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booshank wrote: ↑Tue May 25, 2021 23:28
Oh yes, well any significant change to that would need a complete rebuild. Perhaps it could be done by building the new pillars outside the existing viaduct and the upper deck over it, then demolishing the existing viaduct and building the lower deck.
That brings you back to the original problem, which is that there’s barely space to put anything outside the width of the existing viaduct, and even if you could do that, all the underground services are buried below the footways of the A4 to get them out of the way of the existing structure. You’d have to move those first, and to where?
EpicChef wrote: ↑Wed May 26, 2021 10:14
Also, how will signage and VMS be fitted on the lower deck?
I think that’s a fairly minor problem with this proposal TBH!
The most obvious thing, if the existing viaduct had to be replaced, would be to put the M4 in tunnel and remove the highly unsatisfactory situation of having it above the A4 altogether.
I’d have a west-facing exit on the surface at Boston Manor leading to the A4, serving traffic for the A4 around Brentford and the South Circular. I’d then bore a dual three lane tunnel from just west of where the viaduct begins. It would split underground into a pair of dual two lane tunnels. One would emerge to join the A4 into London and the other would join the A406.
I’d then hold a SABRE Members’ Walkabout on the viaduct just before it was demolished. There would be a parade, live music, and flying pigs would leave smoke trails across the sky
This is all strictly fantasy of course. But:-
The deck width of of a D4 double deck structure should be no more than the existing D2 viaduct deck. The wiki gives the width as 59 feet between parapets. With 12 ft (3.65 m) standard lanes, four lanes should not be wider than this.
The footprint of the existing viaduct appears to be directly over lanes 2 and 3 of the A4. The concrete pillars either side of the lower deck would therefore be well within the area taken up at the moment by lane 1 of the A4.
In short, I see no reason why a D4 double deck structure like the Johannesburg example wouldn't fit within the current A4/M4 footprint.
booshank wrote: ↑Wed May 26, 2021 20:31
This is all strictly fantasy of course. But:-
The deck width of of a D4 double deck structure should be no more than the existing D2 viaduct deck. The wiki gives the width as 59 feet between parapets. With 12 ft (3.65 m) standard lanes, four lanes should not be wider than this.
The footprint of the existing viaduct appears to be directly over lanes 2 and 3 of the A4. The concrete pillars either side of the lower deck would therefore be well within the area taken up at the moment by lane 1 of the A4.
In short, I see no reason why a D4 double deck structure like the Johannesburg example wouldn't fit within the current A4/M4 footprint.
And as pointed out in the real world issues such as access to the existing buildings and the services they need make it practically and politically impossible. Nobody is going to sign off on the idea of building a D4 motorway above a live road. Even if they were insane enough to do that how do you physically do it ? There are no magic skyhooks to hang heavy loads from.
booshank wrote: ↑Wed May 26, 2021 20:31
This is all strictly fantasy of course. But:-
The deck width of of a D4 double deck structure should be no more than the existing D2 viaduct deck. The wiki gives the width as 59 feet between parapets. With 12 ft (3.65 m) standard lanes, four lanes should not be wider than this.
The footprint of the existing viaduct appears to be directly over lanes 2 and 3 of the A4. The concrete pillars either side of the lower deck would therefore be well within the area taken up at the moment by lane 1 of the A4.
In short, I see no reason why a D4 double deck structure like the Johannesburg example wouldn't fit within the current A4/M4 footprint.
And as pointed out in the real world issues such as access to the existing buildings and the services they need make it practically and politically impossible. Nobody is going to sign off on the idea of building a D4 motorway above a live road. Even if they were insane enough to do that how do you physically do it ? There are no magic skyhooks to hang heavy loads from.
Access to existing buildings? Presumably between the pillars like this. These buildings look closer to the pillars than the London ones would be. If the requirement to move services were a dealbreaker, much of the world's urban infrastructure would never have been built. That's presuming they are under lane 1 of the A4 and not under the pavements/parking.
I wouldn't get too hung up on my theoretical idea of building over the existing road (rather than closure, demolition then building), though it's surprising what has been done.
booshank wrote: ↑Wed May 26, 2021 20:31
This is all strictly fantasy of course. But:-
The deck width of of a D4 double deck structure should be no more than the existing D2 viaduct deck. The wiki gives the width as 59 feet between parapets. With 12 ft (3.65 m) standard lanes, four lanes should not be wider than this.
The footprint of the existing viaduct appears to be directly over lanes 2 and 3 of the A4. The concrete pillars either side of the lower deck would therefore be well within the area taken up at the moment by lane 1 of the A4.
In short, I see no reason why a D4 double deck structure like the Johannesburg example wouldn't fit within the current A4/M4 footprint.
And as pointed out in the real world issues such as access to the existing buildings and the services they need make it practically and politically impossible. Nobody is going to sign off on the idea of building a D4 motorway above a live road. Even if they were insane enough to do that how do you physically do it ? There are no magic skyhooks to hang heavy loads from.
Access to existing buildings? Presumably between the pillars like this. These buildings look closer to the pillars than the London ones would be. If the requirement to move services were a dealbreaker, much of the world's urban infrastructure would never have been built. That's presuming they are under lane 1 of the A4 and not under the pavements/parking.
I wouldn't get too hung up on my theoretical idea of building over the existing road (rather than closure, demolition then building), though it's surprising what has been done.
On another practical level I think you'd hit the immediate barrier of Highways England having kittens at the idea of so many pillars being relatively unprotected - to make them an integral and impossible to knock over part of the damn thing you'd end up with a structure so ghastly to look at it everyone would sharpen the knives for whoever proposed it within an atto-second.
Bryn Terminally cynical, unimpressed, and nearly Middle Age already. She said life was like a motorway; dull, grey, and long.
There's almost certainly room for D4 double deck (ALR) with the pillars on each side rather than in the middle, with the A4 as S4 underneath at ground level between the pillars, like this.
Not that it would be a popular solution, but I can't see a technical obstacle.
Well maybe but you would have to do a complete rebuild that would disrupt traffic on the A4 and M4 for a prolonged period.
Oh yes, well any significant change to that would need a complete rebuild. Perhaps it could be done by building the new pillars outside the existing viaduct and the upper deck over it, then demolishing the existing viaduct and building the lower deck.
Attached to the pillars I expect, perhaps duplicated on both sides so car drivers dont miss stuff behind lorries.
KeithW wrote: ↑Tue May 25, 2021 23:21
Well maybe but you would have to do a complete rebuild that would disrupt traffic on the A4 and M4 for a prolonged period.
Oh yes, well any significant change to that would need a complete rebuild. Perhaps it could be done by building the new pillars outside the existing viaduct and the upper deck over it, then demolishing the existing viaduct and building the lower deck.
Constructing a new deck over an existing live Motorway which in turn is above the A4 seems impractical to say the least and would be a non starter politically. This is what things look like at ground level. https://www.google.co.uk/maps/@51.49103 ... 8192?hl=en
Pillars on what is now lane 1 of the A4 each side.
Oh yes, well any significant change to that would need a complete rebuild. Perhaps it could be done by building the new pillars outside the existing viaduct and the upper deck over it, then demolishing the existing viaduct and building the lower deck.
Constructing a new deck over an existing live Motorway which in turn is above the A4 seems impractical to say the least and would be a non starter politically. This is what things look like at ground level. https://www.google.co.uk/maps/@51.49103 ... 8192?hl=en
Pillars on what is now lane 1 of the A4 each side.
Easier said than done given you'd have to keep the A4 running whilst the M4 was closed. That's capacity reduced by 60% before you've got a single bit of lateral safety zone in place, so in essence that's 2 of 10 lanes available for use during construction; that won't fly.
Any fix here will end up underground, people don't like a single level flyover here - double decking will be like a pork pie at a vegan wedding.
Bryn Terminally cynical, unimpressed, and nearly Middle Age already. She said life was like a motorway; dull, grey, and long.
And as pointed out in the real world issues such as access to the existing buildings and the services they need make it practically and politically impossible. Nobody is going to sign off on the idea of building a D4 motorway above a live road. Even if they were insane enough to do that how do you physically do it ? There are no magic skyhooks to hang heavy loads from.
Access to existing buildings? Presumably between the pillars like this. These buildings look closer to the pillars than the London ones would be. If the requirement to move services were a dealbreaker, much of the world's urban infrastructure would never have been built. That's presuming they are under lane 1 of the A4 and not under the pavements/parking.
I wouldn't get too hung up on my theoretical idea of building over the existing road (rather than closure, demolition then building), though it's surprising what has been done.
On another practical level I think you'd hit the immediate barrier of Highways England having kittens at the idea of so many pillars being relatively unprotected - to make them an integral and impossible to knock over part of the damn thing you'd end up with a structure so ghastly to look at it everyone would sharpen the knives for whoever proposed it within an atto-second.
In what sense? Concrete pillars of the size needed to support a D4 double deck viaduct are going to be hefty enough to survive being reversed into or the odd collision. Otherwise the Johannesburg double decker would have fallen down long ago as South African drivers are pretty bad at avoiding obstacles judging by the scrapes all over many concrete barriers etc The existing M4 viaduct central pillars are just protected by a kerb.
Or from a driver safety point of view? On a 30 mph road it doesn't seem unacceptably dangerous.
This is strictly a rerto-futuristic fantasy of course. But the practicalities can't be that much of a dealbreaker as hundreds of viaducts of various types have been built above busy urban roads around the world.
Access to existing buildings? Presumably between the pillars like this. These buildings look closer to the pillars than the London ones would be. If the requirement to move services were a dealbreaker, much of the world's urban infrastructure would never have been built. That's presuming they are under lane 1 of the A4 and not under the pavements/parking.
I wouldn't get too hung up on my theoretical idea of building over the existing road (rather than closure, demolition then building), though it's surprising what has been done.
On another practical level I think you'd hit the immediate barrier of Highways England having kittens at the idea of so many pillars being relatively unprotected - to make them an integral and impossible to knock over part of the damn thing you'd end up with a structure so ghastly to look at it everyone would sharpen the knives for whoever proposed it within an atto-second.
In what sense? Concrete pillars of the size needed to support a D4 double deck viaduct are going to be hefty enough to survive being reversed into or the odd collision. Otherwise the Johannesburg double decker would have fallen down long ago as South African drivers are pretty bad at avoiding obstacles judging by the scrapes all over many concrete barriers etc The existing M4 viaduct central pillars are just protected by a kerb.
Or from a driver safety point of view? On a 30 mph road it doesn't seem unacceptably dangerous.
This is strictly a rerto-futuristic fantasy of course. But the practicalities can't be that much of a dealbreaker as hundreds of viaducts of various types have been built above busy urban roads around the world.
Driver protection more than structure protection - as you say you'll need some giant supports. We'd need barriers protecting them, which would have to be concrete as well - by the time you've turned the A4 into this dystopian arrangement you probably would have been better just digging a tunnel.
Double decker motorways have a place, but this isn't it.
Bryn Terminally cynical, unimpressed, and nearly Middle Age already. She said life was like a motorway; dull, grey, and long.
booshank wrote: ↑Thu May 27, 2021 18:12
In what sense? Concrete pillars of the size needed to support a D4 double deck viaduct are going to be hefty enough to survive being reversed into or the odd collision. Otherwise the Johannesburg double decker would have fallen down long ago as South African drivers are pretty bad at avoiding obstacles judging by the scrapes all over many concrete barriers etc The existing M4 viaduct central pillars are just protected by a kerb.
Or from a driver safety point of view? On a 30 mph road it doesn't seem unacceptably dangerous.
This is strictly a rerto-futuristic fantasy of course. But the practicalities can't be that much of a dealbreaker as hundreds of viaducts of various types have been built above busy urban roads around the world.
And hundreds have been knocked down or in some cases fallen down. The existing flyover is in bad condition and has simply been patched up due to the high costs of a proper repair but its close to end of life at this point.
The two-lane M4 is elevated along a 5 .5km stretch through Chiswick, above the A4. These two roads carry more than 138,000 vehicles each day into and out of London. De-icing salts used on the elevated section during winter had seeped past the asphaltic plug joints and penetrated the crosshead concrete beams that form the soffit of the road deck. This penetration had caused the near-surface reinforcement to corrode, weakening the elevated M4 section and posing a risk to the A4 below.
Highways Agency trials had shown that wide-scale replacement of the beams would require a 9.1km detour of all M4 traffic onto the A4, which would itself require major traffic management to enable the works. This solution was impractical, with the cost estimated at £5million per beam. VolkerLaser were appointed to apply our specialist knowledge to this complex problem.
There is a similar age flyover on the Freeway section of SR 315 through Columbus Ohio, that was essentually rebuilt in the 1990's and you may recall what had to be done with the A14 Huntingdon viaduct.
The existing M4 viaduct central pillars are just protected by a kerb.