V-Thread Anchors
A drilled thread in rock is a low-impact way to anchor for recreational and rescue purposes without leaving behind bolts.
The technique is well established in ice climbing, and works in a very similar style in rock.
Pros
- Cheap - saves on expensive hardware (bolts, hangers, rings, etc).
- Easy to replace - In the event of being damaged, the cord can be cut-out and replaced. This makes it suitable for anchors in the path of floods and avalanches.
- Easy to decommission - much easier and cleaner to decommission a no longer required anchor station.
- Flexibility - the strength of the anchor can be adapted by changing the hole spacing, hole diameter, and cord diameter (whereas bolts are a fixed diameter & depth).
Cons
- Requires specific tools & equipment - a threading tool & dyneema cord may not be in a standard canyoners pack.
- Anchor movement - compared to a bolt anchor, the anchor manager will feel the cord shifting as the rappeller loads and unloads the rope. A backup removable bolt is useful for the anchor manager to avoid this.
Tools & Material
There's a few v-threading tools (designed for ice climbing) on the market.
- Jsnare https://www.jsnare.com/shop
- Petzl Multihook (two versions)
- Grivel Candela
Alternatively it's straightforward to create your own from a coat-hanger. Just use pliers and a file to create a pointed hooked tip.
Technique
(To expand).
The drill will not want to bore at an angle initially. Instead start by making a small vertical hole ~1cm deep, then reposition the drill to the desired angle. The small vertical hole will help keep the tip in place while drilling at an angle.
A 60° angle makes the depth calculation easier, as all sides are the same length. e.g. Two 60° holes drilled 7cm apart, will meet 7cm deep.
Backup
A backup anchor can be wise choice to use until the v-thread has been tested for real. A removable bolt provides both a good backup to a v-thread, and also helps manages the anchor. It can then be removed by the last person before they descend (LAMAR).
Important note: removable bolts often need very specific holes, requiring precise drill bits (e.g. 12mm not 1/2") with 4 cutting heads. Know how to use them before placing them for real.
Position the removable bolt hole such that an alpine quick-draw can loosely link the bolt and rappel quick-link. The anchor manager can also load the removal bolt with their weight, allowing the cord to only be pulled in a single direction (by the rappeller).
Load Testing
Over the Edge Rescue in New Zealand have done lab tests on many types of threads & cords.
Below is an exert, testing 5mm dyneema cord, 75mm spaced holes, 12mm diameter holes, 50 degree junction.
Various Links
- Backing-up a v-thread, described for ice, but same principles apply.
- Rock Thread and V-thread Testing by Over The Edge Rescue (PDF Report)
- Beal 5mm Pure Dyneema from Adventure Plus
