Drilling Abalakov threads

[aricooperdavis]

This week I've been caving in the Vercors with the Shepton (amazing trip, thanks so much all!), and have enjoyed my first forays into rigging with dyneema cord.

We saw quite a few approaches to this, but the most interesting for me was the use of "Abalakov threads", where two holes are drilled in the rock such that they intersect. Dyneema cord is then passed through these holes, and used to anchor the main rope (by means which are also interesting but for a different post).

I have a few questions arising from this for the UKCaving community, particularly for cavers who have actually drilled/prepared such anchors.

- How do you ensure that the drilled holes meet? Is it just practice, or is a jig of some form used? Does anyone have any examples of one?

- I've read that the intersection of the drilled holes are smoothed to remove sharp edges that may excessively abraid the dyneema cord. How is this done? Does this impact the choice of drilled hole diameter?

- Experiments in ice suggest that strengths are higher when the holes are drilled in parallel with the direction of pull (rather than perpendicular to it). Why, then, do cavers tend to use a horizontal arrangement?

I would love to hear any other experiences you've had with this type of anchor!

 

[wellyjen]

There was a thread (sorry!) about these last year.
https://ukcaving.com/board/index.php?threads/berger-rigging-change-of-“style”.29732/

 

[aricooperdavis]

Thank you, wellyjen, that thread is well worth a read but does still leave my unanswered questions above I think.

Having had a bit of an experiment this morning, I may have an answer for why a horizontal arrangement makes more sense that a vertical one for cavers. A vertical arrangement results in a higher bend angle and lower bend radius for the dyneema cord where the higher drilled hole exits the rock. This increases the wear on, and reduces the breaking strength of, the dyneema cord. This factor is less important in ice where the pressure of the cord on the ice causes it to melt and smooth off the edge, and where the ice is the weakest component in the system rather than the dyneema cord.

 

[mikem]

Horizontal is easier to line up by eye, when drilling, than vertical. Threads have the advantage over epoxy bolts that they can be used immediately & are cheaper for the installer, but it will be interesting to see whether they are as economical over the long run, & as safe...

 

[aricooperdavis]

Horizontal is easier to line up by eye

I hadn't thought of that as all my testing has been in boulders on the ground, but of course that's a big factor in the cave!

Threads have the advantage over epoxy bolts that they can be used immediately & are cheaper for the installer

My interest is in Cornish metal mines where almost any fixed metalwork rapidly dissolves due to the metal ions in the rock and prolonged exposure to salt water. Rock quality, particular given that shafts have usually been driven using explosives, will be a big issue though.

 

[mikem]

You'll need a longer threading tool - bolts in mines tend to be bigger than those used elsewhere, although a lot of that is to do with many being traverse lines or zipwires, that transfer a much greater load onto them (& the flakey rock).

Chemicals may still be an issue, as few cords are pure dyneema (& blasting may also have created edges within the rock):

Harmful substances for ropes and slings | EDELRID

This EDELRID Knowledge Base article is about how chemicals affect textile PPE and which substances are particularly critical.

edelrid.com

 

[ChrisB]

A vertical arrangement results in a higher bend angle and lower bend radius for the dyneema cord where the higher drilled hole exits the rock.

That occurred to me too. It would, however, be quite easy to put a larger radius on the bottom edge of the upper hole, or even drill a very short hole at 30º upwards to intersect it.

Cavers may be assuming there's no benefit in using the vertical orientation; the tests mostly failed by fracture of the ice, so most rock will be strong enough that the sling would be the weak part. In weak or fractured mine rock, however, there might be a benefit. The choice of orientation may also depend on the rock structure; if there are vertical planes of weakness, put the anchor horizontally.