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
"Although it pains me to say it, in this case Jeff is right. Plan accordingly." --Andy Armstrong