The maximum depth of the Channel Tunnel is 114.9 meters below sea level and the crossing takes about 35 minutes. Obviously nitrogen narcosis isn’t a problem since people make the trip every day without incident. I’m curious about how close it is to being a problem.
A little digging seems to indicate that a 114 meter scuba dive with a total bottom time of 17 minutes (start of dive to beginning of ascent) might be a situation requiring decompression. Is the tunnel experience different because the passengers aren’t immersed in water and/or using scuba equipment? Perhaps the relatively gradual ascent is sufficient decompression for this relatively short “dive”. Are the train cars decompressed (relative to the outside air) similar to the way airplane cabin pressure is compressed?
What happens if something goes wrong? If a train is stopped at the deepest point for any length of time and then evacuated, might that increase nitrogen narcosis concerns?
Which raises an interesting question itself: how deep in air would you have to go before risking decompression problems if raised to 1 atmosphere very quickly. Or is it even an issue? Anyone?
FTR, the EuroTunnel train is underground for only about 20 minutes.
After further reading…
Nitrogen narcosis becomes a problem starting around 30m (in water). If my 1200x factor from above is correct, that would be over 22 miles in air. The deepest mine so far seems to be about a tenth of that.
Just for clarification: nitrogen narcosis and decompression sickness (DCS, or “the bends”) or (more generally decompression illness (DCI), which encompasses the various embolisms and barotraumas that can result from rapid pressure changes) are two entirely distinct syndromes. The former is the result of nitrogen at high pressure in the central nervous system, which is poorly understood, and the latter the effect of gas coming to equilibrium in tissues and then not being able to outgas sufficiently with the pressure drop during ascent.
Travellers using the Channel Tunnel don’t have to worry about either because of the reasons mentioned above; the air in the Chunnel is essentially at ambient surface pressure. However, workers operating in pressurized vessels like caissons can and, before the seminal work on the biological effects of high pressure respiration by John Scott Haldane, did become ill or die from DCS (though the pressures weren’t high enough to result in nitrogen narcosis).
Correct. The only way to deal with narcosis is either to stay above the depth at which it occurs (which varies widely for different people; I’ve been down to recreational limits without suffering the effects, other people are affected at <80 feet) or switch to a breathing gas that has a lower fraction of nitrogen in it.
Nope. The layer of rock above (and around, and below) the tunnel makes it a pressure vessel, although unlike your standard PV like a propane tank, the pressure is lower inside rather than outside. the PC apeman had it right in post #3 (though I haven’t checked his numbers); the pressure is determined by the weight of the air (or in the case of being submerged, the water) above you. Since the tunnel is holding up all the water, all you have to do is hold up the 60 miles or so of air above you. Quite a balancing act, non?
The concrete and other structure is certainly under a great deal of pressure. This does not pressurize the interior any more than a sky scrapper pressurizes the air down at the first floor. The Chunnel has structural integrality that does not require pressurized air within to keep it from collapsing. If it was pressurized, the air would just run out either end.
Unlike a SCUBA diver, the Chunnel does not need to expand and contract to get air into its lungs. It’s a ridged structure.
I have never been ‘narked’ out. I’ve never felt the effects up to about 120 deep in recreational diving.
The bends are caused when you assend and nitrogen in your blood begins to expand and can cause bubbles in your blood. Narcossis is the effect that nitrogen has at depth when it is in solution in your blood.
Decompressing allows the nitrogen that is in soulutin in your blood to slowly be released. If you don’t decompress, these gasses become bubbles, which can cause a number of different problems.
I can’t find my copy of Life at the Entremes, but IIRC it has a discussion of the effects of decompression when being raised from sea level to higher altitude.
There is also a ventilation system installed in the tunnel, with massive fans on land at either end of the tunnel.
But it is not normally needed – the passage of the trains thru the tunnels pulls enough fresh air along with them. So this fan system is officially known as the SVS - secondary ventilation system.