Chads93GT wrote:If there are no rebelays, and it is a simple case of rigging a tensionless hitch around a tree for a pit drop, then it will be fine...................
Then it's definitely OK to rig with a polyester rope, yes...provided that the people USING the rope are sufficiently experienced not to allow slack to accumulate above them.
What's simple for you may not be simple for someone who is inexperienced.I recall a trip where someone who'd had a good deal of training had a bit of difficulty climbing up a 60' drop, because it was awkward at the top--tight, with some stuff jutting out. Nothing on the surface can perfectly capture what it's like to do SRT in a cave. So he free-climbed about fifteen feet. Which is fine. Except that he didn't advance his ascenders while doing so. I had told everyone on the trip, before we went into the cave, that they needed to always advance their ascenders and make sure no slack accumulated above them. But when you're first starting vertical caving, there's a lot to remember, and some people will forget things.
When I saw him near the lip, and I saw that he had not advanced his ascenders, I yelled for him to do so and he did. At that point there was about 8 feet of rope (not including slack) between him and the tree where the rope was rigged with a tensionless hitch.
If he'd fallen before pulling the slack through, then going by the amount of slack accumulated divided by the total rope between him and the anchor, he would have taken a 15-foot long fall of about factor .6. He might have scraped along the wall some, reducing the fall energy...but not reducing it to something small. The rope had a nylon core, so if this had happened then his Croll would likely have cut the sheath of the rope and pulled it down, exposing the core and scaring the living daylights out of him. But it would probably not have cut the rope.
On the other hand, if the rope had a polyester core, his Croll would probably have cut the rope in two and he would have fallen 60' to the ground.
In another simple rigging scenario that wyandottecaver brought up (see the link in my previous post), where you have a rope rigged to a single, bomber feature in a cave near the ground and run it along the floor for a significant distance before the drop, you have another plausible mechanism for a shockload. Caver A climbs up the drop and slack accumulates above. Caver B is standing at the top and steps on the rope, pinning the slack between his foot and the anchor. The friction along the floor prevents the slack from being pulled out, even when the rope is weighted by Caver C. Feeling the tension on the rope from Caver C, Caver B looks down, thinks "my bad," and steps off the rope. Then the slack quickly comes out of the rope, resulting in caver B taking a fall.
Chads93GT wrote:I can see how it would cause issues for in cave pit drops from multiple rig points.
Sometimes. But just as people not being careful can result in shockloads with a single anchor point, you can rig carefully and thoughtfully with multiple anchor points and avoid shockloads of sufficient magnitude to be problematic with a polyester rope.
A polyester rope does have some stretch. You can generally rig a Y-hang off two anchors with a polyester rope...so long as you make sure that the extension will be minimal if one fails (so don't rig with the dynamic load-distributing system presented in
On Rope and in the NCRC level-3 curriculum--rig a static load-sharing anchor instead, like you'd probably do anyway). Rigging from two bolts on the same wall is even easier--you can do it the same way. You can even sometimes use polyester rope to rig a traverse line with a single (i.e. non-redundant) pitch-head anchor...so long as the distance between the pitch-head anchor and the closest traverse anchor is sufficiently small. I've been told that you can even sometimes rig a rebelay, so long as there is a whole lot of rope out (i.e. the rebelay is much lower than the top anchor or next highest rebelay...I'm not sure how much lower it has to be though), and it is rigged with no more slack than necessary to progress on the rope above and step into the rebelay loop.