Rappelling Off Boulders

One of the few things that I don’t enjoy about climbing is getting to the top and having to rappel off a sketchy natural rappel anchor. I’m not talking about tat condition, I can fix that easy enough, but the natural anchor itself. When the anchor is sketchy, we need to follow best practices of backing up the anchor, if possible, and sending the lightest down last. Setting that aside for a moment, I am curious about rules of thumb for natural rappel-anchor size.

I have heard rules of thumb for rappelling off trees: as big around as your thigh, alive, and well rooted. I haven’t heard rules of thumb for detached boulders. For detached boulders, I am assuming they need to have good contact with the ground, be well balanced, and be of a minimum size. But what is the minimum size? And how would you judge it in the field?

I did some half-baked calculations using the unit weight of different rock types and rappel forces I have come across. I am not an engineer or physicist, but my very naïve results look like this:

I’d appreciate your input on this. Have you come across any rules of thumb on minimum boulder size, contact area, or other related stuff?

EDIT to add a few rules of thumb people came up with:

• It should be as big as a refrigerator, well planted, not wobbly.
• It should be as big as a coffin, or you might go home in one.

And things that I'll be considering:

• Given any doubt, back up the rappel anchor and send the lightest down last.
• The boulder length, width, and depth, in feet, should multiply to a number greater than 20.
• The boulder should be well planted, and the attachment to the boulder should be as low to the ground as possible, to minimize tipping.
• The contact surfaces should be rough, and should not slope towards the edge, to minimize sliding.
• When rappelling, contact between the rappel rope and the edge will cause friction and take force off the anchor.
• When rappelling, a steady descent speed will minimize peak forces on the anchor.

When i was taught to build top rope anchors, i was instructed to use things that look to be about the size of a refrigerator or larger and are planted on the ground well (not teetering or wobbling).   

But are you bounce testing your boulders?

In seriousness, it looks like you converted the weight of the boulder to kN and listed that in the last column? That would be the force needed to lift the boulder vertically, not slide it across a surface. You can make some idealized assumptions about friction to get a lateral force, but in reality this is highly variable and trying to estimate it is borderline pointless. Nick's rule of thumb is likely a better bet, and solid assessment skills are invaluable. Even a small-ish boulder can work if it's wedged and seated properly, and a huge boulder is a death trap if it's on a sloping ledge.

Nick A wrote:

When i was taught to build top rope anchors, i was instructed to use things that look to be about the size of a refrigerator or larger and are planted on the ground well (not teetering or wobbling).   

And which way gravity is trying to pull it.

Easiest Solution: Go Canyoneering and desensitize yourself. They love rapping off sketchy shit.  

Nick, thanks. I had heard that about top rope anchors as well. Unfortunately, I have out of necessity broken that rule dozens of times for rappels. I have lost sleep but not yet died, thankfully.

Red, I had initially gone down the road of looking up static friction coefficients, but gave up due to all the assumptions. And Red and Andrew, thanks for the important insight about the slope of the surface. That should definitely be a condition of a general rule. For now, let's assume zero slope.

Randy, indeed. I am trying to find something between a refrigerator and this:

Tim Meehan wrote:

Nick, thanks. I had heard that about top rope anchors as well. Unfortunately, I have out of necessity broken that rule dozens of times for rappels. I have lost sleep but not yet died, thankfully.

Red, I had initially gone down the road of looking up static friction coefficients, but gave up due to all the assumptions. And Red and Andrew, thanks for the important insight about the slope of the surface. That should definitely be a condition of a general rule. For now, let's assume zero slope.

Randy, indeed. I am trying to find something between a refrigerator and this:

Are the pebbles on top supposed to help?

The adage I'd always heard is 'if it's smaller than a coffin, you may be going home in one', but I assume that's a pretty conservative statement. How many blocks have been pulled off flat ledges (IE in cases where the block wasn't sitting precariously on edge already)?

Hey, Tim. Looks like you have some extra time on your hands.  ; )

Maybe we should consult the Egyptians about huge stone blocks on inclines, friction coefficients, etc  

Hope you are well!

Dan! Yup, taking a sanity break from work. I think we can learn from ancient Egyptians never to rap off a block on rollers.

Matt, I think the coffin adage is a powerful one. Seems like a sandstone block shaped like a coffin might have a volume around 20 cubic feet and take around 7.5 kN to lift. That's a nice buffer for a relatively high 2.5 kN rappel force.

Rasputin NLN wrote:

Are the pebbles on top supposed to help?

It's called a cairn anchor. It's a canyoneering thing, but I have encountered it on climbing routes. I think it's a cool trick, maybe not first choice, but yeah basically enough small rocks = one big enough rock.

J C wrote:

It's called a cairn anchor. It's a canyoneering thing, but I have encountered it on climbing routes. I think it's a cool trick, maybe not first choice, but yeah basically enough small rocks = one big enough rock.

Shit id hammer in some button heads if I came across some bs like that. 

I thought I was YGD. Wonder how canyoneerere walk around with balls that big. 

I mean I've seen real cairns I'd have no problem rapping from. But the rock stack pictured seems really scary. Do they generally take that form?

I don't know about canyoneering outside of the video I watched on those, but the one I found was probably ok. It consisted of a bunch of loose rock in a crevice/canyon. I didn't dig through to see what was ultimately tied off. It was obviously old webbing, and I found another way down.

Which was slinging a kinda fractured horn. It held, but I think honestly considering what happens if a failure occurs. In this case, it was obvious that any type of backup was useless, so I rapped carefully and made sure my partner had the second rope. So some mitigation effort at least.

Big Red wrote:

But are you bounce testing your boulders?

In seriousness, it looks like you converted the weight of the boulder to kN and listed that in the last column? That would be the force needed to lift the boulder vertically, not slide it across a surface. You can make some idealized assumptions about friction to get a lateral force, but in reality this is highly variable….

All jokes aside, this is definitely a case where you DO want to bounce test it!   The coefficient of friction (mu) is larger for static conditions than kinetic ones.  Meaning that if it’s just barely sitting there under static friction forces, (or even just barely balanced under gravitational forces) a little jostle could start it moving, so a good series of cyclic bounces is a better test to see if it’s in a solid position and won’t move.  

Of course if all you have is tat, then you’d be Fucked, because we all now know to never bounce tat.  Only option then is to sit there and call for rescue. They’ll understand.  

well based on some quick google searches, static coefficient of friction for granite on granite is somewhere between 0.45-0.60. Assuming 0.50 for easy math, that means it takes about 3.4 kN to start your notional 3x3x1 ft block of granite sliding. Might be good enough under ideal conditions, but not much margin. So maybe 3x that, or a roughly 3x3x3 ft boulder? Seems to check out with a lot of the rules of thumb mentioned here.

If I can move it with my body, I won’t sling it. Pretty basic. Ive rapped off many a boulder, general rule of thumb I use is larger than a toilet, that thing has been there long before you and it will be there long after youre gone. Or maybe not, depends on how big a toilet you use. 

I only rapped off a snow bollard once but that was enough to not want to do it  again.

Gerald Adams wrote:

I only rapped off a snow bollard once but that was enough to not want to do it  again.

Why?

If considering really marginal boulders with no backup anchors it might be worth top climber being ready to try holding back the boulder if it started moving. Would have to release it if worst was about to happen, with at least a slight chance of warning abseiler to try and unweight rope and get out of fall line ASAP. 

Alternatively top person sits/stands on boulder ready to leap off to save themself (eg one climber = several rocks in cairn anchor photo upthread??). Obviously that person has then to abseil on an even less secure anchor.

Thankfully I can propose daft solutions from the comfort of my armchair and expect never to put them into practice.

I have threaded a low, drystone wall to ab down a seacliff; rope running over several feet of grass before the edge probably increased safety. If I'd had a partner I suppose it would have been simple for one person to backup the first man while also testing the anchor by using their body behind the wall to spead the load, with a slightly slack connection to the primary anchor.