Solution for retaining a scree slope

AlexR

Active member
This is mainly as an "archive" post if anyone ever encounters the same problem and wonders how to deal with it.

CJ and I are were faced with a very pesky scree slope, which immediately gave our dig the name "Sisyphus". Effectively the scree trundles into the dig (a tube in the side wall of the scree sloped passage, itself containing scree), and the scree slope is also the only real place for putting any spoil. There are some other holes, but I'd rather not fill them up at this point. The slope can only support itself up to an angle of around 45?, any more than that and it all just comes back. That and it's not a good foundation to put more spoil on.

Found a surprisingly effective solution:
Bits of wood plank ca. 40cm long were driven into the scree with a sledgehammer. Scree was then excavated downslope of these piles. Only about 5cm of the plank needs to remain burried to hold back the scree (bit delicate). The scree was put into sandbags and used to build a retaining wall. The process can be repeated downslope, changing an angle of 45? to around 70?, effectively gaining a lot of space for spoil. The wooden piles can actually be jiggled out and re-used, but after 2 rounds they're fairly knackered so we left them in.
We also used some mushroom trays as a sort of geogrid in the structure to improve load bearing capacity.

Just ignore the spoil tower visible on the left, that bloody thing still needs completely taking down an replacing with the same sandbag structure.
 

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royfellows

Well-known member
A neat solution.
I think a bit more security and permanency could be added by supporting the top of the timbers with an outer cross stemple driven downwards at each end.?
It may be necessary to do a bit of rock work to make up the sockets.
 

AlexR

Active member
Thanks Roy,
I completely agree that timber across the passage would provide additional support, but it's all softwood and I think of them as temporary aids only. There is water running down inside the scree, so I fear they're not long for this world. Life could be extended with various coatings, but I'm not sure it's worth the effort.
In the long term, we're considering to do pretty much what you said but with scaffolding between the passage walls. As the weight of spoil increases on the top the risk of sliding/ shear failure (e.g. the entire thing sliding down on the bedrock underneath) becomes a pretty uncomfortable thought.

EDIT:
Also forgot to say, if you can use any form of metal for the sheet piles, I'd imagine that'd be much easier to hammer in. Provided they're not too deep you can just pull them out again after the putting sandbags on the other side. I just happen to have a lot of planks, so that's what we use.
 

CJ

Member
I was skeptical at first, but by the time I emerged from my hobbit hole (the dig) I was rather impressed with Alex's work and how well it was holding up. There was very little scree egress, and frankly we will be left needing a new name for the dig...  The only time scree poured into the dig was during the staking process with the sledgehammer. In fairness to the spoil tower, it held up well until my clumsy-self got involved  :ang:
 

CJ

Member
Cap'n Chris said:

We don't quite have the big picture as to what lies below all the spoil. Eventually it will make sense to cement/mortar/postcrete/concrete over the scree but we are being cautious until we're sure it's not going to bite us in the ass further down the line. I was pushing for concrete early on, but I now think it's best holding off, just in case.
 

royfellows

Well-known member
Something that I have been thinking about for some time is the use of grout, basically a mix of PVA which is quite cheap these days, and cement, being pumped or forced into loose ground.
Many years ago I was at Wheal Cock  near Botallack, Cornwall when there had been a run of ground around the collar of the massive clifftop shaft. Contractors had pumped gallons of the stuff down into the run and then back filled. It had made a right mess down in the shaft where timbering had collapsed releasing backfill that had made iup part of the shaft down in the stoped ground, but it did work. Give it that.
 

mrodoc

Well-known member
Willie Stanton did similar with liquid cement in Reservoir Hole to stabilise the top of excavated shafts and then worked down. This technique is described elsewhere and seems to have stood the test of time.
 

AlexR

Active member
Could you point us towards the reference for that mrodoc? Whoever reads this in the future will be grateful for it, I think  :)

Like CJ says we considered concrete/cement but decided against it, in part because I was concerned we'd mess things up for ourselves further down the line, and in part because I have no experience with it. Even though we have water, transporting the amounts of cement we'd likely need to the site would be interesting (but feasible).

Any experience, especially with pictures, would be great as it's another tool in the arsenal.
 

pwhole

Well-known member
It's a shame that plastic decking planks are so expensive, as we used them to great effect in holding back one choke in Longcliffe, as they're in flowing water pretty much constantly. Luckily these were spares from a member's garden project, otherwise it would have had to be steel, but even that would have quickly degraded in that environment - but we did have some stainless rebar in reserve if it came to it! The scaff tube brackets (joist hangers) were covered in concrete and the whole thing later reinforced by a stone buttress/staircase on the downslope side, and it should hold it back forever.

As you saw lower down, the rockmesh and its cover of perforated rubber doormats also make a good (if pricey) solution to holding back lots of scree in the same flowing water!
 

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Pitlamp

Well-known member
pwhole - please could you give a little detail about how you fixed the ends of the horizontal scaff tubes in that particular example? Ta.
 

CJ

Member
Pitlamp said:
pwhole - please could you give a little detail about how you fixed the ends of the horizontal scaff tubes in that particular example? Ta.

I could be wrong but I believe it's this: https://www.wickes.co.uk/Wickes-Maxi-Speedy-Joist-Hanger-50-x-145mm/p/156726  Apologies if that wasn't what you were after
 

mrodoc

Well-known member
AlexR said:
Could you point us towards the reference for that mrodoc? Whoever reads this in the future will be grateful for it, I think  :)

I had a copy of the article in my files but it took a little while to find. Rather than post the whole thing (interesting as it is) here is the source: : Studies in Speleology, Vol. IV, September 1983, 77
 

pwhole

Well-known member
I could be wrong but I believe it's this: https://www.wickes.co.uk/Wickes-Maxi-Speedy-Joist-Hanger-50-x-145mm/p/156726  Apologies if that wasn't what you were after

Yep, that's the one - with 8mm concrete screws fixing it to the wall and then concreting over. They're 50mm, so take a scaff tube perfectly. The lower retaining wall was done the same way, but the rockmesh was sufficient to hold this back without decking boards - and we didn't have any more anyway :)
 

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Pitlamp

Well-known member
Thanks CJ - and thanks pwhole - they look just the job.  (y)

(Mental note made, for when that situation next arises.)    ;)
 

Mark Wright

Active member
When we were digging our various sites in Rowter and later down Eldon, we simply drilled a couple of 12mm holes either side of where the tube needed to fit and put a stainless steel pin in both holes. The scaffold tube was simply put over one pin and, by cutting a small slot in the other end of the tube with the hack saw, it could be located on the other pin and then twisted to stop it coming off. It was a bit loose at first but soon became solid with half a ton of rubble behind it.

Half as many holes to drill as Phil's method and probably stronger than the joist hangers and no cement needed.

Mark
 

pwhole

Well-known member
I also had to use that method on the choke below, as the wall was too irregular and the wrong angle to get the joist hangers on, so hammered in some stainless bar to hang them on, and used joist hangers at the near end, which didn't need concreting. We were concreting down there anyway, so it wasn't much extra hassle to seal them all up.
 

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SamT

Moderator
Agreed.

I once suggested that Croesor/Rhosydd needed a big clean up operation to pull out all the tat/crap that had accumulated.  Someone else suggested why bother pulling it all out, just put it all in a tonne bag and sink it in one of the lakes.  Initially, I thought that was insanity, but he then went on to suggested that if we lugged it all the way out, down the hill, into a fossil fueled van, into a skip and then on via various fossil fueled lorries.. to where?..... probably dumped in a disused slate quarry that was being backfilled with land fill. 

Back to the OP though..  We were working in a rift with a big unstable slope of mud and rock with our exit at the bottom.. plenty of opportunity to be trapped on the wrong side.

We bolted a ton bag to each wall so it was virtually suspended off the 4 bolts (rift happened to be exactly the right width) and filled it.  There was a lot of 'slop' so we mixed cement dust through it, which hardened.  We ended up with two of these, forming two giant steps. 

Haven't been back for a long time now, so not sure how they're holding up.  :-\
 

pwhole

Well-known member
This section was the most complicated to fix - the previous photo showed the rockmeshed pack above the small climbing-shaft, the bottom of which is just visible behind Alastair. The pack of deads at left is supporting roughly 5m depth of backfill, 5m X 2m in plan, comprising the floor of the main shaft. The miners built a superb self-draining structure, and the water discharges down the 'staircase' that I'm standing on. However, us re-opening the shaft massively increased water-flow, and our regular traffic was making this staircase disintegrate - if the pack of deads collapsed, everything above it would too, and we'd never get out alive - not via this route anyway.

So we built two massive reinforcement steps, with scaff-tube and rebar, spare deads and masses of concrete, and the top one is at the bottom of the shot. There wasn't much we could do with the pack itself, other than slather it in rockmesh, but it wouldn't really do much given the anchor points, and it did seem a step too far in that instance. As long as no-one touches it :)

The second photo shows the small climbing-shaft from above, before we rockmeshed the packwall - this is the only way in or out of the extensions, so keeping it open was sort of critical - and it has steps built into it, so it's very cool.
 

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