So we have a GREAT find and a GREAT dig going, and you all being expert diggers, I thought I would ask for some advice to make sure we are not missing something. I don't want to give too much away in case nefarious local cavers google for info So anyway...the facts:
1) HUGE sink takes lots of water, runs even when the weather is dry pretty well, at the right level/contacts/etc and we know it goes because we have a hole and see at least a 100-foot drop. So we know it's good and well worth digging. Not the issue.
2) One side of the dig (half of the wall) we got to solid rock - yay! Other half, locally 2 meters give or take of mud, wood, and rock (gravel to cantalope size). We have a chockstone and will bang it however the mud half of our dig hole of 2 meter depth collapses (it takes water, water eats it away quickly, it collapses). Currently we left it when it became unsafe and will let some good rainstorms take care of it and stabilize, but the entire room we are working in is mud-filled like this.

So my major question would be thus: how do you recommend to best stabilize the 2-meter tall bank? We have plenty of rock in the fill we could build a wall with it (I have visions of the beautiful walls  going down to frozen deep!) , however, I doubt it would hold and not just collapse as well without additional stabilization due to the water intake and constant erosion.

If it would help, I am documenting this project and simply do not want to post video - but if you believe it would help with suggestions, PM me privately.

Help it collapse. Sounds like you have room for some mud at the bottom of the hole.

Best option would be to brace it with 2x4s; check out the Eldon Hole "trip report" for pics/ideas. Obviously that takes a fair amount of kit, but it's the safest thing.

We had reasonable success in a similar situation by filling fertiliser bags (plastic bags made from heavy gauge plastic, previously used to hold around 50kg each of powdered material) with the mud and stacking them to create a wall which sloped backwards slightly from the bottom to to the top. This held back the wall of mud behind them. A double row on the bottom few layers makes the wall more stable.

Plastic bags are best if conditions are wet, but you need to orient them so that the opening is slightly downhill and then the water will run out over time and they set fairly solid. The plastic makes them very resistant to being washed away since external water just bounces off.

If the spoil is gravelly rather than mud, or if the stack is not directly exposed a flow of water, then woven polypropylene bags are better since they drain better and seem to mesh together as they dry out and the weight of the upper layers squashes the layers below.

Polypropylene bags ! If underground you can use White ones (no sunlite).
If on the surface use coloured one's the darker the better,White ones will rot in UV
and could collapse and kill you !

Help it collapse is #1 on the idea list of course, but there is a LOT of fill, and in the event it is a canyon beneath at risk of getting plugged by sending it down, stabilizing would be choice #2. Thanks for the bag idea! That will make it nicer and light to carry up a steep side of the mountain bushwhacking through briars and underbrush in the summer heat hitting 100F.  8)

And if you want the bags to become more solid. You can always mix a percentage of cement in the mud. When the water runs out then the mud becomes more like a "stabilizer".

BR

Dagobert - Sc Avalon

As mention here before in the UK you can buy rot proof sausage shaped bags that are filled with sand to hold down sheets on silage pits. Better looking than tyres. We used them very successfully in The Silo.

Yer tis-

http://ukcaving.com/board/index.php?topic=11894.0

100f sounds lovely to be fair. I'd second (or third) filling bags and making a wall.