Dan Henry wrote:This seems wrong on a couple of levels. For one, there's no redundancy as both ends are tied to the same anchor. Wouldn't you want a seperate anchor to have true redundancy? Like backing up to a different tree?
For true redundancy, yes, you'd want a separate anchor. But true redundancy, in the sense that the failure of any single component would not cause catastrophe, is not the goal here. And by the way, for true redundancy, you'd need two ropes too.
Often we rig redundantly in the limited sense that failure of any single
anchor point (e.g. tree, bolt, formation) will not cause catastrophe. This is frequently prudent and the right thing to do. And sometimes it is pointless.
The example I like to give is of a huge, well-seated boulder by the entrance of a cave. Tying the rope directly to the whole thing (or using webbing) is non-redundant. Putting two bolts in the boulder and rigging to them
is redundant. Which is better?
Well, in some cases, bolting it might be better--maybe the edges of the boulder are sharp and could damage the rope or webbing, or maybe you don't have much extra rope or webbing, but you do have a bolting kit and a fast and proficient bolter. (From a conservation perspective, of course, I would suggest that adding a bolt to a rock should not happen unless it is not safe to rig it in some less impactful way...but not everyone agrees with that.)
My point is that failure of the boulder will cause both the redundant and non-redundant anchors to fail. I would suggest that rigging off a large, live tree is more secure than rigging off of any number of bolts placed in the same wall, or even nearby walls.
The notion that it is necessary to rig redundantly, even when you have a suitable anchor point that is totally "bomber", might be an unfortunate filtering in of top-roped climbing techniques to vertical caving. In top-roped climbing, you want to place your anchor directly over the top of the rock or ice climb. So even if you have bomber trees, you'll usually use two of them, because having two allows you to place the anchor "power point" anywhere in between them. But when we're caving, especially with American-style rigging, this is silly and unnecessary. If there's a super-strong, super-reliable tree next to the cave entrance, and it's safe to approach the tree, and the lip that the rope will go over isn't too bad, and the rope will run down the hole in a way that provides for a safe rappel,
just use it! (That's a long list...but those conditions are, more often than not, easily fulfilled.)
The goal of this rigging recommended in
Alpine Caving Techniques seems to be (1) to avoid having a taut rope around a tree, with the idea that the tree can somehow abrade the rope (which seems silly to me...ropes damage trees plenty, but I've never seen a rope that appeared to have taken damage from a tree). And (2) to back up
the sling, with the idea that the sling is less reliable than the rope.
I still want to know
why someone's slings are less reliable than their ropes...and why they're not doing something about
that. If you have any unreliable life-support equipment, that's a problem. And while some pieces of equipment are, by their very nature, less reliable than others--for instance, used and stored correctly and inspected frequently, ropes and webbing and slings virtually never break, whereas carabiners can come unclipped and ascenders can slide against some element of the environment and be removed from the rope--I don't see any reason why a sling that's less reliable than a rope would exist...or why a caver should ever tolerate the use of such a sling.
Dan Henry wrote:Secondly, the knot in the rope weakens the rope strength by about half. You're going to do this anyway the second you rig your first rebelay, but what's the point of doing it in a situation like this where you could do a simpler full strength tensionless tieoff and use less equipment?
When rappelling and ascending are what you're doing with your ropes, load-bearing knots tied in them don't impact the strength in any significant way:
On 1/29/09, I wrote:...If the rope runs over any edges, even if padded, it will likely break at the edges instead of the knot. If the rope runs (in single strand) through a carabiner or pulley at a high angle, there is some chance that it will break there instead of at the knot.
If you think about the kind of fall that would be necessary to produce forces high enough to break the rope at the knot (i.e. forces of about 2/3 the strength of the rope, since knots in kernmantle rope tend to weaken rope by 1/3 on average), well, take such a fall on ascenders and the ascenders sever the sheath and pull it down, limiting the loading and preventing knots from breaking. Or the ascenders would simply cut the rope--then the rope would break at the ascender-rope interface, and not at the knot. Now suppose you're descending. Then in a shockload your descender would slip, and hopefully you would be able to maintain control as the speed increases above what you had intended, perhaps while swinging against a wall. Suppose you have an auto-braking descender (or a descender with a braking assist, if that is the politically correct term for it these days..since they do not always break by themselves). Well, that will still slip. They are designed to slip...and it would be hard to design one that didn't slip anyway, even if you tried, because you would either have to have an exceedingly long handle or need to apply a tremendous force to move. Sometimes they do damage rope, though. They could even, like an ascender, cut the sheath and pull it down. While this is unlikely, it is theoretically possible for a descender to sever the rope in this situation. (Please note that, aside from the possibility in a shockload of swinging or falling into walls, or dislodging falling rocks, this is actually a safer situation than a free-fall rappel terminated by a release of the brake handle. In a very fast rappel, the friction surfaces of the auto-braking descender become very hot, so then the descender could potentially cut the sheath and melt the core. That is unlikely here.) Whatever happens, like in the case of ascenders, the force applied is limited to being way below what would break a knot.
So when could a knot actually be broken in a vertical caving situation? Well, it's unlikely, which is perhaps why it has (as far as I know) never happened. It is possible. I think the most likely cause would actually be exposure to the part of the rope with the knot in it to dangerous chemicals, e.g. sulfuric acid. However, if we're talking about a properly functioning rope, then no correct use would result in such a breakage. Incorrect use, i.e. really big shock loads, probably having to be bigger than FF1, could do it. But when ascending or descending normally, as detailed above, this cannot happen. Something else will give or break first. Well, there are two situations where the rope could break at the knot. One is when you are actually tied to the rope--in this post, I have already detailed one situation where I would tie a knot in the rope and clip myself to it. If there were another knot above this, that knot might break if the rope were overloaded. But the knot I'm tied to would be just as likely to break, and in any case the actual difference in the rope's strength would be minuscule.
The other situation, which may be more common, is that the descender is hard-locked off. Well, guess what, this is essentially another knot, and I think that the rope would be much more likely to break as it enters a hard-locked descender (of any type) than at another knot along its length. But it could break at the knot. Again, even if it did, at a very slightly higher load on the rope entering the descender, it would be able to break there. So again, the overall strength is not significantly reduced by the knot being in the rope.
I conclude this argument with a statement that is common sense to many cavers and rope users: Except in highly technical and particular situations (i.e. some highlines), knots in a rope do not actually weaken it significantly in a real world situation.
Getting back onto the topic of this interesting rigging technique recommended in
Alpine Caving Techniques: There are plenty of situations where I would strongly prefer to rig to a tree with a sling or webbing, rather than directly with the rope--when the tree is sappy. But then I wouldn't tie the rope to the tree behind it, because that would defeat the point. Single slings and single pieces of webbing are trusted in rescue...it's odd that the authors of
Alpine Caving Techniques are unwilling to trust them fully in sport use.
Stelios, I am interested to hear your reasoning for believing that a sling should be trusted less than a rope.
There is only one argument that I can devise for this claim: You're trusting the rope anyway, and there's nothing you can do about that (besides, of course, purchasing ropes of appropriate type and high quality from respectable manufacturers, using and caring for them properly, and inspecting them frequently and retiring them immediately when they are no longer safe). But every component besides a rope does not
have to be trusted by itself--every other component can be "backed up" and you can still do Single Rope Technique. So back everything else up.
I don't agree with that reasoning, because I think that complexity often presents a significant danger...a much more significant danger than the risk that a sling, piece of webbing, or carabiner, properly used, will break.
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