most cost effective?

1. Build an entirely new tunnel to modern standards.
2. Build a rail bridge over the Severn.
3. Build a smaller crossing further upstream to save costs (e.g. Sharpness)
4. Don't bother and send everything via Gloucester.

Also, would your answer be the same if the Severn Tunnel had never been built at all?

 

[MarkyT]

I read somewhere that generally the maintenance costs for a bridge over whole life are many times more than a similar length tunnel. I expect the capital cost of either would be highly variable, and dependeant on specific site conditions, navigational clearance requirements and ground characteristics. Also the more modern lighter cable supported types of bridge construction have not in the past been considered suitable at all for the very high peak loads and forces typical of rail traffic, although the short stayed bridge section on the Oresund strait link between Denmark and Sweden and the much longer 25 de Abril suspension bridge over the Tagus in Lisbon both carry trains today.

 

[DerekC]

Assuming there was a major structural disaster with the Severn Tunnel that was beyond economical repair, which replacement option would be the most cost effective?

1. Build an entirely new tunnel to modern standards.
2. Build a rail bridge over the Severn.
3. Build a smaller crossing further upstream to save costs (e.g. Sharpness)
4. Don't bother and send everything via Gloucester.

Also, would your answer be the same if the Severn Tunnel had never been built at all?

It's got to be (1) or (2). My guess is that it would be a bridge. The capital cost is generally lower and although @MarkyT is right that maintenance would usually be higher, I am not sure that applies here A modern tunnel on a slightly different alignment could probably avoid the "Great Spring" problem, but the geology is complicated, I think, and who knows what else lies under there!!

If the Tunnel hadn't been built - then a bridge would certainly have been built instead. How about a copy of the Forth Bridge, on the alignment of the first Severn Road Bridge. That would have been spectacular!

It occurs to me that the first Severn Bridge is at or near the end of its design life - so how about a combined road/rail replacement?

 

[PeterC]

It's got to be (1) or (2). My guess is that it would be a bridge. The capital cost is generally lower and although @MarkyT is right that maintenance would usually be higher, I am not sure that applies here A modern tunnel on a slightly different alignment could probably avoid the "Great Spring" problem, but the geology is complicated, I think, and who knows what else lies under there!!

If the Tunnel hadn't been built - then a bridge would certainly have been built instead. How about a copy of the Forth Bridge, on the alignment of the first Severn Road Bridge. That would have been spectacular!

It occurs to me that the first Severn Bridge is at or near the end of its design life - so how about a combined road/rail replacement?

The alignments would need to diverge at Beachley with separate crossings of the Wye. The alignment on the English side would be problematic, I suspect that housing north of the GWML in the triangle between the M4 and the M5 would be an issue. Either you diverge east of Parkway and run a little north of the M4 or west of the M4 with some speed restricted curves.

 

[HSTEd]

Given the examples in China, we could build a ~1100m span cable stayed high speed rail bridge with two or four tracks and a top speed of 250km/h.
It can also carry road lanes.

 

[AndrewE]

It's got to be (1) or (2). My guess is that it would be a bridge. The capital cost is generally lower and although @MarkyT is right that maintenance would usually be higher, I am not sure that applies here A modern tunnel on a slightly different alignment could probably avoid the "Great Spring" problem, but the geology is complicated, I think, and who knows what else lies under there!!

I suspect that today they would just use ground-freezing ahead of a TBM and install a waterproof tube. The Severn Tunnel was originally built when tunnelling was in its infancy.

If the Tunnel hadn't been built - then a bridge would certainly have been built instead. How about a copy of the Forth Bridge, on the alignment of the first Severn Road Bridge. That would have been spectacular!
It occurs to me that the first Severn Bridge is at or near the end of its design life - so how about a combined road/rail replacement?

Given the examples in China, we could build a ~1100m span cable stayed high speed rail bridge with two or four tracks and a top speed of 250km/h. It can also carry road lanes.

No more road traffic provision, please!

 

[Bald Rick]

Given that there is already sufficient road capacity across the Severn, it would definitely be a tunnel. A new rail tunnel would be cheaper up front than a new rail bridge over that distance. A 7km twin track tunnel to latest standards would be in the region of £500m-£700m. Compare to the cost of the Queensferry crossing, of £1.35bn for a shorter crossing.

If more road capacity was required, there might be a case for a joint road/rail bridge, but it’s not, so there wouldn’t be.

 

[squizzler]

I am not a trained geologist but my intuition is that an immersed tube tunnel would be more appropriate than an bored tunnel for the estuary conditions. For the uninitiated, this consists of concrete tubes lowered from barges into a trench that is dredged in the sea floor, and I suspect the ooze that makes up the base of the river Severn would make an excellent substrate for such a construction.

As for alignment, I have speculated before on these forums that the best route would be from Weston Super Mare in Somerset to Penarth in South Wales. A new approach line would separate from the Berks and Hants line near Frome and run along the South Mendips via Wells to the tunnel portal, with a four way junction on the line between Bristol and Taunton. In due course the Berks and Hants, and the link from this route to Bath and Bristol, would be upgraded to HSR standards so as to become the principal fast route to Bristol and South wales.

Amongst other service improvements, my proposed tunnel would allow for some of the XC services between Exeter and Birmingham to go via Cardiff and Newport whilst others can continue to go via Bristol. It would also, most importantly of all, upset the highways agency by providing a more downstream route across the Severn Estuary than what they got.

Having said all of that, I would also rebuild the legacy Severn Tunnel (immersed tube again) for freight and commuter trains. The approach tracks are already in place. There are two suspension bridges and I think two pairs of tracks under the Severn, especially as they are at different locations, would not be excessive as the area becomes increasingly economically integrated.

As far as the OP's final question is concerned, if designing the railway network of that area from scratch I would build the downstream (Weston - Penarth) tunnel as four tracks. I probably wouldn't bother with the existing route (if the Severn tunnel had never been built the line Badminton line from Wooton Basset would not exist). I might also build an international airport on an artificial island in the middle of the Severn rather than two pointless little aerodromes near Bristol and Cardiff..

 

[johnnychips]

In the 80s, consideration was given to an electricity-generating barrage across the Severn. This would have run from Brean Down just south of Weston to Lavernock Point near Cardiff. Obviously it hasn’t been built, but one of the selling points was a road and a railway on top of the barrage.

 

[The Ham]

In the 80s, consideration was given to an electricity-generating barrage across the Severn. This would have run from Brean Down just south of Weston to Lavernock Point near Cardiff. Obviously it hasn’t been built, but one of the selling points was a road and a railway on top of the barrage.

Whilst it would have produced a lot of power it wasn't liked due to the impact on the mud flats (i.e. significantly reducing their size due to the low tide not being as low for as long.

 

[randyrippley]

..............As for alignment, I have speculated before on these forums that the best route would be from Weston Super Mare in Somerset to Penarth in South Wales. A new approach line would separate from the Berks and Hants line near Frome and run along the South Mendips via Wells to the tunnel portal, with a four way junction on the line between Bristol and Taunton. In due course the Berks and Hants, and the link from this route to Bath and Bristol, would be upgraded to HSR standards so as to become the principal fast route to Bristol and South wales.........................

Well............thats a good plan for reopening the north end of the S&D, what are your plans for the southern end?
You would have a major issue with drainage / flooding west of Wells - you're out on the levels there

 

[The Planner]

As for alignment, I have speculated before on these forums that the best route would be from Weston Super Mare in Somerset to Penarth in South Wales. A new approach line would separate from the Berks and Hants line near Frome and run along the South Mendips via Wells to the tunnel portal, with a four way junction on the line between Bristol and Taunton. In due course the Berks and Hants, and the link from this route to Bath and Bristol, would be upgraded to HSR standards so as to become the principal fast route to Bristol and South wales.

So you have to go back on yourself to get to Bristol? how much time would that add? Presumably anyone who wants anywhere west of Cardiff is changing?

Amongst other service improvements, my proposed tunnel would allow for some of the XC services between Exeter and Birmingham to go via Cardiff and Newport whilst others can continue to go via Bristol. It would also, most importantly of all, upset the highways agency by providing a more downstream route across the Severn Estuary than what they got.

There is no way that Bristol would get dropped off a XC service. Why do we need to upset Highways England, surely it isn't one mode of transport vs another as that is stupid?

Having said all of that, I would also rebuild the legacy Severn Tunnel (immersed tube again) for freight and commuter trains. The approach tracks are already in place. There are two suspension bridges and I think two pairs of tracks under the Severn, especially as they are at different locations, would not be excessive as the area becomes increasingly economically integrated.

So you are rebuilding two tunnels, why not just rebuild the current one and make it 2 bores with twin track in each? There is only one suspension bridge there which carries the M48 and only two tracks under the Severn.

 

[Llanigraham]

If Squizzler thinks that the Severn Estuary is all silt to bury his immersed tube tunnel then can I suggest he looks at a maritime map of the area and perhaps drives over the Prince of Wales Bridge at low tide. There certainly seems to be a lot of rocks visible in both instances.

 

[randyrippley]

In the 80s, consideration was given to an electricity-generating barrage across the Severn. This would have run from Brean Down just south of Weston to Lavernock Point near Cardiff. Obviously it hasn’t been built, but one of the selling points was a road and a railway on top of the barrage.

Didn't the calculations show that it would have silted up before the investment costs could be recouped?

 

[squizzler]

So you have to go back on yourself to get to Bristol? how much time would that add? Presumably anyone who wants anywhere west of Cardiff is changing?

Which route are you referring to? If you are referring to London - Bristol traffic, I speculated that a proposed lower Severn Tunnel would make the logical route for any future Western Region HSR line be along the axis of the Berks and Hants, as the same trunk could carry traffic from London towards Bristol, the Southwest and South Wales via the tunnel. Obviously this route would be designed so Bristol trains did not have to reverse en route (duh!). In the meantime, London - Bristol traffic would continue to use the existing route rather than reverse at Westbury.

There is no way that Bristol would get dropped off a XC service. Why do we need to upset Highways England, surely it isn't one mode of transport vs another as that is stupid?

Perhaps if you allow me to explain the same proposal in different terms, the current XC services to the SW via Bristol could continue as before. The Birmingham - Gloucester - Cardiff XC route could be extended through the tunnel and continue to Exeter or beyond.

A shorter distance between the Southwest and South Wales would definitely be a selling point that the rail operators can stress in their marketing. So yes, it is one mode against another. And those with long memories will recall that the highways people didn't co-operate with rail to share a causeway built to take the A9 across the Dornoch Firth in Scotland, despite it being a no-brainer at the time.

So you are rebuilding two tunnels, why not just rebuild the current one and make it 2 bores with twin track in each? There is only one suspension bridge there which carries the M48 and only two tracks under the Severn.

I suspect that modern safety standards would require any future tunnel of this length to have one track per bore, so four new tracks would require four new holes. Nonetheless, why put four lines in one place when you can separate them to create new connections elsewhere? Maybe the geology of the riverbed mitigates against this, but all other things being equal I still prefer my idea!

 

[HSTEd]

Didn't the calculations show that it would have silted up before the investment costs could be recouped?

That depends on your assumptions, in addition what costs you assign for simply dredging the relevant portion of the barrage.

Improved "dual generation" barrages can be expected to have drastically less issue with silting because the tidal range behind the barrage is preserved to a greater degree (its analogous to an AC electrical system where the voltage is largely unchanged and power is extracted by a phase-difference across the barrage)

 

[Tomos y Tanc]

It occurs to me that the first Severn Bridge is at or near the end of its design life - so how about a combined road/rail replacement?

I think the old bridge has a fair amount of life in it yet. It underwent a massive programme of strengthening in the 1990s as a stop-gap prior to the opening of the second crossing.

Even if the old bridge was closed to traffic, the bridge itself would probably have to remain as it's a Grade 1 listed structure.

 

[ashkeba]

[...] If the Tunnel hadn't been built - then a bridge would certainly have been built instead. How about a copy of the Forth Bridge, on the alignment of the first Severn Road Bridge. That would have been spectacular!

It occurs to me that the first Severn Bridge is at or near the end of its design life - so how about a combined road/rail replacement?

A bridge was built - and what you call "the first Severn Bridge" was built about 90 years after the Severn Railway Bridge. It was destroyed by ships in fog.

But I think any replacement would be a new tunnel, or a bridge close to the tunnel alignment.

 

[HSTEd]

A bridge has the advantage of greater economies of scale for multiple tracks.
You could have dedicated freight/slow lines across the bridge so a Class 66 on an overweight train doesn't get in the way of the IEP doing 140mph for the few kilometres it can over the bridge......

 

[MarkyT]

So you are rebuilding two tunnels, why not just rebuild the current one and make it 2 bores with twin track in each? There is only one suspension bridge there which carries the M48 and only two tracks under the Severn.

Bore your new double track tunnel alongside. Create cross passages to old tunnel. Close old tunnel for rebuilding and install new waterproof lining. Reinstate one line in old tunnel (insufficient space for two tracks due to new lining) and use the remaining space for a fenced safe access route capable of accommodating small road vehicles for maintenance and escape. The two tunnels would act as emergency egresses for each other via the cross passages, which would be equipped with secure fire and waterproof doors. Three tracks would provide overtaking facilities in one direction at a time. Reconfigure pumping and ventilation facilities at Sudbrooke as appropriate.

 

[randyrippley]

Something about the tunnel I've never understood....
is the "great spring" simply the river leaking into the tunnel, or did they actually hit an underground waterway underneath the river?

 

[ashkeba]

Something about the tunnel I've never understood....
is the "great spring" simply the river leaking into the tunnel, or did they actually hit an underground waterway underneath the river?

The second. It is fresh water which I think was pumped out for industry use. Maybe still is.

 

[Bald Rick]

The second. It is fresh water which I think was pumped out for industry use. Maybe still is.

It is. 50million litres a day, or using SI units, 20 Olympic swimming pools a day. Some is sold to the local water company AIUI.

 

[Llanigraham]

Something about the tunnel I've never understood....
is the "great spring" simply the river leaking into the tunnel, or did they actually hit an underground waterway underneath the river?

The Great Spring is an underground "river" that seems to be flowing the opposite way to the river above. It is fresh water and surprisingly pure and unpolluted, and is used in the local water supply and I believe at a small brewery.

 

[Lucan]

The second. It is fresh water which I think was pumped out for industry use. Maybe still is.

What would be so hard about stopping the leak into the tunnel? Or is the sale of the water worth enough to keep pumping it.

 

[Peter Kelford]

Upgrade the Gloucester line.

 

[HSTEd]

What would be so hard about stopping the leak into the tunnel? Or is the sale of the water worth enough to keep pumping it.

Brickwork is notoriously hard to waterproof.

You'd probably have to rebore the tunnel with modern lining to get it to stop.

 

[Peter Kelford]

Brickwork is notoriously hard to waterproof.

You'd probably have to rebore the tunnel with modern lining to get it to stop.

Well, you'd get concrete jets to line the tunnels, rather than rebore them, but I get what you're saying.

 

[AndrewE]

Well, you'd get concrete jets to line the tunnels, rather than rebore them, but I get what you're saying.

You would have to stop the water coming through first, and that's if you can cope with the loss of diameter taken up by the new lining anyway...

 

[Bald Rick]

What would be so hard about stopping the leak into the tunnel? Or is the sale of the water worth enough to keep pumping it.

It’s a river, not a leak! 50,000,000 litres a day is roughly the equivalent of 5000 domestic hosepipes on at the same time.

Almost every bored tunnel in the world is ‘wet’ in some way, this one more than most, but it’s not unusual. Eg the Channel tunnel is wet, and has plenty of water pumped out of it.