Thanks to expert canyoneer Kevin Clark for technical advice on this post.
Note - This post discusses techniques and methods used in vertical rope work. If you do them wrong, you could die. Always practice vertical rope techniques under the supervision of an experienced climber, and ideally in a progression: from flat ground, to staircase, to vertical close to the ground before you ever try them in a real climbing situation.
While out on a long multi pitch climb, your rope gets damaged by rock fall. What can you do to get down quickly and in relative safety?
Here are a few Crafty Rope Tricks (CRT) to help you do this.
Ouch! Probably time to head down.
First off, if the damage is near the end of the rope, you may be able to simply cut off the damaged part and continue upwards, provided your rope is pretty much long enough to make it every pitch.
This will require a careful reading of the route topo map, perhaps some alternative intermediate belay anchors, and maybe some simul-climbing, which is hopefully doable if you’re on a moderate alpine route.
However, if the damage is more in the middle part of the rope, you probably can’t continue upwards and need to start rappelling. (Hopefully this is painfully obvious, but if you were to tie a knot to isolate the damaged part and try to keep going up, that knot will get stuck in the gear placed by the leader.)
First, assess the damage.
Is the core of the rope (white part) severely damaged? If yes, you may want to actually cut out the damaged part and retie the rope with a stout knot. (A Flemish bend is a good choice.) And you do have a knife with you, right?
If the rope has mild damage, mostly to the sheath, you can probably type a butterfly knot to isolate the damaged part. The butterfly knot is a great choice for this, because it retains pretty much the full strength of the rope (minus a bit for the knot, of course) and can take a pull in any direction. Tie the knot so the damaged part is isolated in the loop.
Butterfly knot isolating the damaged part of the rope.
So, you've isolated the damaged part and now need to head down. What now?
The more traditional old-school approach is to start doing standard twin strand rappels with the knot in the rope, and have everyone “pass the knot”. This can be time-consuming, dangerous, and, if you have two strands of rope to work with, probably completely unnecessary.
There are actually very few situations in which you really need to pass a knot for real, but they pretty much all involve tying two ropes together for a long single strand rappel, and not when you're doing a normal double strand rappel. For example, cavers on a very long rappel, or big wall climbers fixing a rope to the ground or rapping back to a high camp after fixing two pitches come to mind.
But, you may be thinking, what if you DO have two rope strands and there IS a knot in one of them? No problem, you should never need to deal with passing the knot.
One way to tackle this is to use a carabiner block or knot block (aka Reepschnur, Google it). This is common in the canyoneering world but not so much in rock climbing. This method essentially uses the damaged strand of the rope as a pull cord, with a carabiner or knot “blocking” the damaged side of the rope from pulling through the anchor.
This is definitely a valid technique if you are experienced with it and absolutely know what you're doing. However, there are some subtleties to setting it up correctly. You need an anchor point with a very small diameter connection, such as a chain or quicklink. If you rig it wrong, the blocking knot could pull through your anchor and you could die. This has happened.
The knot block falls into the category of advanced ninja rope tricks that you will not be learning from this website. If you're curious, Google it and practice it in a safe environment with someone who knows how to rig it properly, before you ever try it for real.
Here's another method that, in my opinion, is preferable for most climbers.
(In the example and photos below, we’ll assume that the damaged part of the rope is in the right hand strand.)
Step 1: Put the middle of the rope at the anchor and isolate each strand.
Thread the rope for a normal rappel. Put a locking carabiner on one or both anchors, and clove hitch the rope to the carabiners. Now each strand is fixed independently. (If you’re a crafty canyoneer and are very familiar with the Stone knot, you can use that here as well.)
Here is a post of how to do this in more detail.
Because the damaged rope is on the right hand side, everyone can rappel on the single strand on the left side. Sweet, no knot to pass!
Note that when you rappel on one strand, you'll have a lot less friction, and the rappel will be faster. Everyone needs to be aware of this, and possibly use some simple techniques to add more friction so you can rap in complete control. Gloves are recommended if you have them.
Step 2: Use a “ground anchor” to secure the rope for the last person.
When the first person down arrives at the anchor or the ground, they can tie off the damaged strand of the rope (right strand) onto an anchor, a tree if they're on the ground, or even themselves. Everyone else in the group continues rappelling on the left strand.
The last person at the top removes the carabiners and clove hitches so the rope is running normally through the rappel anchor. Then, the last person raps on the left, undamaged strand of rope, and is counterbalanced by the ground anchor.
Note that this puts a load on the top anchor that’s about twice the weight of the last person just like if you were toproping. So, the anchor needs to be reasonably strong. If the top anchor is less than perfect, ideally the last person is fairly lightweight, does not have a heavy pack or a lot of gear, and knows how to rappel slowly and smoothly, without a lot of bouncing.
If you have the misfortune to be doing this from an extremely marginal rappel anchor, it might be best for the last person down to avoid the ground anchor, and just deal with passing the knot. However, most of the time this should not be an issue, because the maximum force you can put on a rappel anchor is about 2 kN, or about 440 lbs, and that’s if you are really jumping and bouncing around.)
When the last person is down, untie the ground anchor, pull the damaged (right) strand of the rope, and you're done. Repeat if necessary until you get to the ground.
Doing this avoids all of the mid-air shenanigans of having to pass a knot on rappel.
(And, the next time you’re at your local mountaineering club’s training to “rappel past a knot”, you can ask the instructor to tell you in what situations you might ever have to do this. Who knows, you might just teach them something. =^)
Here's a diagram of how the ground anchor works: