Box Tunnel, track lowering, would 3rd rail not have been better?

[AM9]

Isn't it a case of the older tunnels being built of porous materials where sealing the inside would be pointless as the hydrostatic pressure would just force the lining into the tunnel hollow. More modern structures have waterproof outer surfaces in their construction so that the core of their walls remains dry.

 

[snowball]

The original lining of the westbound Penmaenbach road tunnel on the A55, built in the 1980s, wasn't waterproof. The tunnel was very wet and apparently referred to by locals as "the carwash" until it had secondary lining panels installed. Presumably this was still dependant on the tunnel's drainage system.

 

[Elecman]

What's the going rate for 1kWh in industrial use? 10p? So £45 / hr. Just over £1000 a day.

And what's the going rate for selling decent spring water? 20p/tonne? (Compare to the price of Evian, or even tap water). 10m gallons is 45,000 tonnes. So £9000 a day.

Net £56k/week. Nearly £3m a year.

Nice little earner.

Your in the ball park for the price per unit of electricity averaged out including the standing charges for an HV Supply. Your figures for the units/cost per day are however largely understated

 

[DynamicSpirit]

Isn't it a case of the older tunnels being built of porous materials where sealing the inside would be pointless as the hydrostatic pressure would just force the lining into the tunnel hollow. More modern structures have waterproof outer surfaces in their construction so that the core of their walls remains dry.

OK, so from that I'm guess that for many rail tunnels, it's a case of the tunnels not being waterproof because they are so old, and a newly built tunnel, built with modern materials, wouldn't have the same problem of needing to continually pump water. Yes?

But that begs another question in my mind... Why does the hydrostatic pressure get so high? I can see that if a particular water channel was blocked and there was nowhere else for the water to go, then pressure would quickly build up. But I would have naively imagined (admittedly without any knowledge of underground geology so I could be wrong) that the water is flowing through porous rocks and that the tunnel would occupy a very small fraction of the total volume available for the water to flow in - and therefore the water could easily find other places to go, so there would be no pressure build-up. Your and other posts indicate there is actually very high pressure, which implies there is something wrong with my reasoning, but what? Is the pressure just naturally very high because the tunnels go very deep below the water table, or is it something else?

 

[najaB]

Is the pressure just naturally very high because the tunnels go very deep below the water table, or is it something else?

Yes, the tunnel is making a hole through an area where the water is under very high pressure.

 

[Murph]

In SCUBA terms, every 10m of water above you adds 1 atmosphere (1 bar), or 14.7 psi. While not exactly the same thing, the same overall effect is in play. You've got a mostly static body of water, and it's basically just the weight of the water itself above the tunnel that is creating the pressure.

 

[Bald Rick]

OK, so from that I'm guess that for many rail tunnels, it's a case of the tunnels not being waterproof because they are so old, and a newly built tunnel, built with modern materials, wouldn't have the same problem of needing to continually pump water. Yes?

No. The Channel tunnel is not waterproof, except for a few km at the French end where it had to be due to the geology.

 

[QueensCurve]

I've just seen on TV that they lowered the track level of the Box Tunnel in Wiltshire to accommodate future electrification.

I'd have thought it would have been too costly and very risky and as this was once done to a tunnel in Scotland which lead to its collapse burying 2 workers who 35 years later still remain buried.

https://en.m.wikipedia.org/wiki/Penmanshiel_Tunnel

Would it not have been a lot safer and cheaper to use 'third rail' for the tunnel and adapted trains with brushes to use third rail for tunnel sections?

That would kill 2 birds with one stone as it would also allow those trains to operate on the South Eastern networks as well as overhead electric sections.

Current thinking is that all new electrification should be 25kV OHLE.

Network rail resisted the argument that they should use third rail in Sydney Gardens Bath. Apart from anything else, it would commit future operators to need dual current stock.

The tunnel collapse you refer to is the Penmanshiel Tunnel Collaps of 1979. That work was being carried out for guage enhancement, not electrification.

The box tunnel track lowering has now been completed safely. As have similar works at other tunnels including Ipswich and Winchburgh.
--- old post above --- --- new post below ---

We have had to be extra careful with the OLE geometry, the strategic steam reserve locomotives have particularly tall funnels fitted for some reason.

Are the strategic steam reserve locomotives a fact?

 

[najaB]

Are the strategic steam reserve locomotives a fact?

Of course they are. Now stop asking questions, they are bad for your health.

 

[QueensCurve]

was worried about the strategic steam reserve secret under ground bunker but apparently the new connection was installed without problem and is hidden behind that in no way odd looking door marked SSR - SECRET about 2/3rds of the way through.
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Oh not sure i am supposed to say that

I thought the door was alongside the Eastern portal on the Up side?

 

[Murph]

Are the strategic steam reserve locomotives a fact?

Of course they are, although some modernisation has been going on, so there's now the new strategic diesel reserve joining them. Where do you think all the 37s have been going?

 

[AM9]

OK, so from that I'm guess that for many rail tunnels, it's a case of the tunnels not being waterproof because they are so old, and a newly built tunnel, built with modern materials, wouldn't have the same problem of needing to continually pump water. Yes?

But that begs another question in my mind... Why does the hydrostatic pressure get so high? I can see that if a particular water channel was blocked and there was nowhere else for the water to go, then pressure would quickly build up. But I would have naively imagined (admittedly without any knowledge of underground geology so I could be wrong) that the water is flowing through porous rocks and that the tunnel would occupy a very small fraction of the total volume available for the water to flow in - and therefore the water could easily find other places to go, so there would be no pressure build-up. Your and other posts indicate there is actually very high pressure, which implies there is something wrong with my reasoning, but what? Is the pressure just naturally very high because the tunnels go very deep below the water table, or is it something else?

The pressure at a point, (assuming no influence from geothermal or volcanic action) is directly proportional to the height of water above the point, either as clear water or saturated ground (i.e. below the water table). If the available flow of water is constrained by the cross section of its path in the porous ground, the pressure will drop as it passes through which is then completely released when it reaches the open space of the tunnel. A pressure gradient will also exist across the porous material of the tunnel wall. If the flow is stopped completely, the pressure throughout will rise to the maximum, dependent on the depth of water above it.
If the inner surface of the tunnel is retrospectively sealed, the substrate material, (100+ year-old rock/concrete?) will be under a mechanical imbalance, i.e., the pressure from the outside will also appear at the interface between the original porous rock/concrete and the new lining. It is unlikely that the lining will be rigid enough to resist all of the pressure, (it would need to be of similar thickness to the original wall) because that would severely reduce the usable internal diameter of the tunnel, and hence probably create structure gauge problems. If the lining was not strong enough to resist the full pressure, it would part company from the old material and collapse into the interior of the tunnel. Ideally the tunnel should be sealed on its outer surface but that is virtyually impossible on an established structure.
The problem is usually greater when the tunnel outer skin is directly exposed to clear water, and the available flow rate of water is virtually limitless. Nevertheless, there are also strata of land where underground water pressure and available flow is similarly high.

 

[Radedamer]

I have it on reliable authority that current security thinking requires the provision for a "zero reliance scenario". The Strategic Steam Reserve has therefore been updated with a Strategic Pit Pony Reserve.

 

[ainsworth74]

I have it on reliable authority that current security thinking requires the provision for a "zero reliance scenario". The Strategic Steam Reserve has therefore been updated with a Strategic Pit Pony Reserve.

Good lord man don't spread that about it's supposed to be held in even more secrecy than the existence of the Strategic Steam Reserve which is something of an open secret these days.

 

[kieron]

Apparently once the tunnel was finished the drain was sealed off, leaving the water of the Great Spring sealed behind the tunnel walls. The pressure built up to about 200PSI and bricks started to pop out of the walls. At which point it was decided that pumping the water out might not be such a bad idea after all.

If it's under that sort of pressure, would you need to pump it out? Or could you just insert a long pipe into the tunnel, and let the water pressure do the rest?

 

[najaB]

If it's under that sort of pressure, would you need to pump it out? Or could you just insert a long pipe into the tunnel, and let the water pressure do the rest?

You would need to pump it out because without pumping the water pressure would only be high enough to fill your pipe up to the top of the water table.

 

[QueensCurve]

The original lining of the westbound Penmaenbach road tunnel on the A55, built in the 1980s, wasn't waterproof. The tunnel was very wet and apparently referred to by locals as "the carwash" until it had secondary lining panels installed. Presumably this was still dependant on the tunnel's drainage system.

y golchi ceir?

 

[Llanigraham]

Good example of the "power" of underground water.
A friend is having to have a new bore hole drilled for their water supply as their current shared one runs dry with 3 houses using it.
Water diviner walks their fields and says "Drill here", and marks spot with an orange plastic stick.
Couple of weeks later contractor arrives with his drilling rig and starts drilling a 6" bore hole. At approx 30 metres they hit water and it comes out of the bore to a height of about 2 metres. Stop drilling and they then time how long the "spurt" lasts. Gradually it reduces and contractor says that should give you X litres per hour from now on and you will need a ??gpm pump.
They now have a good and continuous water supply, but have needed to up-grade their treatment system as their flow rate is now higher.

The bore spurted because the water at 30 metres was being held under pressure and this was released by the drilling. Now it has an exit the pressure has been released but still needs to be pumped as the water has now formed a level with the natural water table level in the surrounding ground.

 

[QueensCurve]

I think it I heard it said that they pump ten million gallons a day!

4M I believe.

 

[najaB]

4M I believe.

The article that DarloRich attempted to quote says that it is 14 million: Railnews article.

 

[QueensCurve]

The article that DarloRich attempted to quote says that it is 14 million: Railnews article.

Astonishing.

I understand that Merseyrail has increased water ingress since the water table has raised following deindustrialisation of the area in the 1980s.

Has a similar phenomenon affected the Severn Tunnel?

 

[HowardGWR]

Good lord man don't spread that about it's supposed to be held in even more secrecy than the existence of the Strategic Steam Reserve which is something of an open secret these days.

Judging by the number of steam engines that were rescued from Barry Island, the number being hauled out of museums and the number of new build projects, we will have a SSR soon, anyway.