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Thread: How not to build a tunnel...

  1. #21

    Re: How not to build a tunnel...

    Quote Originally Posted by El Salsero Gringo
    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

  2. #22
    Registered User El Salsero Gringo's Avatar
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    Re: How not to build a tunnel...

    Quote Originally Posted by Andy McGregor
    Ahh, civil engineering, I wondered where ESG got the idea he was a smug know-it-all?
    Erm, because I'm (almost) never wrong?

  3. #23
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    Re: How not to build a tunnel...

    Quote Originally Posted by ESG
    Erm, because I'm (almost) never wrong?
    Really ...

    Quote Originally Posted by Lounge Lizard about ESG's post
    not quite right,

  4. #24

    Re: How not to build a tunnel...

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

  5. #25
    Registered User El Salsero Gringo's Avatar
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    Re: How not to build a tunnel...

    Quote Originally Posted by Lounge Lizard
    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 an interesting technical article on the cause of the collapse, from the New Civil Engineer website.
    Last edited by El Salsero Gringo; 9th-July-2005 at 07:08 PM.

  6. #26
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    Re: How not to build a tunnel...

    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 ...

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