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  1. #1
    The swimming rodent Treerat's Avatar
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    Airline use (land use) - any DCI risk at depth?

    An odd question that I probably should know the answer to but.......

    On a course last week we were looking at sewer and combined space rescues. For the practical exercises were were lowered 10m underground, carried out search and rescue then retrieved the casualty to the surface.

    Although we usually work fairly shallow our kit allows us to drop a maximum of 50m with a working duration (9l cylinder) of 47 minutes. However we are looking at extending this with a surface supply.

    Is there a risk of DCI working below ground but out of water? I seem to remember that there were a number of deaths when the London sewers were built that they put down to the bends? Is there a profile that can be followed as per our scuba ones to help reduce any risks?

    I asked the question on the course but was told that the risk didn't exist, however this doesn't seem to fit with what I can remember.

    Ta
    Andy
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    www.gafirs.org.uk
    If it moves - canoe it, if it doesn't dive it!

  2. #2
    Gimme a medal BenL's Avatar
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    At ground level, the pressure difference is approx 1mBar/10m. For an open shaft, I expect this to remain so. Otherwise all those miners would get bent at the end of every shift.
    I don't want to get technical or anything, but alcohol IS a solution

  3. #3
    The swimming rodent Treerat's Avatar
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    Quote Originally Posted by BenL View Post
    At ground level, the pressure difference is approx 1mBar/10m. For an open shaft, I expect this to remain so. Otherwise all those miners would get bent at the end of every shift.
    They are breathing air forced down from the surface which pressurizes the mine rather then self contained air?
    Andy
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    www.gafirs.org.uk
    If it moves - canoe it, if it doesn't dive it!

  4. #4
    Gimme a medal BenL's Avatar
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    Quote Originally Posted by Treerat View Post
    They are breathing air forced down from the surface which pressurizes the mine rather then self contained air?
    Sometimes, yes. But then due care in those cases is made. But in your case, it's all millibars, surely? With a surface supply using a demand valve, then should be no problem? An instructor friend of mine uses Felix Baumgartner's record skydive to illustrate: he fell from 40km and covered approx 1000 milliBar. Descend 30m in the water and you've covered 4000 millibars.
    I don't want to get technical or anything, but alcohol IS a solution

  5. #5
    Last of the Mohicans gobfish1's Avatar
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    Quote Originally Posted by Treerat View Post
    An odd question that I probably should know the answer to but.......

    On a course last week we were looking at sewer and combined space rescues. For the practical exercises were were lowered 10m underground, carried out search and rescue then retrieved the casualty to the surface.

    Although we usually work fairly shallow our kit allows us to drop a maximum of 50m with a working duration (9l cylinder) of 47 minutes. However we are looking at extending this with a surface supply.

    Is there a risk of DCI working below ground but out of water? I seem to remember that there were a number of deaths when the London sewers were built that they put down to the bends? Is there a profile that can be followed as per our scuba ones to help reduce any risks?

    I asked the question on the course but was told that the risk didn't exist, however this doesn't seem to fit with what I can remember.

    Ta
    some of the digging will have hit water , they used to pump in air to keep the water at bay , so the workers would be under Pressure , ie caisson worker',s

    had tables be4 divers that s were are tables started ,

    http://www.google.co.uk/url?sa=t&rct...,d.ZGU&cad=rja
    Last edited by gobfish1; 31-07-2014 at 09:18 PM.

  6. #6
    Nicotine, valium, vicodin... notdeadyet's Avatar
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    Yeah, tunnels would have been pressurised with air locks at the entrances while they were digging them. Same for bridge footings in big rivers like the Hudson in New York, they'd build a wooden dome on the river bed called caissons, blow it with air and the workers could construct the foundations in the dry. Workers used to suffer caisson disease, it got known as the bends because a dance called the Grecian Bend was in fashion at the time.

    Unpressurised you'd have to go to a hell of a depth underground to risk a bend.
    Caliph Hamish Aw-Michty Ay-Ya-Bastard, Spiritual leader of Scottish State in England

  7. #7
    Last of the Mohicans gobfish1's Avatar
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    Quote Originally Posted by notdeadyet View Post
    Yeah, tunnels would have been pressurised with air locks at the entrances while they were digging them. Same for bridge footings in big rivers like the Hudson in New York, they'd build a wooden dome on the river bed called caissons, blow it with air and the workers could construct the foundations in the dry. Workers used to suffer caisson disease, it got known as the bends because a dance called the Grecian Bend was in fashion at the time.

    Unpressurised you'd have to go to a hell of a depth underground to risk a bend.
    over 100 bent doing the footings on one new york bridge ,

    think thats also were the deco beer came from ,

  8. #8
    Prior Member Tim Digger's Avatar
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    The very deep high altitude mines like those in the Andes (remember the trapped miners a few years ago) are supplied with pressurised air for ventilation, the combined increase in pressure in the confined space caused by the emergency air supply without obvious provision for exit of gas, and the extreme difference between exit altitude and working depth required them, when rescued to be decompressed slowly.
    Otherwise unless there is some similar reason why there is a significant pressure differential (>50kpa , half a bar, 380mmHg) between working place and surface then no problem. Caisson working on river footings is pressurised to keep out the water and is the original described cause of DCI (St Louis Mississippi bridge mid 19th C).
    Evolution is great at solving problems. It's the methods that concern me.
    Tim Digger

  9. #9
    Old but keen Mark Chase's Avatar
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    Quote Originally Posted by Treerat View Post
    An odd question that I probably should know the answer to but.......

    On a course last week we were looking at sewer and combined space rescues. For the practical exercises were were lowered 10m underground, carried out search and rescue then retrieved the casualty to the surface.

    Although we usually work fairly shallow our kit allows us to drop a maximum of 50m with a working duration (9l cylinder) of 47 minutes. However we are looking at extending this with a surface supply.

    Is there a risk of DCI working below ground but out of water? I seem to remember that there were a number of deaths when the London sewers were built that they put down to the bends? Is there a profile that can be followed as per our scuba ones to help reduce any risks?

    I asked the question on the course but was told that the risk didn't exist, however this doesn't seem to fit with what I can remember.

    Ta

    As above ask them the diferance with the caisson workers?

    I can only asume its the breif time your down with makes the exposure negligable. Those guys were probably don 12 hours or something daft.

    ATB

    Mark

  10. #10
    Formerly sbc23cam Steve Clark's Avatar
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    Traditionally, caissons were sunk under pressure to prevent water filling the excavation. Men worked inside these and everything came in and out through an airlock. 'Dives' such as 15-20m for 8hours, then instant decompression! Lots of people injured and killed. Brooklyn bridge in NY was one of the first major examples.

    Occasionally, this technique is still used today. One of the shafts on the Jubilee line extension was converted into a sealed pressurized caisson when they were inundated from an aquifer. Modern techniques such as cased piling and diaphragm walling, pressurized using bentonite and tunneling with earth pressure balance tunneling machines have made working in compressed air less common. (Although fitters still need to maintain the tunneling head by passing through an airlock)

    If you are working at atmospheric pressure (i.e. in a sewer), there are no deco issues. You just need enough air to get out if the atmosphere is un-breathable.

    Steve


 
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