How not to build a tunnel...

[David Bailey]

Pictures of the Gerrards Cross tunnel collapse here (http://freespace.virgin.net/guy.gorton/collapse.html).

So comforting to know that we entrust our lives to these people...

[JoC]

Don't know where you got those pics from DJ(M) but very interesting! The two sides of the tunnel look really badly aligned in one of thise middle pictures don't they, I wonder if those joints were properly locked before they finished construction?

Amazing that every little scrap of land has got sufficiently valuable to make this kind of engineering viable.

[philsmove]

My be we can persuade David Reid’s to post here

http://www.cerocscotland.com/forum/showthread.php?t=5717&page=2&pp=20

[Pammy]

I wonder if those joints were properly locked before they finished construction?

Does "properly locked" matter to Mr Tesco when he's assessing important decisions about where to place one of his countless new stores... :whistle:

[Robin]

Does "properly locked" matter to Mr Tesco when he's assessing important decisions about where to place one of his countless new stores... :whistle:

Not considering the amazing 1p in every 8p in the UK is spent there.
Makes you wonder when they'll be launching a takeover bid for The Government© and Great Britain(UK) PLC

[Pammy]

Not considering the amazing 1p in every 8p in the UK is spent there.
Makes you wonder when they'll be launching a takeover bid for The Government© and Great Britain(UK) PLC

You mean they haven't? :confused:

I think it's just not "public knowledge" :whistle:

Then again, we were talking about Tesco's, now if we'd said Asda's, that'd be a different kettle of fish... :wink:

[Ladybird]

I'm quite surprised how thin the concrete sections are, in comparison to the amount of stuff on top of them. No matter how strong arch constructions can be, surely there's got to be a "give" point when too much weight is placed on top?

Not to mention being surprised that someone wants to photograph tunnels for a hobby (with a ladder?) for a personal website - or am I just lacking something in my life?

[David Bailey]

Not to mention being surprised that someone wants to photograph tunnels for a hobby (with a ladder?) for a personal website
Trainspotters have their uses :)
Although if I and my family were travelling through there, I might take a passing interest in the structural safety of tunnels I travelled under...


or am I just lacking something in my life?
Dunno - but welcome to the Forum :)

[El Salsero Gringo]

I'm quite surprised how thin the concrete sections are, in comparison to the amount of stuff on top of them. No matter how strong arch constructions can be, surely there's got to be a "give" point when too much weight is placed on top?

Not to mention being surprised that someone wants to photograph tunnels for a hobby (with a ladder?) for a personal website - or am I just lacking something in my life?Arches are incredibly strong because they divert a vertical load (the weight of the material on top) sideways, loading the arch material in compression (concrete is only strong in compression) and placing a sideways load on the abutments, which consequently have to be very strong. Note the comment about a possible cause of the collapse being "insufficient side-filling before increasing the top load." Also it takes four hinge points to collapse an arch which is why arch bridges remain very stable even when the foundations shift.

[Ladybird]

Originally Posted by Ladybird
Not to mention being surprised that someone wants to photograph tunnels for a hobby (with a ladder?) for a personal website
Was meant to be a bit of a funny - didn't move on from quick post to advanced to use the smilies, which I guess I should have.

Although if I and my family were travelling through there, I might take a passing interest in the structural safety of tunnels I travelled under...

Good point - forgot that people generally take far more (and detailed) interest in stuff local to them/likely to impact immediately on them (not literally, I hope :) )


but welcome to the Forum

Cheers - have been "lurking" a while (read the FAQ - there's so much to learn!). Recently tried first post on Leroc-in-Bristol forum (in any forum, ever) and it went awol (after a lot of nerves/nervous energy expended), so have been "trialling" quietly here. Best trundle off to the "hello" thread next, and then see if I can get any better/more confident at it.

[Ladybird]

ESG - thanks. Only know a bit about arches from historical/archaoeological books/visits/documentaries. Kind of know theory (like buttresses) but they tend to use v big chunks of stone in most of the examples I've paid any attention to.


Note the comment about a possible cause of the collapse being "insufficient side-filling before increasing the top load."

whoops - that'll teach me to read the words as well as looking at the pics :grin: .


Also it takes four hinge points to collapse an arch which is why arch bridges remain very stable even when the foundations shift

don't quite understand what you mean by "hinge points"? Guess I was (in my iggerance) working on the idea of some kind of ratio that says
x top weight > y side filling and z arch material (type/thickness) = collapse?

[bigdjiver]

Arches are incredibly strong because they divert a vertical load (the weight of the material on top) sideways, loading the arch material in compression (concrete is only strong in compression) and placing a sideways load on the abutments, which consequently have to be very strong. Note the comment about a possible cause of the collapse being "insufficient side-filling before increasing the top load." Also it takes four hinge points to collapse an arch which is why arch bridges remain very stable even when the foundations shift.Dime-store-guru mode. It seems to me that different filling materials have been used, which might allow different rates of water penetration. It is possible that a period of heavy rain could place a considerable differential stress on the tunnel.

If there was a pressure differential between the two sides of the arch the thin female sides of the joint would be taking it all, and a diagonal crack there would force one side under the other. I did not like reading the report of the two sides being banged together. If the joint was slightly out of line and impacted it could cause such a crack. All this is idle speculation.

[cerocmetro]

saw this thread and fascinated by the techy talk :worthy:

But am I missing something obvious or am I just being an ex-surveyor who teaches dancing, This is not a classic arch fro two reasons, 1) it is squashed so there is a much higher loading going through the centre and not diverted down the sides and 2) I have never seen an arch that is not either a) a continuous fabrication or b) built symetrically with the loading through a centre stone.

In this case the centre and the main focal point of the load is a join :what:

I am not convinced the side filling had much to do with it, but I am waiting for them to find the black box. As they say and particularly on this occasion "keep watching this space"

Adam

[Stuart]

I was just looking up some train times on the National Rail enquiries website, and this is described as a "Technical Problem" on that!

[Lounge Lizard]

saw this thread and fascinated by the techy talk :worthy:

But am I missing something obvious or am I just being an ex-surveyor who teaches dancing, This is not a classic arch fro two reasons, 1) it is squashed so there is a much higher loading going through the centre and not diverted down the sides and 2) I have never seen an arch that is not either a) a continuous fabrication or b) built symetrically with the loading through a centre stone.

In this case the centre and the main focal point of the load is a join :what:

I am not convinced the side filling had much to do with it, but I am waiting for them to find the black box. As they say and particularly on this occasion "keep watching this space"

Adam
:yeah:
The design of the 'link beam' seems to allow the arch to buckle and collapse inward, I am amazed that this design ever got through the approval stages- unless it was not meant to take much loading.

Above the 'link beam' there appears to be a reinforcing cage to form an insitu beam tying the two sections together, but it seems this has been left as exposed re-bar, not sure why that would be unless it was part of the following.

Could it be the tunnel was intended to be covered with a heavily re-inforced ground floor slab, which meant its final loading would be minimal, but no one took account of the construction loading occurring whilst work was in progress.

It is interesting to see the 'digger' on the tunnel roof in one picture, looking at the collapse and the minimal amount of fill on top of the tunnel I wonder if it was caused by plant & machinery working on top it.
Point loading from a single item of plant is far far greater than evenly distributed loading over the same location.

The fill and the cover to the tunnel roof does not seem to aid distribution of loading, I wonder if the area was zoned to avoid working directly above the tunnel
Peter

[Robin]

I was just looking up some train times on the National Rail enquiries website, and this is described as a "Technical Problem" on that!

Well its obviously a technical problem - just not one to do with trains !
:grin:

[El Salsero Gringo]

saw this thread and fascinated by the techy talk :worthy:

But am I missing something obvious or am I just being an ex-surveyor who teaches dancing, This is not a classic arch fro two reasons, 1) it is squashed so there is a much higher loading going through the centre and not diverted down the sides and 2) I have never seen an arch that is not either a) a continuous fabrication or b) built symetrically with the loading through a centre stone.

In this case the centre and the main focal point of the load is a join :what:

I am not convinced the side filling had much to do with it, but I am waiting for them to find the black box. As they say and particularly on this occasion "keep watching this space"

Adam
The only thing that an arch requires is that the underside of the arch remains in compression along its whole length: it can still be very flat. Brunel's 1837 bridge for the GWR across the Thames at Maidenhead has two spans of 128 feet and a rise of only 24 feet. It carries trains 10 times heavier than that for which it was designed.

Details and pictures below are from a very good book called "Structures, or why things don't fall down" by J. E. Gordon (Penguin, 1978) which goes into a lot of detail on arches.

Gordon also says that "if one wants to make a really thin arch - as is the custom with modern reinforced concrete bridges - then one had better get the shape just right, for there is very little room for the thrust line to wander about."

I hope all that's of interest to someone.

[Lounge Lizard]

The only thing that an arch requires is that the underside of the arch remains in compression along its whole length:


yup
but if you introduce what is effectivly a 'slip joint' at the point of maximum compression, the integretry of the arch must become flawed methinks

you would not expect to see a key stone in the centre of a flat arch would you?

[El Salsero Gringo]

you would not expect to see a key stone in the centre of a flat arch would you?According to Prof. Gordon, "The voussoir at the top or crown of the arch is called the keystone and is sometimes made larger than the rest. Although poets, politicians and other non-technical people have attributed special qualities to real and figurative keystones, in fact the keystone is functionally no different from all the other voussoirs and its distinction, if it has any, is purely decorative."

I understand from this that the presence absence of a visible keystone is less influenced by the shape of the arch than by the architect's sense of aesthetics.

[Andy McGregor]

Ahh, civil engineering, I wondered where ESG got the idea he was a smug know-it-all? :wink: :devil:

[Lounge Lizard]

According to Prof. Gordon, "The voussoir at the top or crown of the arch is called the keystone and is sometimes made larger than the rest. Although poets, politicians and other non-technical people have attributed special qualities to real and figurative keystones, in fact the keystone is functionally no different from all the other voussoirs and its distinction, if it has any, is purely decorative."

I understand from this that the presence absence of a visible keystone is less influenced by the shape of the arch than by the architect's sense of aesthetics.not quite right, the keystone was originaly bigger because as the load increased it would in effect strengthen the arch
this was designed out of the arch as expertise grew
but in a flat arch (which is what we are discussing here) the key stone is not present, and the principle of remaining in compression is removed by the join in the middle - we are discussing a collapse here!!

a voussoir is another name for a brick
a guaged voussoir is a shaped brick designed for an arch to keep the mortar joints a constant thickness, the builders of old would 'rub' the bricks to create the correct shape
peter
sorry I do not have any books to refer to so can't include pictures

[El Salsero Gringo]

Ahh, civil engineering, I wondered where ESG got the idea he was a smug know-it-all? :wink: :devil:Erm, because I'm (almost) never wrong? :wink: :devil:

[Andy McGregor]

Erm, because I'm (almost) never wrong? Really ...


not quite right,

[Lounge Lizard]

Originally Posted by Lounge Lizard about ESG's post
not quite right,what is not quite right Andy
peter

[El Salsero Gringo]

but in a flat arch (which is what we are discussing here) the key stone is not present, and the principle of remaining in compression is removed by the join in the middle - we are discussing a collapse here!!
It does seem rather too flat at the top for its own good, doesn't it? And the "knitting" along the top seam doesn't look like it would take any serious load in bending to hold the arch up.

So your first three hinge points are the centre joint and the two places where the slabs sit on the foundations. Then they added a load of weight on top, the thrust line wandered out of the material of the arch, which developed some serious tensions in it's underneath surface, leading to cracking and the development of another hinge point (or two) causing the arch to collapse.

Maybe they did something silly like build the foundations a wee bit too far apart?

hehehehe

I'd love to know what the report of the enquiry says.

Edited to add:

here's (http://www.nceplus.co.uk/b_bank/search_results_details/?report_ID=6943&report_num=0&channelid=6) an interesting technical article on the cause of the collapse, from the New Civil Engineer website.

[bigdjiver]

Before I read the report - I figured out why the arch has to be so flat. Whilst the infill is being deposited virtually all of the force will be directed laterally inwards, and the tunnel has to withstand that. It seems very important that the construction is carefully controlled and monitored to ensure that the filling process does not place undue stress on the concrete. Now to read it ...