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There is a reason that the number of railway-only suspension bridges is almost zero; but, I submit, it could work on principle. However, questions:

  1. Are there any precedents to such a buoyant bridge-tunnel for transport?

  2. What would give the buoyancy? Something like baloons?

  3. How would the watertightness of the tubes be ensured? (Double, triple tube?)

*Lunatic*, n.
One whose delusions are out of fashion.
by DoDo on Mon Dec 21st, 2009 at 03:29:05 PM EST
[ Parent ]
  1. boats and submarines?  I don't know of any archemdies tunnel in existence, but there are plenty of structures in the ocean with the same principle - oil platforms often use buoyant legs to reduce the total strength requirements.

  2. Displacement of water: if the tunnel is say 10m in diameter it displaces roughly 80m^3 per metre, which is much much more than any train weights (that would be a lot more than 80tonne axle load!), even with the tunnel itself weighing a few tonnes per metre there is far more lift available than any train would require.

  3. I think so.  We can hypothesize various simple strategies: double wall with inner pair of train tubes and outer service tube, fast closing doors every few km, automatic breach detection and autopilot system (if a breach is detected behind, accellerate away, before, decelerate and reverse at maximum rate), automatic detactment and floating in the case of severance, safety tanks where people can climb in and float to safety. etc.
by njh on Tue Dec 22nd, 2009 at 05:11:52 PM EST
[ Parent ]
  1. None of those is an example of something with a vehicle running across it, e.g. an example for mastering the vibrations of the elastic structure in practice. (I don't say it's impossible, just untested.)

  2. I don't think the tunnel would be buoyant all by its own displacement. The typical cross section of a single-track tunnel is somewhat less (c. 50 m³), but the per metre displaced water is still an order of magnitude above the per metre weight of an European train (I think you can get as high as 100 tons for 10 metres = 10 t/m in Britain [25t axleload self-emptying car for mined stuff]). However, I would estimate the tunnel walls to be much heavier. for example, the immersed tube of the Marmaray Tunnel comes in at 140 tons per metre (or 70 tons per track), and a 'double-hull', buoyed tunnel with two tubes for high-speed tracks should be heavier than that.

*Lunatic*, n.
One whose delusions are out of fashion.
by DoDo on Tue Dec 22nd, 2009 at 06:13:17 PM EST
[ Parent ]
  1. I imagine there are plenty of vibrations in km tall drilling platforms.

  2. But the Marmaray is meant to sink!  As I suggested to JakeS, his (indirect) suggestion of ferrocement is very appealing, given its lightweight, toughness, and proven record with maritime applications.  There are 100 year old boats still floating in the sea.  AAC (aerated concrete) with a ferrocement outer would be even better, given its positive natural buoyancy.  We know that positive buoyancy is possible, given that there are ships that carry trains.  It's then just a question of diameter and materials choice.
by njh on Tue Dec 22nd, 2009 at 06:34:08 PM EST
[ Parent ]
But the Marmaray is meant to sink!

Actually, it is meant to float: before the interior is completed, just at the limit of buoyancy so that it can be towed into the right position before it is sunk to its place. Buoyancy doesn't matter once it is at the bottom and covered over.

*Lunatic*, n.
One whose delusions are out of fashion.

by DoDo on Tue Dec 22nd, 2009 at 06:49:56 PM EST
[ Parent ]
1) And in skycarpers and radio towers and suspension bridges and cable-stayed bridges too. But the typical vibrations and the critical components in all of these are different and need to be tested.

Ferrocement, and/or larger tube diameter for buoyancy, now that I don't see why it can't work. You should patent it :-)

*Lunatic*, n.
One whose delusions are out of fashion.

by DoDo on Tue Dec 22nd, 2009 at 06:57:31 PM EST
[ Parent ]
A cubic meter of cement weighs something on the order of ten times as much as a cubic meter of water.

"A few tons per meter" is unlikely to cut it.

- Jake

Friends come and go. Enemies accumulate.

by JakeS (JangoSierra 'at' gmail 'dot' com) on Tue Dec 22nd, 2009 at 06:13:47 PM EST
[ Parent ]
Sorry for nitpicking, but
  1. cement is actually rather light, c. 1.5 t/m³;
  2. concrete is more dense, up to 2.5 t/m³;
  3. however, methinks the buoyant tunnel would be made of steel, which is much more dense than concrete: 7.8 t/m³.

*Lunatic*, n.
One whose delusions are out of fashion.
by DoDo on Tue Dec 22nd, 2009 at 06:24:42 PM EST
[ Parent ]
portland cement has a density of 1.44 (that is, m^3 of portland cements weighs 1.44 times that of a m^3 of water).  Concrete can go as high as 3 with very dense aggregate.  Are we talking about trainloads of cement here, because any load will be subject to the axle loading of the rails which sets an upper bound on the train linear density.

Or are we talking about the tube itself?  I was considering the tube to be made of steel alone, but now you mention it, a ferrocement design would be much lighter (steel has a density of about 8).  There are many ferrocement boats out there and ferrocement is lighter than concrete.

by njh on Tue Dec 22nd, 2009 at 06:27:27 PM EST
[ Parent ]


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