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Dep
Guest
Here is something that happened to me the other night on an SRT trip which proved instructive...
The site in question is a circular brick-lined shaft 50ft deep and about 8 feet in diameter. Flush with the ground it has a heavy metal grill over it for safety and a sliding hatch in the top.
There are two heavy girders parallel about four feet apart that lay across the top and form the main structure. The rest is filled in with metal bars about 4 inches apart. The opening is a square hole in the centre about 4 ft square.
A very simple SRT site - a Y-hang is rigged between the two girders and a re-belay to get down onto the main pitch.
The site in question is a circular brick-lined shaft 50ft deep and about 8 feet in diameter. Flush with the ground it has a heavy metal grill over it for safety and a sliding hatch in the top.
There are two heavy girders parallel about four feet apart that lay across the top and form the main structure. The rest is filled in with metal bars about 4 inches apart. The opening is a square hole in the centre about 4 ft square.
A very simple SRT site - a Y-hang is rigged between the two girders and a re-belay to get down onto the main pitch.
I rigged a large Y-hang from the two main girders tied with the standard bowline-on-the-bight. From here I ran a re-belay loop up and over the lip of the shaft and off to a tree. To avoid dirtying the rope I left it coiled in the bag and tied it off about 4 metres below the Y-hang to be released by the first person down – me. A fairly tried and tested rig at this site. I did all of this sitting at the top of the shaft, I guesstimated the lengths of the Y-hang arms; attached them to the girders, rigged the re-belay and then started to lower the whole assemblage into place.
I lowered in the heavy rope bag first, let the line slide through my hand until I got to the knot, let this go and continued to pay out on the re-belay line until the Y-hang eventually came tight onto its intended belays. All simple no-brainer stuff. Just as I was letting go of the re-belay loop to drop it into place, there was a twang and the whole thing went out of shape.
The knot appeared to have capsized allowing one loop to run straight through the knot into the other; the Y-hang then slid across until it was all at one end – one giant loop. I have seen this happen before (from below and had to prussic up it!); the rig is still safe in so far as it will not come undone, but it puts all the load onto a single belay which is definitely not ok.
I was rather concerned as I was certain I had tied the knot correctly (I can tie this one with my eyes closed – literally) so I hauled it up and checked it. It all looked ok except that the knot had capsized. After a while I decided I must have made a mistake so played safe and took it all apart and started again; this time making very sure I tied the knot correctly; checking and double checking. I lowered the whole assembly into place and twang – it happened again!
This time I knew the knot was correct so the fault had to lie with the way I was loading it whilst lowering it into place. The weight of the bag (with 45m of rope in it) is pulling down on the main rope. As soon as I start to lower on the re-belay loop the strain is across the two knot-tails at 180 degrees, which inverts the knot.
So, confident that I had found the flaw I rigged it all again, and this time I lowered the whole assembly in by the Y-hang loops of the knot. With the load only on the main rope and no load at all on the re-belay loop it al dropped into place as intended.
So an unusual and not entirely obvious potential flaw in the standard Y-hang rigging method to watch out for. Using a figure-eight-on-the-bight is better, the knot is less prone to capsizing but this is still possible. The only sure solution is to be careful how you load the knot when it is not in its usual working position, and where possible lower the Y-hang into place by its ‘bunny-ear’ loops not by the re-belay loop.