Digswell Viaduct build a bypass tunnel?

[Aureol Colin]

Given how much of HS2 is being built in tunnel to keep Nimbys in their box. Perhaps a similar solution could be applied to the Digswell viaduct bottleneck by constructing tunnels for the fast lines from south of Stevenage, under the Mimram valley and Welwyn, before re-joining the current formation close to the River Lea.

 

[D365]

Goodness gracious, I wouldn’t want to imagine the gradients involved.

 

[Bald Rick]

This has been looked at, I think. A few points:

1) gradients aren’t actually an issue, because...
2) the tunnels (it would have to be twin tubes) would be nearly 6 miles long, but...
3) that’s makes it around £2-3bn

It’s all chalk round there so relatively easy tunnelling, although the tunnels would be wet.

There’s also the point that you would be dealing with a similar number of protestors at either end of the tunnels, and at the emergency escape shafts (at least one, maybe two), than you would get from the viaduct / extra bores at Welwyn North.

Also, coming up at the South end in the middle of a high risk flood plain is not normally a good place for tunnels.

 

[zwk500]

This has been looked at, I think. A few points:

1) gradients aren’t actually an issue, because...
2) the tunnels (it would have to be twin tubes) would be nearly 6 miles long, but...
3) that’s makes it around £2-3bn

It’s all chalk round there so relatively easy tunnelling, although the tunnels would be wet.

There’s also the point that you would be dealing with a similar number of protestors at either end of the tunnels, and at the emergency escape shafts (at least one, maybe two), than you would get from the viaduct / extra bores at Welwyn North.

Also, coming up at the South end in the middle of a high risk flood plain is not normally a good place for tunnels.

On the cost point, what would be the approximate cost (excluding the purchase of the houses required) for a parallel Viaduct & 2 Tunnels? I suspect that the South tunnel and viaduct would be west of the current line and the north tunnel east of the current line, so the gap between the two would also need some work. And then there's the two bridges at Woolmer green to rebuild...

 

[Bald Rick]

Complete guess, but around £1bn - £1.5bn. It’s a long time since the estimate was done. You can’t exclude the cost of the houses.

There’s no way the new tunnels can be on alternate sides incidentally, unless you want the fast lines down at 50mph or something.

 

[zwk500]

Complete guess, but around £1bn - £1.5bn. It’s a long time since the estimate was done. You can’t exclude the cost of the houses.

There’s no way the new tunnels can be on alternate sides incidentally, unless you want the fast lines down at 50mph or something.

Excluding the houses was because their costs will rise a lot quicker than that of the infrastructure. Today it'd probably be not far off £15-20m by the time you've gone through the full process.

I was thinking the tunnels would be shortened so that there was a much longer transition between the two. Although whether that's cheaper than just buying the houses in Woolmer Green next to the current tunnel is another question. There probably wouldn't be too much in it either way.

 

[Bald Rick]

Excluding the houses was because their costs will rise a lot quicker than that of the infrastructure. Today it'd probably be not far off £15-20m by the time you've gone through the full process.

Railtrack bought many of the houses 20 years ago. Many sold by NR at a nice profit!

The process for gaining consent will be broadly the same for either option, including the land cost. Clearly the actual property may cost more if you are buying housing rather than farm land, but that’s only a small proportion of the process.

 

[malc-c]

My guess is that if those who make these decisions thought it warranted four tracking it would have been done already. I was born and bred in Welwyn, and recall the subject being raised ever since I was a kid back in the late 60's. Back then the north bound divergence into a down and up slow occurred shortly after the viaduct at Robbery Bottom lane, but was then remodeled as part of the HST / Electrification programs, with the points being relayed after the Heath Road bridge, probably as the track is straighter ?

If you look at the land around the line between Woolmer green and WGC, it might be possible to change the path of the existing track so that two new tracks were laid to the east of the lines rather than run new down slow and up slow as there would be less need to purchase houses in Broom Hill /Woodland way area as there is just farmland to the east of the lines. However the big issue is Welwyn North Station, a grade II listed building... You might get permission to remove the existing platform on the down line and move it west to accommodate two new lines, but this would mean that the new tunnels would be to the west of the existing tunnels so the earlier idea of running the lines to the east of the existing lines wouldn't work, and compulsory purchase of homes in Broom hill would be needed.

I don't think it's the cost of building materials that's held up this idea, more the costs of changes to a grade II listed building, purchasing the houses back and given the area that is affected the complaints likely to come about on the environment. On a practical note, they could follow the Victorian example and use spoil from the tunnel to create the new embankments either side of the viaduct. The viaduct would also need to be built in the same style as the existing one and presumably side by side rather than widening the existing viaduct. One possibility would be to use the Hertford loop to divert trains and then widen the existing viaduct with a bus service between WGC, Welwyn North, Knebworth and Stevenage whilst this work is done.

Would be an interesting project to vlog

 

[Bald Rick]

I don't think it's the cost of building materials that's held up this idea, more the costs of changes to a grade II listed building, purchasing the houses back and given the area that is affected the complaints likely to come about on the environment. On a practical note, they could follow the Victorian example and use spoil from the tunnel to create the new embankments either side of the viaduct. The viaduct would also need to be built in the same style as the existing one and presumably side by side rather than widening the existing viaduct. One possibility would be to use the Hertford loop to divert trains and then widen the existing viaduct with a bus service between WGC, Welwyn North, Knebworth and Stevenage whilst this work is done.

It would be a separate 2 track Viaduct, and not necessarily in the same style.

Diverting via Hertford for several years would be.... interesting.

 

[ABB125]

It would be a separate 2 track Viaduct, and not necessarily in the same style.

Diverting via Hertford for several years would be.... interesting.

If the new viaduct is completely separate, why would it be necessary to divert via Hertford?

 

[Bald Rick]

If the new viaduct is completely separate, why would it be necessary to divert via Hertford?

It wouldn’t, except for a couple of weekends to tie in.

 

[ABB125]

It wouldn’t, except for a couple of weekends to tie in.

Ah right, I thought you were implying it would, but having re-read your post it appears that was for if the existing viaduct was being widened.

 

[malc-c]

If the new viaduct is completely separate, why would it be necessary to divert via Hertford?

I think he was referring to my comment of closing the line whilst they widened the existing viaduct, if ever that was such an option...

 

[ABB125]

I think he was referring to my comment of closing the line whilst they widened the existing viaduct, if ever that was such an option...

Yes, I see that now!

 

[D365]

Railtrack bought many of the houses 20 years ago. Many sold by NR at a nice profit!

Network Rail profiting from property sales? Wonders will never cease.

I take it that you mean to say a second Digswell Viaduct was a very serious consideration?

 

[Bald Rick]

Network Rail profiting from property sales? Wonders will never cease.

I take it that you mean to say a second Digswell Viaduct was a very serious consideration?

Absolutely, about 20 years ago.

 

[SynthD]

Is there a suitable non marshy area for a south portal in a longer tunnel?

 

[Hey 3]

Given how much of HS2 is being built in tunnel to keep Nimbys in their box. Perhaps a similar solution could be applied to the Digswell viaduct bottleneck by constructing tunnels for the fast lines from south of Stevenage, under the Mimram valley and Welwyn, before re-joining the current formation close to the River Lea.

Gosh, that solution can be done by widening Welwyn Tunnels and widening Disgwell Viaduct, both if them will not happen under Shapps beacause of NIMBYS and if he signed off a bill to do that infrastructure, he would be commiting politcal suicide. Of course a cheap but not ideal solution is to close Welwyn North, but that would save 2-4 paths an hour and would make Grant Shapps(SOSFT) even more unpopular in his consticuency(Welwyn Hatfield).

Network Rail profiting from property sales? Wonders will never cease.

I take it that you mean to say a second Digswell Viaduct was a very serious consideration?

Well it could have been once, but certainly not now under Shapps, with my aforementioned politcal suicide.

 

[Bald Rick]

Gosh, that solution can be done by widening Welwyn Tunnels and widening Disgwell Viaduct, both if them will not happen under Shapps beacause of NIMBYS and if he signed off a bill to do that infrastructure, he would be commiting politcal suicide. Of course a cheap but not ideal solution is to close Welwyn North, but that would save 2-4 paths an hour and would make Grant Shapps(SOSFT) even more unpopular in his consticuency(Welwyn Hatfield).


Well it could have been once, but certainly not now under Shapps, with my aforementioned politcal suicide.

Closing Welwyn North releases two paths an hour, at little capital cost, with a net gain in revenue (the revenue on the paths released is more than the revenue lost from Welwyn North customers abandoning rail). But is politically almost impossible.

4 tracking the viaducts / tunnels releases, err, 2 paths an hour, plus perhaps extending a couple of existing paths from Welwyn (as more can’t can’t fit in anywhere else), at a cost of about £1-£1.5bn (educated guess). More net revenue, clearly, but not anywhere near that much more.

This is the conundrum.

 

[Hey 3]

Closing Welwyn North releases two paths an hour, at little capital cost, with a net gain in revenue (the revenue on the paths released is more than the revenue lost from Welwyn North customers abandoning rail). But is politically almost impossible.

4 tracking the viaducts / tunnels releases, err, 2 paths an hour, plus perhaps extending a couple of existing paths from Welwyn (as more can’t can’t fit in anywhere else), at a cost of about £1-£1.5bn (educated guess). More net revenue, clearly, but not anywhere near that much more.

This is the conundrum.

I did highlight that in my post and I said it is not an ideal solution.

 

[D365]

Here's a completely off-the-cuff thought: How about burying Welwyn North? (i.e. replacing with an underground station)

 

[Hey 3]

Here's a completely off-the-cuff thought: How about burying Welwyn North? (i.e. replacing with an underground station)

No, it will just be inconvienient, expensive and wouldn't solve the bottle neck.

 

[D365]

No, it will just be inconvienient, expensive and wouldn't solve the bottle neck.

WLW underground would be the slow lines, the existing surface tracks would be retained as the fasts.

 

[Bald Rick]

Here's a completely off-the-cuff thought: How about burying Welwyn North? (i.e. replacing with an underground station)

That would be the same cost as building a new fast line tunnel, but add another Billion.

One (small) benefit of exra tracks, whether alongside or as a long new tunnel throughout, is that the speed restrictions on the section can be removed and it made 125 throughout. Would save 12 seconds down, 17-18 seconds up. Every little helps.

 

[Hey 3]

WLW underground would be the slow lines, the existing surface tracks would be retained as the fasts.

Oh right, now I understand your plan and it is a good one. It can be done as long as it is not really expensive but, it will not solve all the ECML bottlenecks and for that plan to work and relies on the King's Cross throat work(to be done as part of the East Coast upgrade), junctions at Ally Pally, Werrington(to be done as part of the East Coast upgrade),
2 tracking in the Huntingdon area, 2 tracking north of Stoke Tunnel to Grantham, Newark flat crossing, 2 platforms and 3 tracks at Durham, and power supply north of Newcastle.

 

[Doctor Fegg]

4 tracking the viaducts / tunnels releases, err, 2 paths an hour, plus perhaps extending a couple of existing paths from Welwyn (as more can’t can’t fit in anywhere else), at a cost of about £1-£1.5bn (educated guess). More net revenue, clearly, but not anywhere near that much more.

Given that this is Speculative Ideas, my inner crayonista says build a Welwyn Parkway on the valley floor at A1(M) j6, and extend the Moorgate-WGC services there. Then close Welwyn North.

(But I suspect the Mimram valley is protected in some way or other, and you'd need at least one road crossing with all the attendant palaver.)

 

[HSTEd]

I assume there is some reason you couldn't at least rebuild Welwyn North with through lines?

There would naively appear to be room to move both platforms back enough to make space, and there is enough room on the south end for a relatively high speed turnout.
But were it so easy....

 

[Bald Rick]

I assume there is some reason you couldn't at least rebuild Welwyn North with through lines?

There would naively appear to be room to move both platforms back enough to make space, and there is enough room on the south end for a relatively high speed turnout.
But were it so easy....

Usual reason - it doesn’t provide any extra capacity.

With three minute headways, we have 18tph, with two stopping. The stopping trains use two paths.

With a passing loop, and three minute headways, with two stopping, each stopping train uses two paths, and it’s 18tph.

 

[43096]

Given how much of HS2 is being built in tunnel to keep Nimbys in their box. Perhaps a similar solution could be applied to the Digswell viaduct bottleneck by constructing tunnels for the fast lines from south of Stevenage, under the Mimram valley and Welwyn, before re-joining the current formation close to the River Lea.

Isn't HS2 the solution to the Welwyn North problem anyway? Move the fast London-Leeds/Newcastle/Edinburgh services away to HS2 and you free up capacity on the southern ECML.

 

[bramling]

With three minute headways, we have 18tph, with two stopping. The stopping trains use two paths.

With a passing loop, and three minute headways, with two stopping, each stopping train uses two paths, and it’s 18tph.

Does Welwyn North not have 4tph stopping in the peak-flow direction? Or at least, it did!

I agree Welwyn North is politically very difficult to close. It serves a completely different catchment area to Welwyn GC, with some of the villages being rather well-to-do. Having said that, the residents of these villages could just as easily drive or be dropped off at Knebworth or Hertford North, albeit that parking is an issue at both of these. This just leaves the housing immediately surrounding Welwyn North station itself.