I'm my opinion, if the viaduct is unrepairable, the solution is to drop it, suffer the pain of a short term D3 with crappy junctions and work towards a D3+2 lane tidal flow following the peaks, improving junctions along the route and minimising delays by localised means... Replacing the viaduct is logistically too difficult to do over a live road

Huge tangent but relevant to the thread title, what was the 80t weight restriction on the M4 near Newport last year for?

Abnormal load ban... Most motorways can take much larger loads... 480t seems to ring a bell

They will not let it get to the state where it becomes a danger to the general public - hence the plans to install netting to catch the bits that are falling off it onto the A4 below. Yes if it gets too bad then it will have to be demolished but we are some way from that happening yet.

On the subject of CPO's the highways agency, central govenment or local councils cannot simply waltz up to somebody and say "here's some money - your home / business is now ours to buldoze". Indeed I beieve anything needing CPOs, especially road schemes has to go to public enquiry - and in this day and age we can all imagine how that would go. It would take years to get through the process and even if it did eventually get aproval the ammount and scale of protests would be imesnse.

As for funding, while any works to the M4 itself would come from the highways Agency (i.e. Central Goverment), the alternative routes (the A40, A30, A4, A316, A312, etc are all funded and maintained by TfL (i.e. the GLA and London's taxpayers). Like I said there is no money or political will to upgrade these alternative routes to acomadate extra traffic if the M4 elevated section had to be closed.

I could be misunderstanding it but is it not the Boston Manor section which is causing all the problems - which is the section where, ironically, there is plenty of space along most of the route to build some sort of temporary structure, as with A34 Peartree, to route vehicles on whilst the old structure is demolished and rebuilt.

Concrete delamination, whilst concerning, is pretty much to be expected if they keep insisting on using salt on reinforced concrete viaducts. I'd love to think they'd see the light and invest the relatively small sum of money into bridge heating rather than reactive repairs, but I somehow think that's a folorn hope!

http://www.fhwa.dot.gov/BRIDGE/hbrdeck.pdf

To be fair Phil, most of TfL's funding is from the central pot... Council Taxes generally only contribute 25-35% towards the running costs of a local authourity

Gav - catastrophic failure of structures is very rare, catastrophic failure causing death is even rarer... A structure is most likely to collapse due to external influences like earthquake or explosion, rather than disrepair.

I agree, it's a big problem and I'm sure a team of people are currently having sleepless nights wrestling over the short term issues and hopefully the longer term solution.

Hell its London ad we have those pesky terrorists about so the mere mention of a bomb is scary.... Mind you it would push things on somewhat if thy did something like that.

The idea of a tidal flow wouldn't work as it would still have to deal with cross flow traffic.

Could the A4 traffic not be routed else where ? the viaduct will need to be resolved sometime - do we need Junction 2 ? dont think we actually do. the whole structure could be redone.....

Junction 2 is the means of access to the M4 from the A406.

So how was it built? There must have been a live road under it while it was going up?

Derek

In comparison to today the A4 probably carried three cars and a cart horse a day when the elevated section went up...

I've got some photos from the National Archives showing the elevated M4 under construction somewhere - as far as I can recall, the A4 Great West Road was D3 in both directions, but narrowed to D2 with a width restriction in the outside line while the M4 was being constructed.

The A4 was very busy then, as it was the route from West London to Bristol and to the A30 to the West Country and Southampton areas.

The A4 would, these days, have to be reduced to 1 lane each way whilst remedial works took place to support columns - would jacking with temporary supports be feasible whilst the existing concrete is taken down and replaced?

In the interim, HGVs might have to endure being diverted off at J3, and onto the A4 underneath. I'm sure a 7.5t restriction could keep the M4 above open whilst work took place on columns - but the effects on traffic all around would be horrendous and very, very damaging to that area of London.

Signs would need to divert traffic via the A40, A316, and perhaps even go as far as saying stay on the M25 and avoid trying to cross London from this angle...

Tunnelling is the only way. Dive down after J3 then surface at Chiswick roundabout. It would be devastating to that area to build the expanded junction but on the bright side it was devestate that area.

The vacated space on the surface road would make the A4 a lush boulevard with a wide grassy verge.

But if the M4 is a No Go then they will have to use it. The M3/A316 isn't that bad, at least as far as Twickenham.

Apart from the 40 / 50 limits. And after Twickenham there's lights, roundabouts, crossings, 30 limits, side roads to cul-de-sacs, you name it.

Never said it was a perfect alternative, just the one M3 traffic might have to take to avoid the M4.

I reckon there are probably quite a few anti-motorway people in London and the GLA in particular that would love to see less motorway inside the M25. Removing the elevated M4 and replacing it with surface A4 would be their preferred solution, as it would to the bean counters as it would be a much cheaper option than rebuilding the viaduct. Wouldn't be terribly popular with London motorists though.....

I think the idea of gradually weening traffic off the elevated section is the only sensible and realistic option, both educating drivers to using alternative routes and reducing stress to the structure. It wont take people long to find alternative options, coming up from the west country we often park at Hammersmith and catch the tube beyond avoiding traffic and the congestion zone. A park and ride near Heston services linked in with the general London train network would certainly reduce a lot of traffic on the elevated section, especially if was subsidised for that very purpose. Wouldn't be as fun as driving into London but very practical for those of us heading in from further afield!

Digging a whopping tunnel under the whole lot would be idealistic at best, going deep enough to avoid disrupting the existing structure (and other foundations and utilities) would be mind boggling, cut-and-cover would be senseless for the purpose of replacing the structure (other than the aesthetics) as it would still involve a lengthy closure of the route. I suppose the whole point of a tunnel is that it doesn't have to follow the existing corridor, any alternative lines it could take?

Remove the M4 elevated section completely - Realign the A4 with minimal cross flow junctions as possible Terminate the M4 to a surface roundabout and link this to the existing road network in such a way as to minimise the overall impact. Its not going to be the same run in to London but then again were changing and the requirement needing such a high quality link in isnt all that great now - the M4 ends anyways and its not like it is ending the motorway for it to start again.

Have a park and ride around the Heston area. Yes it will eat into the green space at the side of london by refinishing the M4 - Last thing we need is another M62 style start here....