Shelley was a good friend, a great person and we all miss her. Not a single day of climbing has gone by that she is not in my thoughts. In the weeks and months and even years following the accident I desired to be in relative privacy regarding the matter. I spoke about the events that day to her family, my close friends, and the authorities only. I believe that decision was selfish- It was what I could do at the that time- and you were all looking for answers, maybe now, still, looking for closure. In response to all the inquiry and investigation that was done and the accident analysis here is my account and observations to help provide some clarity and understanding- sorry it took a long time to respond to this thread.
There were two pieces of blue 1" tubular webbing ~15' linear, tied with water knots to make 2 individual slings. The two slings were wrapped around a large diameter single pine tree and probably girth hitched. I did not closely inspect the anchor Shelley built that day, something I always regret.
From a distance of about 25' - 30' It was apparent the slings were wrapped around the tree, and appeared to be girth hitched. Shelley tested the anchor by locking the rope in the belay device and pulling on the system with body weight before stepping over the edge of the cliff. The anchor system passed the initial weigh test. She fed slack into the system and moved towards the cliff edge. Shelley positioned, applied tension and began to rap down and then both slings slipped off the tree. Both slings were found at the bottom of the canyon connected to a locker and to the rope by a figure 8, and to Shelley by her belay device and harness.
I returned to the site again, years later, to try and reproduce the anchor failure. Using webbing similar to the kind Shelley used - it was in good used condition, with some fuzzy texture- but still within normal wear parameters. I began by girth hitching some long slings around the pine tree.
The tree is large enough in diameter that you can't reach around it. The slings have to be tossed around the tree and caught with the opposite hand, it’s a bit awkward to do. I tossed the slings around the tree many times. The fuzzy texture of the used nylon webbing sticks to the pine bark and sap a little bit like Velcro; enough that you can let go of one sling momentarily and it will stick to the tree, and hang there - on some tries and not all the time.
Using two slings of the same color it's hard to differentiate the top sling from the bottom sling. If you toss both slings around the tree they can get crossed behind the tree. Then if you accidentally girth hitch the top sling into the bottom sling and the bottom sling to the top sling and lock it with a locker there is not a secure attachment to the tree. If a force is applied to the anchor at this point it can fail, but may resist initial detachment because of friction. The friction generated from the nylon webbing crossing- one sling onto the other- behind the tree, and the pine bark, will resist a powerful tug to the anchor. This effect was not easy to reproduce but after some trial and error with a few attempts it is reproducible.
I think that is the most likely way the anchor failed, but there have been other plausible explanations.
It was a good mild weather day, we climbed three routes, and were both going to be rapping back in to have a short rest break and cool off, on the canyon floor, at our packs. I was a having a rest catching my breath near the edge of the canyon at the belay spot, where we just topped out. Shelley set off with the rope to set up the rappel for us both, she wanted to get down to her pack, take a rest, eat and drink some water, I was to follow.
Shelley had mentioned to me earlier that morning on the drive in that she had on occasion mild anemia that she treated with molasses, but just before setting up the rappel, she did not appear to be lethargic, look flush or seem ill. She was a bit anxious to get to the pack, and get to a place to urinate, but to address what has been suggested- we were not in a big rush to get down either. We were about three routes into our day and both ready for a short break but It did not look like Shelley was too fatigued from climbing to safely set up the next rappel. I will always wish that I did walk over and double check the rap system.
Always double check, and check each other, no matter how experienced you are, take care of each other, and love each other. Shelley you are loved and missed.