Kilsby and Watford tunnels

[AlastairFraser]

Cut and cover would require removal of all the soil and replacing it so would be much worse for the soil formations than a bored tube of soil with local settlement. You'll never put the soil back on top the same way it came out, and disturbance is the biggest issue.

Doesn't it depend on whether retaining walls were built to adjudicate whether surrounding soil formations were disturbed?

 

[Peter Sarf]

Doesn't it depend on whether retaining walls were built to adjudicate whether surrounding soil formations were disturbed?

Think hes talking about the soil etc that is put back over the top of the railway - cut and cover.

 

[AlastairFraser]

Think hes talking about the soil etc that is put back over the top of the railway - cut and cover.

Oh yes, maybe crossed wires. I was mainly talking about the impact of bored tunnels on local formations through vibrations, I maybe wasn't clear enough about this.

 

[MattRat]

But how would it ever be considered ok to demolish all the existing properties above Watford Tunnels?

Because those properties are just asking for trouble. What happens if the tunnel collapses? I'm sure it's very safe and unlikely to happen, but if it ever did....

Better safe than sorry, in my opinion.

 

[Elecman]

Because those properties are just asking for trouble. What happens if the tunnel collapses? I'm sure it's very safe and unlikely to happen, but if it ever did....

Better safe than sorry, in my opinion.

Black Harry Tunnel did collapse and took a house with it

 

[AndrewE]

Because those properties are just asking for trouble. What happens if the tunnel collapses? I'm sure it's very safe and unlikely to happen, but if it ever did....

Better safe than sorry, in my opinion.

Why not box clever? (i.e. apply a little ingenuity to the problem?)

If it should be a significant risk then infill the tunnels and re-bore them in turn. I'm sure some sort of skeleton timetable could be devised, especially if alternative routes were used for some of the freight etc.
People in the buildings up above probably wouldn't even be able to feel it. Then de-tunnelise the approaches as far as was justified - and then a bit further if you could get good money by flogging the real estate you had created. I'm sure there are lots of examples, just not over newly-created tunnels. Wembley Central and the car park over the approach to Plymouth spring to mind.

A decent-size newly-created tunnel would be drier and more stable than a cutting in loose rocks.

 

[zwk500]

Why not box clever? (i.e. apply a little ingenuity to the problem?)

If it should be a significant risk then infill the tunnels and re-bore them in turn. I'm sure some sort of skeleton timetable could be devised, especially if alternative routes were used for some of the freight etc.
People in the buildings up above probably wouldn't even be able to feel it. Then de-tunnelise the approaches as far as was justified - and then a bit further if you could get good money by flogging the real estate you had created. I'm sure there are lots of examples, just not over newly-created tunnels. Wembley Central and the car park over the approach to Plymouth spring to mind.

A decent-size newly-created tunnel would be drier and more stable than a cutting in loose rocks.

If it isn't broken don't fix it comes to mind. Doing exactly this to Farnworth tunnel ended up with a large vein of sand collapsing into the tunnel. I also don't understand how you create real estate by opening out a tunnel - there will now be a big cutting instead of flat land?
At Watford you could reduce the height of the cutting between the fast and slow lines, but I don't think there's much room for the edge of the cutting to existing housing on the outer boundary to ease the outer cutting gradients.

 

[AndrewE]

If it isn't broken don't fix it comes to mind. Doing exactly this to Farnworth tunnel ended up with a large vein of sand collapsing into the tunnel. I also don't understand how you create real estate by opening out a tunnel - there will now be a big cutting instead of flat land?
At Watford you could reduce the height of the cutting between the fast and slow lines, but I don't think there's much room for the edge of the cutting to existing housing on the outer boundary to ease the outer cutting gradients.

Quite, but I was responding to

Because those properties are just asking for trouble. What happens if the tunnel collapses? I'm sure it's very safe and unlikely to happen, but if it ever did....
Better safe than sorry, in my opinion.

and then thinking further ahead... I haven't suggested opening out a tunnel at all in this thread.

If there is a reason to "do something" about Watford tunnel (and I don't think there is, although there was a derailment because of the wet and corroded rail fixings during the war) or the approach cuttings (which might need something doing) then why not think big? I do know that the bores are quite tight and poorly ventilated though, from personal work experience.

So I suggested instead of opening it out with the associated costs of acquiring the properties above, repair/expand the tunnels and then gain land for sale.

Maybe you don't know the areas. Farnworth is surrounded by coal seams, the varied strata associated with them and (probably) a lot of stuff ground up by the ice sheets and then washed down out of the Pennines - and which has filled up earlier valleys or river channels. Which could have been the source of the sand. Not easy to see or guess what is where under the ground in an area like that.

Watford to (almost) Cheddington is solid chalk all the way through the Chiltern ridge. You might remember that the Channel Tunnel engineers chose to follow one of the chalk bands all the way because it was easy to dig and relatively watertight? I don't know what has destabilised the cuttings N of Watford tunnel, maybe they are just too deep and steep-sided, so groundwater comes out and weakens the sides. Anyway, if there is an ongoing problem I was just saying it could be solved by putting the railway inside a protective structure, covering it over and then selling the new land above.

 

[Philip]

To go back to Watford Tunnel, are the ventilation shafts over the older fast tunnel or over the newer slow tunnel?

 

[zwk500]

To go back to Watford Tunnel, are the ventilation shafts over the older fast tunnel or over the newer slow tunnel?

On google maps, there appears to only be 1 ventilation shaft and it's over the Newer/Slow Line tunnel (between the M25 carriageway and slip road). There may be others I couldn't see on a fly through of the satellite images.

 

[Mcr Warrior]

Looking at old pre WW1 Ordnance Survey maps, there seem to have been a total of five air shafts above the "Slow Tunnel" on the North-Eastern side of the Watford tunnels, and then another five air shafts on the "Fast Tunnel" on the South-Western side. Not so easy to tell whether that's still the case now that much of the land above the tunnels has been partly built on.

Side by side georeferenced maps viewer - Map images - National Library of Scotland

maps.nls.uk

 

[swt_passenger]

Looking at old pre WW1 Ordnance Survey maps, there seem to have been a total of five air shafts above the "Slow Tunnel" on the North-Eastern side of the Watford tunnels, and then another five air shafts on the "Fast Tunnel" on the South-Western side. Not so easy to tell whether that's still the case now that much of the land above the tunnels has been partly built on.

Side by side georeferenced maps viewer - Map images - National Library of Scotland

maps.nls.uk

Not easy to see anything is it, even when opened side by side with Google maps or Streetview. It’s possible some have been blocked since it’s mostly electric traction.

But maybe some of those newer houses just have surprisingly deep garden ponds…

 

[Snow1964]

Looking at the Ordnance Survey, I reckon that the trackbeds in Kilsby and Watford tunnels are roughly 30 metres/100 feet below ground level. That's a lot of rock and soil for Victorian navvies to dig out with picks and shovels.

Nearby is the Great Central London Extension, built in the early 20th century, when construction techniques had advanced considerably. But the GCLE still went through the watershed in a tunnel at Catesby.

The general rule back in 1830s and 1840s was cuttings would be max 60 feet deep, otherwise it was cheaper to tunnel.

The type of rock / ground materials did affect the exact cut off depth, but you will rarely get tunnels with bottom under 50 feet below ground level unless done to protect a view.

 

[Philip]

Thanks for the information re Watford air shafts and the map.