Hi Dave,

Joel tells me you have been trying to get in touch about airlifting. I recall an email from you which I couldn't reply to. If you're thinking of airlifting somewhere, let me know. I'm still at the same e-mail address.  Besides having 300m of 19mm hose which you could use, I learned a hell of a lot in Llygad which I wish somebody could have told me about before. To the best of my knowledge, it's a system thats only been used in Pwll-y-Cwm, which of course, isn't really underground. Angles of lift, volumes of air, size of lift, materials to use, methods of extending as dig progresses. Many factors to consider.

Cheers, Mike Barnes

Many years ago Andy Goddard and I dug out the connection between Aquaflash Pot and Joint hole using lifting bags, so it is a technique which has been practically applied at other sites. Actually they were improvised lifting bags made of tacklesacks lined with bin liners. It worked pretty well, but the main improvement (apart from using proper lifting bags!) which struck me was the importance of rigging up a pulley system to redirect the force without losing so much from friction. You hardly ever want to pull straight up, but that's the only thing a lifting bag will do.

Mark

MSD said:

Many years ago Andy Goddard and I dug out the connection between Aquaflash Pot and Joint hole using lifting bags, so it is a technique which has been practically applied at other sites. Actually they were improvised lifting bags made of tacklesacks lined with bin liners. It worked pretty well, but the main improvement (apart from using proper lifting bags!) which struck me was the importance of rigging up a pulley system to redirect the force without losing so much from friction. You hardly ever want to pull straight up, but that's the only thing a lifting bag will do.

Mark

Click to expand...

I wouldn't for one moment want to say that experience with lifting bags is of no importance, Mark, but I don't think that's what Mike is talking about. An airlift, in my understanding, is like an underwater vacuum cleaner, where the 'suction' is provided by releasing air into the bottom of the suction pipe. If you get the airflow right, you can suck gravel etc. up with it as it the air rises up the pipe. I remember reading about it years ago in a book which was talking about looking for old bottles in underwater rubbish dumps. I seem to recall the book saying, much as Mike has done, that there are some critical factors in the design and use of such a system which make a big difference to how successful it is. 

Personally (as you know), I'm not a diver so my interest is largely vicarious, but I'd be interesting in reading some detailed information on either technique if you and/or Mike would care to tell us more.

Nick.

Am I right in thinking this is the same technique underwater archaeologists use to excavate stuff? The spoil can be passed over meshes to catch the doubloons and pieces of eight, leaving the wreck exposed for study or lifting. If so are there any relevant technical papers that our more academic members could ferret out? Probably totally different and irrelevant, but maybe of use / interest...

Ah, I see. Obviously I misunderstood the original post.

http://www.weymouthdiving.co.uk/equipment.htm  gives some reasonable info and diagrams about both water lifts (purely venturi effect) and air lifts (enhanced venturi effect due to the air expanding as it goes up). Marine archaeology is indeed a key application of this technology.

From what I could assimilate in a quick read, airlifts require a reasonable head of water abover the dig site (minimum 6 metres), but are more powerful under optimium conditions. That figures. In the wikpedia entry (wikpedia covers everything these days!)  it further mentions that the air flow should be pulsed for best effect. I don't quite understand why this should be so.

In a commercial context an "airlift dredger" is a quite different design, with a closed chamber which is sent down to the floor. This is first filled by suction and then vented to the surface by high-pressure compressed air. Thus it is partly a forced flow system rather than a passive suction system.

It was pretty cool stuff actually and I enjoyed reading about it. I remember being thwarted by a sand choke in a sump which this technology would be able to tackle.

Mark

Which sump?

Big sea dredging companies use it quite effectivly as well. especially in dubai

surely a pure air lift would only get your material to the surface.
to get above the surface and spray over a grid for archaeology etc, i would have thought you needed a pump. Certainly the documentaries that i've seen on building the artificial islands in Dubai used pumps to spray the water / sand / gravel mix to build up the island

Yeah they airlifted it up into the holds of the dredger, moved it and then used pumped water canons to shoot the dredged material in the right places. you could probable make one using a hose a small compressor and some bits of pipe.

Air lifts just take the gravel (or what ever) from one end of the pipe to the other. The "outlet" end can still be underwater, just away from the dig site (though the air will obviously head to the surface once out of the end of the pipe). Not used one myself, but the principle is fairly straight forward. The devil is, as they say, in the detail!

Cheers,

Marcus

In fact, an airlift can lift the water and whatever is in it, 10 percent of the lifts length above water level. I think this is down to the momentum of the water rising up the pipe, combined with the continuous water/air going in at the working end. They really are incredibly powerful tools. In Llygad, armfulls of gravel dissappeared up the pipe with each breath. I likened it to hovering out a lorry load of rice crispies with a powerful cleaner like a Dyson, but without ever needing to empty the bag. A head of 6m is also not necessary, though the more the better. Indeed, most fish tanks in the country have an airlift in the corner, even though they are not there to move anything but the water itself, oxygenating it in the process.

Theoretical situation:

Passage blocked by gravel at bottom of 12m deep shaft reached through a shallow passage maybe 50-60m long. If we ran a pipe all the way from the dig to the sump pool, would an air lift have sufficient power to suck the gravel all the way? We don't want to dump all the gravel in the shallow passage since it is quite small.

Second question: rought how many litres/minute of air are needed? I'm wondering whether it's realistic to power such an airlift from big (read 15 litre) cylinders if you don't make the hose diameter too large. According to  http://www.weymouthdiving.co.uk/equipment.htm  a 75 mm diameter hose requires 300-600 litres of air a minute. If you could get away with 300 litres/hour a 15 litre 232 bar cylinder would last about 12 minutes. Not fantastically long, but maybe long enough to get something done.

Unfortunately I would hazard a guess that my requirement to move the gravel 50m horizontally will require both a bigger hose and more air....

Any tips?

Mark

Hi Mark,

Interesting, you'd get excellant power rising up the 12m shaft but don't know if an airlift would do the horizontal bit. A water dredge would work though haven't used one myself. How far is the sump from the entrance, and whats it like getting to the entrance. Carrying a 15cfm petrol compressor at llygad the 100m or so, nearly killed me. I had nonstop problems using tanks. I thought connecting 2 15ls at the entrance would give me 20 odd minutes at -15m. No chance. Leaks, 1st stages not supplying enough air, connecting 1st stages to 19mm pipe, a list of problems to long for me to list. my dive log says it all, 'NEVER use tanks again, they DO NOT work'. I must have used thousands of cfm's, my dive shop would have loved me if I'd pursued with tanks.

Does the sump resurge? if you dumped at the top of the shaft, would any current clear the route. A water dredge needs a lot of water to work and of course, you'd need a supply at the entrance for the pump. Friction could also be a big problem if it has far to travel. I found tables on the net, but can't remember where. for an airlift, I wonder if  you had a bigger diameter pipe in the shaft to creat the power, which goes to a smaller pipe in the level. It might block at the reduction part. Is the shaft nearly vertical? I found this gave huge power, which at Llygad I started with. I dumped about 20m up fom this point with the pipe running at a very gentle rise. using flexible hose was a mistake, it kept blocking. Wickes mini downpipe was ideal.can't remember if it was 68 or 58mm. Of course, thats only suitable for small gravel. I found that as the dig progressed, i had to put the lift itself into the 'Slot' where the angle dropped to maybe 30-35 degrees. This dropped the power a lot, but could still dump along the 20m levelish pipe. You might be Ok going the full 50-60m bit because of the power created in the 12m bit. Why not knock up a test rig simulating the conditions in a quarry somewhere. I've still got the 300m of 19mm hose, you're welcome to use it.

The sump is not that far from the entrance (15 minutes caving maybe), BUT it's in northern Sweden, so it's pretty remote. The shaft is completely vertical. I think we could dump some material in the passage at the top, the problem is to not block it! I think material lifted to the top of the shaft will be swept away in flood, the main problem is that I can't keep going there (it's 1500km away!). We need an intensive "siege" over a few days, not a week, by week campaign.

I have a portable compressor for diving tanks as does a friend, so filling tanks is not so expensive. However, I take your point about problems with the first stages delivering sufficient flow.

Why does flexible hose block? Were you using corrugated hose or smooth sided?

Thanks for the offer of the hose, but it's a long way for me to come and get it

Mark

Mark, been thinking about your possible project. Reckon that if you stuck with the same size pipe, you wouldn't have any blocking problems. The flexi hose I used was ribbed but the inside was fairly smooth. the problem arose by the billions of pieces of gravel, some of which were produced by various blasting/chieseling ops over the years, inevitably meeting inside the pipe. With the slight ribbing, it was enough to snag and of course, very quickly create a major jam. This didn't happen  as soon as I started on the rigid downpipe. The slot was at a right angle to the passage where the stuff was dumped, so here I used a very short piece of flexi pipe to connect the 2 rigid pipes. 1 connecter was left untaped so any blockages here could be quickly cleared. However, didn't need it. If you can keep the 'horizontal' pipe rising, even if at a very gentle, continous angle, I think it would work. If the top of the pipe in the shaft was kept at floor level, rising to the roof at the surface, you could be in buisness. Any roof bulges may be a problem.  Don't forget that once it's working, the air in it will want to take the whole thing to the roof, so either rocks or lead wieght or any naturel belay will be needed to keep the angle right. I used scaffold pipe for the working end to avoid bouyancy probs. Something I learned which was really usefull was that that excellant stuff, Duck tape can be used underwater, really good for joining the pipework insitu. The cardboard interior roll falls apart straight away, so I transfered enough for the days work onto a small offcut of the downpipe pipe. It was so good, once, when the above mentioned untaped joint pulled apart whilst digging, I realised the joint itself had become a bit loose. At first it seemed like the dive was over, but incredibly, lying there right next to me was a roll that I'd lost 2 weeks before. It worked as well as if it had just come from the factory, despite 2 weeks of emersion. The cheaper imitations were nowhere near as good. The problem with my tank attempts was that I cobbled bits together. If I had got a better valve/hose connector, maybe made to order, there's no reason why it shouldn't work. 3 tanks/1st stages all going to a manifold should be ok, plus giving a reasonable dive/dig time.