Cavechat.org

Jeff Bartlett wrote:

dutchcontractor wrote:Yup, I realize this adds an extra "link" in the chain, but you'll end up with a simple perfectly equalized anchor. I prefer direct rigging but when it gets too complicated I'll go with the extra link (I use 2 biners). Should be good to take 20+kN. See the picture below.

I'm going to ask this question not to be critical, but because I think it's a good question to ask (and this might even be best as another thread): Why?
More clearly stated: why would this be preferable to, say, a double-figure-eight (bunny ear knot) properly stressed-and-dressed in 11mm PMI and connected to two good bolts?

2 good bolts directly rigged with bunny ears is fine. I was just demonstrating what I would do if you wanted to use more than 2 bolts. For whatever reason that's what folks are doing in Surprise pit.

Jeff Bartlett wrote:: Why?
More clearly stated: why would this be preferable to, say, a double-figure-eight (bunny ear knot) properly stressed-and-dressed in 11mm PMI and connected to two good bolts?

Exactly my thoughts. The rigging needs for a 400' pit are no different than for a 40' pit. The only reason there are four 6-8" bomber bolts and a backup boulder is psychological. Surprise has a lot of exposure, and the roar of the waterfall adds to the fear. A pair of 4" bolts for the main anchor and a secured traverse line to reach that anchor is all that should be needed. Just because four bolts have been placed at the main rig point doesn't mean all four need to be rigged using some complex quadruple eight knot.

Extremeophile wrote:

Jeff Bartlett wrote:: Why?
More clearly stated: why would this be preferable to, say, a double-figure-eight (bunny ear knot) properly stressed-and-dressed in 11mm PMI and connected to two good bolts?

Exactly my thoughts. The rigging needs for a 400' pit are no different than for a 40' pit .... Just because four bolts have been placed at the main rig point doesn't mean all four need to be rigged using some complex quadruple eight knot.

EDIT: I didn't account for the 8mm cord being doubled. So all of these numbers are wrong. Proceed at your own risk.

And it's actually a weaker rig, 50% weaker if you're using typical 8mm cord. The rope is the weak point in the anchor system, assuming bombproof bolts in good rock. Since the breaking strength of 8mm rope (assuming the best, i.e. actual 8mm rope like Bluewater II) is around 4,000 lbs (17.7 kN) and we know tying a figure eight knot in rope is going to reduce that by 25-35%, you've built an anchor system that can handle 12 kN, not ~20 kN as previously surmised. If you use 8mm CORD as stated below, well, you're starting at around 3,000 lbs (13.4 kN) according to PMI's specs and you're chopping that down to about 9 kN with the knot.

I know we're splitting hairs since you're pretty much not going to break anything in either anchor system, but I don't want to climb tandem on a 9 kN rig. I've forgotten how to calculate safety factor or I'd do so here -- help me Andy or Jansen or Scott! -- but it's more harm than good. The breaking strength for 11mm PMI Pit Rope is 30 kN (about 6700 lbs); a correctly tied double-figure-eight rig to good bolts in good rock and secured with good carabiners is a 20 kN anchor system. Unless you have very good reason to suspect that one or more of the bolts is suspect, it's a superior rig, and that's without addressing issues like the relative complexity of each rig.

Someone with a lot more rigging/rescue knowledge and experience than I can probably address this much more completely.

dutchcontractor wrote:2 good bolts directly rigged with bunny ears is fine. I was just demonstrating what I would do if you wanted to use more than 2 bolts. For whatever reason that's what folks are doing in Surprise pit.

Yeah, I really don't want to seem like I'm singling you out. It's just that lots of people see these threads out there on the internet, and 2 years from now a Google search for "rigging Surprise Pit" is going to land folks right here (which is, coincidentally, why we often see comments in 2 or 3 year old threads from newer cavechat posters). So I thought it was worth talking this out publicly, for those who might find the thread in the future and be wondering what the best way to rig a deep pit is. I actually appreciate you bringing this three-bolt option up, since it's a good discussion to have. And I appreciate Chad starting this thread in the first place, the idea being literally "hey, I just realized this wasn't the optimal rig, let's put it out there and talk about it."

This is how I get myself into trouble on Cavechat, I start thinking of it as a quasi-permanent public record and people take it the wrong way

Safety factor-smacktor. Common sense and paying attention is a better gauge. I prefer to get rid of as many links as possible. Too many other safety factors to worry about with out throwing in more gear.

For accuracy sake, the cordelette strength would be more than 9 kN. If the strength of a cord is 4000lb, then when you turn it into a loop (assuming no joint), the loop will hold 8000lbs. A loop of 4000lb rope would hold 8000lbs. Twist the loop and double it, you get 16000lbs. On Rope explains this better than I can. So, I would say the equalized anchor with a cordelette should see 15-20 kN.

I rigged to more than 2 bolts when I visited Surprise a while back. I didn't have anyone available who could vouch for the age or condition of the anchors and they were pretty old and unfamiliar looking. Combine that with the fact that someone in the past saw it necessary to put so many bolts in and I was prepared to err on the side of caution.

Think of it this way: they were either knowledgeable bolters/riggers and decided a bolt constellation was necessary to have or they were bolt-happy fools in which case do you trust the job they did? Another possibility is that some bolts became suspect with age and new ones were added without removing the old ones. In that case...which are the suspect ones because they all look like they've been there a long time. Every time I see that many bolts I get nervous. Maybe someone with a firm grasp of the history of those bolts can shed some light.

Here is down, quick and dirty explanation of the strength of a cordelette. Using Jeff's 8mm BW at 4,000 lbs. (17 kN)


To calculate the safety ratio, let’s assume that each caver weights 170 lbs. and they are climbing tandem.
170 x 2 = 340lbs or 1.5 kN
1.5 kN divided by 68 kN = 47 kN for a Safety Ratio of 47:1

Tim White wrote:Here is down, quick and dirty explanation of the strength of a cordelette. Using Jeff's 8mm BW at 4,000 lbs. (17 kN)

...and I've just demonstrated the best way to lure Tim White out of Cavechat hiding -- horrifically botch some rigging math! Tim, I promise I was paying attention in your NCRC class.

Jeff Bartlett wrote:Tim, I promise I was paying attention in your NCRC class.

Hah good posts. And you're welcome Jeff for the thread. I have no problem looking out something I did wrong In afterthought when a trip is over. I like the 8mm loop rig idea. Better than a triple figure 8.

Although it doesn't make much difference, the math is wrong for the rightmost loop. That loop can support 24kN (12+12) not 29kN (12+17). Both sides of the loop will have equal loading up to the point where the weakest link (the 12kN knot) breaks.

Mike

Tim White wrote:Here is down, quick and dirty explanation of the strength of a cordelette. Using Jeff's 8mm BW at 4,000 lbs. (17 kN)


To calculate the safety ratio, let’s assume that each caver weights 170 lbs. and they are climbing tandem.
170 x 2 = 340lbs or 1.5 kN
1.5 kN divided by 68 kN = 47 kN for a Safety Ratio of 47:1

And the angles in the side lobes impact things how much?

(sorry, just had to give you a hard time there

Jeff Bartlett wrote:

Tim White wrote:Here is down, quick and dirty explanation of the strength of a cordelette. Using Jeff's 8mm BW at 4,000 lbs. (17 kN)

...and I've just demonstrated the best way to lure Tim White out of Cavechat hiding -- horrifically botch some rigging math! Tim, I promise I was paying attention in your NCRC class.

Actually it's a general rule of the Internet that the fastest way to get a correct answer is to post an incorrect answer and wait for someone to correct you.