A spin off from the discussion in Tankard Shaft Dig thread.

It was remarked that sandbags might decay when used as supporting walls underground. Well some do and some don't so it depends on what long term effect you want.
We are currently using polypropylene sandbags as when they are hammered flat and chocked with small rocks they can make a good retaining wall. Also there is a useful use for excavated material. If you are worried about aesthetics you can face them off with a rock wall. Polypropylene bags are used in earth building construction though in time they might be prone to UV damage. Underground they may well have an indefinite lifespan as this report suggests.

"  One report said 30-year-old polypropylene bags were recovered from a landfill (which is approx. how long poly bags have been in existence) and they were still in good condition.
- another report said the lifespan could be closer to 500 years: see www.gcswall.com: "An FHWA study sometime later concluded that the half life of polypropylene fabrics in these benign environments could be 500 years or more, which exceeds the service life of about any construction material used in our transportation facilities."

So there may well be no worry in using them for long term support in a cave dig.

Hessian bags will rot away in time so can be used to build mud walls or with contents mixed with a little cement.

Scaffolding, Acro Props , cement walling and polypropylene walling in a Mendip dig.

Polypropylene sandbags etc in a Mendip dig.





Bag filling at the dig face.

Hessian bags are made from hemp or jute and in a damp cave environment may well rot in one or two years. It depends on what long term effect you actually want. Cost wise polypropylene sand bags are much cheaper than hessian bags. In Spring 2016 you could get 20 - 25 polypropylene bags from eBay at under ?7 post free.

The Old Ruminator said:

So there may well be no worry in using them for long term support in a cave dig.

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Excepting abrasion.

We also use polyprop bags for spoil. Almost all the fill we have is silt and sand, there is just no other way to stack it up. What little rock we do find, we wall up to make it look nicer, but it is in very short supply.

In many areas, it is necessary to climb up/ over stacked bags to get through the cave. In these places, the bags do become ragged after a few years...

It is our long term intention (assuming the dig goes) to remove as many bags from the passages as possible, to be stacked in the large chamber we will find, then walled up in front in stone

Which is of course the other benefit of bags; you can relocate spoil very easily as and when the need arises.

The Old Ruminator said:

Cost wise polypropylene sand bags are much cheaper than hessian bags. In Spring 2016 you could get 20 - 25 polypropylene bags from eBay at under ?7 post free.

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I get mine free from a local brewery, who otherwise have to pay to dispose of them. Let me know if you want some.

I normally get about 200 at a time, so with more than a days notice, I suspect you'd get a few more  :-\

Always do a head count after every digging session:

One thing that does need to be considered is the aesthetic impact of using bags underground. 

Very true. The ones in the post above are stabilising a route through a roof-fall so don't detract from the quarry system on either side.

tamarmole said:

One thing that does need to be considered is the aesthetic impact of using bags underground. 

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You are quite right and wherever possible/ practical, if bags are to become a permanent structure, it isn't a bad idea for them to be walled up behind stone to give a more "natural" look.

In the shorter term however, in the case of an active dig, which is after all what we're talking about, the "cave" would not be there without the digging, so anyone upset by the aesthetics need not visit, just as though no digging had taken place.

In the case of The OR's dig, which has been dug from surface, I don't see any concern at all with how it does or doesn't look. The construction and engineering of a dig tells the story of the work that's gone in. The construction work is as much part of the cave as any other feature.
That's not to suggest diggers should deliberately make their dig a mess! 

In the case of spoil from a dig being brought back to "existing" cave, more consideration is needed to preserve the nature of the cave as far as possible, while balancing this against the benefits of digging and potential new discoveries. After all, without digging, we wouldn't have half the caves we have 

The woven polyprop bags used by O.R. are very much preferable to plastic sacks because they allow for draining of water, so any damp clay solidifies.
Just beware of stacking them in a wet environment, wetness may cause them to slump, causing some interesting problems. ---

I still maintain that a couple of handfuls of cement in each bag would be an advantage, providing you are not intending to move the bags in the near future.

Of course any alteration to a cave involves some sort of destruction. That's why we record everything in photos as we go along. I have 1,500 images of the current dig. That's way over the top but I like doing it. At Vurley we are creating new cave out of total blockage and to be honest the bags became a necessity for handling the excavated material as we are keeping the shaft fairly discrete. The more void you create the more problem you have with keeping it all safe. We found it easier to manhandle bags to the upper chamber than trying to maneuver the kibble up there . They could be stacked on ledges and moved a short distance at a time by a small team. I suppose a neat wall of bags is hardly less aesthetic than a load of rusting scaffolding and rotting timber.
Back at Skyfall we used farm feed bags . It was a near vertical upwards dig and we needed to stop excavated material slumping back down the rift and blocking our way in. The bags had also to survive the drop down the ladder pitch which in fact most did.

Ready for the drop at Skyfall.

Traditional sandbags are the right size for lifting up the shaft by hand. We too had access to free brewery polypropylene sacks but found that they were too big and the plastic inner liner was a nuisance. Most sandbags have draw strings so can be tied off when full. I was surprised how stable a sandbag wall became when bags were chocked and hammered flat. We are continuing to use most methods of support in the dig. We would love to emulate the great Willie Stanton with roomy tunnels. drystone walling and neat steps. That all took an enormous length of time and a specialist wall builder. For us its a matter of gaining depth as quickly and safely as possible with the hope of finding a void big enough to accommodate neatly all of our spoil.
Its possible that we are now back on track and going in the right direction. Our wall building team have returned from their winter break so we might not need so many sand bags in the near future.

The Old Ruminator said:

with the hope of finding a void big enough to accommodate neatly all of our spoil.

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You know something's wrong, the first time you realise you only want a breakthrough so there's enough space to stack more spoil! 

Do you think there is therapy for that sort of thing on the NHS? 

A really big breakthrough spoils a good dig.

But it does give you an reason for starting a new dig

The small white bags I get for free from a local seed merchant who supplies farmers and repacks for shops.
Otherwise they are burnt !No plastic liners either.
If the white one's are exposed to direct sun light for several months they fall apart ,under ground they last
possibly for ever ,if not abraded !
If you need to use bags on the surface ,you must use coloured ones .
Our local council now use black ones,has been a slow learning curve  !

You can fill a bag with liquid mud/clay and when stacked and left for a week  it has set like a brick
when the water drains out.has been done by us several times ,even with one ton bags.