The Deterioration of Flyover structures in London

[MotorwayGuy]

Over the past 20 years, I've noticed quite a few of the flyovers on important roads into London have rapidly deteriorated. The Chiswick Flyover is/was one of the visually most shocking examples, with large sections of the concrete supports having spalled with exposed rebar. For over ten years there has been netting fixed to the overhanging sections to prevent debris falling onto the carriageway. Apparently repair works have been carried out since the imagery was taken but these ageing structures will eventually have to be replaced. Even newer structures such as the Westhorne Avenue Flyover have begun having the same issues over the past few years.

It's a fact that all structures have a limited design life, but when the day comes where it's no longer safe for traffic to continue using them, what will happen? I can see politically replacing a structure like the Chiswick Flyover being very difficult to justify, but at the same time without it traffic would grind to a halt. Of course there has been ideas thrown around of tunnelling it, but the chances of that happening before the structure expires seem slim when comparing it to how long things like the Stonehenge Tunnel and Lower Thames Crossing have been in the planning stage.

Then we have things like the A13 Lodge Avenue Flyover. Replacing this means all traffic will be forced through the roundabout while it is replaced as there is no room to build it offline. It's cousin at Movers Lane was replaced in the 90s, which would have had similar issues, does anyone know how traffic was dealt with then?

The similar "temporary" one at Gallows Corner is now reduced to a 20 speed limit, presumably due to similar concerns about the structure. I almost forget the S1 Hogarth Flyover, there are probably others I've forgotten about.

[Helvellyn]

I dispute the idea that all structures have a limited life. As long as you're not talking about major damage of the sort that could occur at any time, or issues like ground movement, a properly-maintained masonry arch should last pretty much indefinitely.

[KeithW]

Well yes but the flyovers in London are for the most part concrete structures and they do have a tendency to have problems, see the Huntingdon Viaduct and of course the Genoa Viaduct. In the case of both of them it was very hard to inspect the internal state of the tendons as it had never been given a means to do so. The expected lifespan of such a concrete viaduct is 100 years or so BUT inspection in the 1950's was rather lax.

See also
https://railuk.com/infrastructure/struc ... frewshire/

The Tees Transporter bridge made of steel also has problems, in that case largely because the foundations are sinking as water for industry was pumped from an underground aquifer.

[Bryn666]

I dispute the idea that all structures have a limited life. As long as you're not talking about major damage of the sort that could occur at any time, or issues like ground movement, a properly-maintained masonry arch should last pretty much indefinitely.

Road salt and water penetration are the two worst things you can routinely subject reinforced concrete to, and that is what most of our major road structures are made from in a climate where we have lots of water. Structures therefore do have a limited life.

[Helvellyn]

I dispute the idea that all structures have a limited life. As long as you're not talking about major damage of the sort that could occur at any time, or issues like ground movement, a properly-maintained masonry arch should last pretty much indefinitely.

Road salt and water penetration are the two worst things you can routinely subject reinforced concrete to, and that is what most of our major road structures are made from in a climate where we have lots of water. Structures therefore do have a limited life.

I'm not disputing that concrete ones do.

When they're built now is there a plan for how they'll be managed when they need replacing without causing absolute chaos?

[Bryn666]

I dispute the idea that all structures have a limited life. As long as you're not talking about major damage of the sort that could occur at any time, or issues like ground movement, a properly-maintained masonry arch should last pretty much indefinitely.

Road salt and water penetration are the two worst things you can routinely subject reinforced concrete to, and that is what most of our major road structures are made from in a climate where we have lots of water. Structures therefore do have a limited life.

I'm not disputing that concrete ones do.

When they're built now is there a plan for how they'll be managed when they need replacing without causing absolute chaos?

I'm sure you already know the answer to that... build and forget seems to be order of the day. It's someone else's problem in 120 years time (even if empirical evidence suggests we're now facing a major structural timebomb!)...

I was idly figuring out how you'd keep everything moving to replace the M6 viaducts in the West Midlands, and the answer is simply you can't. It'd be a decade long disruption as seen in the USA where freeways have been rebuilt from scratch due to similar problems.

[RichardA35]

Road salt and water penetration are the two worst things you can routinely subject reinforced concrete to, and that is what most of our major road structures are made from in a climate where we have lots of water. Structures therefore do have a limited life.

I'm not disputing that concrete ones do.

When they're built now is there a plan for how they'll be managed when they need replacing without causing absolute chaos?

I'm sure you already know the answer to that... build and forget seems to be order of the day. It's someone else's problem in 120 years time (even if empirical evidence suggests we're now facing a major structural timebomb!)...

I was idly figuring out how you'd keep everything moving to replace the M6 viaducts in the West Midlands, and the answer is simply you can't. It'd be a decade long disruption as seen in the USA where freeways have been rebuilt from scratch due to similar problems.

I was also idly thinking of how many UK trunk road structures have been replaced recently due to structural deterioration as opposed to changes in traffic patterns.
My list was quite short (although my memory has probably forgotten loads):
Several footbridges over M3 and M4 where concrete deck replaced with steel truss
A38 Marsh Mills Viaduct
A34 Wolvercote viaduct west of Oxford.
Woodlands Lane Bridge over M3 J2-3 (post smart motorway works)

Most structures are removed when the traffic patterns change and widening of the carriageway is needed above or below the structure. (e.g. M4 J4B - J8/9) or a third party scheme comes along (e.g. HS2)
We seem to be very good at maintaining and strengthening structures although the appearance might be otherwise.

[Bryn666]

I'm not disputing that concrete ones do.

When they're built now is there a plan for how they'll be managed when they need replacing without causing absolute chaos?

I'm sure you already know the answer to that... build and forget seems to be order of the day. It's someone else's problem in 120 years time (even if empirical evidence suggests we're now facing a major structural timebomb!)...

I was idly figuring out how you'd keep everything moving to replace the M6 viaducts in the West Midlands, and the answer is simply you can't. It'd be a decade long disruption as seen in the USA where freeways have been rebuilt from scratch due to similar problems.

I was also idly thinking of how many UK trunk road structures have been replaced recently due to structural deterioration as opposed to changes in traffic patterns.
My list was quite short (although my memory has probably forgotten loads):
Several footbridges over M3 and M4 where concrete deck replaced with steel truss
A38 Marsh Mills Viaduct
A34 Wolvercote viaduct west of Oxford.
Woodlands Lane Bridge over M3 J2-3 (post smart motorway works)

Most structures are removed when the traffic patterns change and widening of the carriageway is needed above or below the structure. (e.g. M4 J4B - J8/9) or a third party scheme comes along (e.g. HS2)
We seem to be very good at maintaining and strengthening structures although the appearance might be otherwise.

There must be some excellent work you don't see as we've managed to avoid the catastrophic failures other countries have had - I'm not counting things like the M20 bridge being demolished by a passing HGV as that wasn't a "failure" per se, the bridge was fine until the driver hit it!

There have been a few overbridges replaced Oop Norf, I can think of a few on the M6 alone, the M62 has had a couple of replacements and will have another when the railway bridge at J20 is done in Autumn.

I think we tend to notice before things fail, regular inspections work wonders but as we learned with, say, Hammersmith Flyover, we have either got to replace in situ or do super costly repairs to the existing - neither is a quick fix. Definitely no rebuild alongside and shift stuff across for us it seems. This is where my suggestion of fit and forget comes from, the inevitable replacement or repairs never have a contingency route available.

Chiswick will be a huge question mark when that needs replacing.

The really ropey looking structures like those thin legged footbridges on the S Yorks stretch of the M1 can't be long for this world though.

[MotorwayGuy]

Well yes but the flyovers in London are for the most part concrete structures and they do have a tendency to have problems, see the Huntingdon Viaduct and of course the Genoa Viaduct. In the case of both of them it was very hard to inspect the internal state of the tendons as it had never been given a means to do so. The expected lifespan of such a concrete viaduct is 100 years or so BUT inspection in the 1950's was rather lax.

See also
https://railuk.com/infrastructure/struc ... frewshire/

The Tees Transporter bridge made of steel also has problems, in that case largely because the foundations are sinking as water for industry was pumped from an underground aquifer.

With regards to the Huntingdon Flyover, how bad was the condition of the structure? I can't seem to find much information online but I wonder how many more years it could have survived, and if indeed the new A14(M) would have been built if this wasn't a concern? It appears as though serious steel reinforcements were installed on a single span, and these rather puny things (you can also see the spalling on the pillars here).

[KeithW]

With regards to the Huntingdon Flyover, how bad was the condition of the structure? I can't seem to find much information online but I wonder how many more years it could have survived, and if indeed the new A14(M) would have been built if this wasn't a concern? It appears as though serious steel reinforcements were installed on a single span, and these rather puny things (you can also see the spalling on the pillars here).

Well you could start with the wiki
https://www.sabre-roads.org.uk/wiki/ind ... on_Viaduct

I recall the large steel reinforcements that were added to prevent prompt failure.
see them here:
https://www.google.com/maps/@52.3298642 ... &entry=ttu

A successful investigation showed that the steel tendons holding the structure in tension had not been properly grouted so were decaying and that could not be fixed at that point. The first intention when it came to upgrading the A14 would have retained the viaduct but it was simply in too poor condition to be considered viable.

This was considered on this site in some depth - it ran to 223 pages
viewtopic.php?t=31860

[KeithW]

There must be some excellent work you don't see as we've managed to avoid the catastrophic failures other countries have had - I'm not counting things like the M20 bridge being demolished by a passing HGV as that wasn't a "failure" per se, the bridge was fine until the driver hit it!

There have been a few overbridges replaced Oop Norf, I can think of a few on the M6 alone, the M62 has had a couple of replacements and will have another when the railway bridge at J20 is done in Autumn.

I think we tend to notice before things fail, regular inspections work wonders but as we learned with, say, Hammersmith Flyover, we have either got to replace in situ or do super costly repairs to the existing - neither is a quick fix. Definitely no rebuild alongside and shift stuff across for us it seems. This is where my suggestion of fit and forget comes from, the inevitable replacement or repairs never have a contingency route available.

Chiswick will be a huge question mark when that needs replacing.

The really ropey looking structures like those thin legged footbridges on the S Yorks stretch of the M1 can't be long for this world though.

We have mandatory inspections for bridges
https://www.standardsforhighways.co.uk/ ... 4e41ccd1a0

As for the M1 in South Yorks see.
https://web.archive.org/web/20081205020 ... 5/tinsley/

[Runwell]

The similar "temporary" one at Gallows Corner is now reduced to a 20 speed limit, presumably due to similar concerns about the structure. I almost forget the S1 Hogarth Flyover, there are probably others I've forgotten about.

There's been discussion on one of the other threads regarding the PFI contract for the A13, back when the other flyovers nearby were upgraded. A tunnel has been discussed along with a bypassed section of road, which would also resolve the other matter of the Renwick Road traffic lights. Whether it ever happens remains to be seen, although it won't be long before the PFI deal expires and the A13 is back in TFLs hands....except they'll probably be too skint to so anything about it. More likely the flyover will be forced to close suddenly after one overnight inspection.

I think the restrictions at Gallows Corner are even stricter than GSV shows now. There are plans to replace it, but only like for like now.

[ForestChav]

I'm not disputing that concrete ones do.

When they're built now is there a plan for how they'll be managed when they need replacing without causing absolute chaos?

I'm sure you already know the answer to that... build and forget seems to be order of the day. It's someone else's problem in 120 years time (even if empirical evidence suggests we're now facing a major structural timebomb!)...

I was idly figuring out how you'd keep everything moving to replace the M6 viaducts in the West Midlands, and the answer is simply you can't. It'd be a decade long disruption as seen in the USA where freeways have been rebuilt from scratch due to similar problems.

I was also idly thinking of how many UK trunk road structures have been replaced recently due to structural deterioration as opposed to changes in traffic patterns.
My list was quite short (although my memory has probably forgotten loads):
Several footbridges over M3 and M4 where concrete deck replaced with steel truss
A38 Marsh Mills Viaduct
A34 Wolvercote viaduct west of Oxford.
Woodlands Lane Bridge over M3 J2-3 (post smart motorway works)

Most structures are removed when the traffic patterns change and widening of the carriageway is needed above or below the structure. (e.g. M4 J4B - J8/9) or a third party scheme comes along (e.g. HS2)
We seem to be very good at maintaining and strengthening structures although the appearance might be otherwise.

There are presumably plenty of other examples where a structure has needed partial or complete temporary closure to allow major repairs. The A52 Clifton Bridge in 2020 being an obvious example round here.

[Chris5156]

The Chiswick Flyover is/was one of the visually most shocking examples, with large sections of the concrete supports having spalled with exposed rebar. For over ten years there has been netting fixed to the overhanging sections to prevent debris falling onto the carriageway. Apparently repair works have been carried out since the imagery was taken but these ageing structures will eventually have to be replaced. Even newer structures such as the Westhorne Avenue Flyover have begun having the same issues over the past few years.

It's worth noting that there is more than one highway authority at play here, and each one has their own issues.

Many of London's major highway structures now belong to TfL, who have cut back spending on highway maintenance drastically in the last decade and have deferred a huge amount of maintenance work. Before they took over, their structures had been through the hands of multiple public bodies that were not necessarily very careful: the GLC did not prioritise spending on highways post-1973, and was then dissolved to leave structures variously to the boroughs and the DTp. The hand-me-down process did not help with maintenance regimes, funding or record keeping. Plus, many of the flyover structures were designed to use under-carriageway heating to prevent ice, but that was rarely reliable so they ended up being salted anyway, and weren't designed to withstand the corrosive effects of salt. As a result of all the above there are a lot of major roads in London that look very threadbare these days.

The Chiswick Flyover, though, belongs to National Highways. They do have a more stable funding structure and have not been deferring maintenance in the same way, but they appear to be battling against the deterioration of a structure that is very difficult to maintain. As you say, the elevated length of M4 seems to suffer terrible spalling, which may or may not be a sign of terrible structural problems, but certainly isn't a reassuring sight.

I was also idly thinking of how many UK trunk road structures have been replaced recently due to structural deterioration as opposed to changes in traffic patterns.
My list was quite short (although my memory has probably forgotten loads)...

I'm sure there are others, but one that springs to mind is that a whole series of footbridges and accommodation bridges over the M4 between Reading and Bristol were replaced with new steel structures in either the late 90s or early 2000s. I never used the road before then but I'd hazard a guess the originals were all of the same design and all suffered the same problem.

[wrinkly]

I was also idly thinking of how many UK trunk road structures have been replaced recently due to structural deterioration as opposed to changes in traffic patterns.
My list was quite short (although my memory has probably forgotten loads):
Several footbridges over M3 and M4 where concrete deck replaced with steel truss
A38 Marsh Mills Viaduct
A34 Wolvercote viaduct west of Oxford.
Woodlands Lane Bridge over M3 J2-3 (post smart motorway works)

A bridge on the A30 (perhaps over the Tamar on the Launceston bypass?) about the same time as the Wolvercote one.

Mossband viaduct on the A74 was replaced for the M6 Cumberland Gap in 2008 but would have had to be replaced very soon anyway. Post-tensioning tendons.

Bridge carrying the westbound Bowdon spur over the M56.

Bridge carrying Thorley Lane over the M56 near J5.

Railway bridge over the M62 near Rochdale, coming up this year.

Marsh Mills which you mention was accompanied by some non-road structures in the SW such as the Royal Devon and Exeter hospital.

[fras]

Road salt and water penetration are the two worst things you can routinely subject reinforced concrete to, and that is what most of our major road structures are made from in a climate where we have lots of water. Structures therefore do have a limited life.

I'm not disputing that concrete ones do.

When they're built now is there a plan for how they'll be managed when they need replacing without causing absolute chaos?

I'm sure you already know the answer to that... build and forget seems to be order of the day. It's someone else's problem in 120 years time (even if empirical evidence suggests we're now facing a major structural timebomb!)...

I was idly figuring out how you'd keep everything moving to replace the M6 viaducts in the West Midlands, and the answer is simply you can't. It'd be a decade long disruption as seen in the USA where freeways have been rebuilt from scratch due to similar problems.

Build the western link between M6 and M5 first, then you can take a bit more time.
A lot of traffic on the Midlands Links (to use a term once commonly used to describe them), is commuter traffic. Public transport in the West Midlands is now hugely better than when I started living in Birmingham in 1971. The Midlands Metro is being expanded and could be expanded a lot more, there is now a very busy local railway network too. So I think the job could be done, but it would certainly take quite a many years. In addition, there is the M6 Toll. This could be made toll-free by government for the duration of the works.

Then there is Spaghetti Junction to deal with !

[wrinkly]

Bletchley flyover (rail over rail and road) was going to get a relatively minor revamp for East-West Rail but its condition was found to require complete replacement of the centre section by a box structure.

[2 Sheds]

I dispute the idea that all structures have a limited life. As long as you're not talking about major damage of the sort that could occur at any time, or issues like ground movement, a properly-maintained masonry arch should last pretty much indefinitely.

That can be the case for arches built of stone. Like cathedrals. Not so for brick arches (which come under the classification 'masonry arches'). Bricks deteriorate with age and given enough time begin to soften and revert to clay (we don't really know though yet for the harder engineering bricks, introduced in the Victorian era). Some brick arches are rather like 'Trigger's broom' and have had extensive replacement of brickwork. (non Fools and Horses fans look it up). Some have been strengthened with precision directional drilling and insertion of steel/stainless steel rods to prevent hinge formation (which is a characteristic of most arch collapses).

[Lonewolf]

The Wentbridge Viaduct between Darrington and Barnsdale Bar on the A1 has had one carriageway closed for months I don't know what the issue is but they finally finished after about 10 months. Then they rearranged the cones and started on the other side

[RichardA35]

I forgot this example of replacement piers as a result of IIRC Thaumasite attack rather than ASR.
Here the deck was sound but the piers were compromised so a transfer structure was constructed underneath.