Fellow people. This article will be a little longer than the previous ones and it will contain more (useful) information. Thus we have to establish a few things:
The term main (system) is used to describe the load bearing equipment in your rig. This includes the slackline, the anchoring system (slings, weblocks, etc.) as well as the anchor points (trees, poles or ground anchors and frames). Depending on your rig, it includes the tensioning system the entire time (hard-pointed) or only during the tensioning and detensioning process (soft-pointed).
The term backup (system) is used to describe the load catching equipment in your rig. These parts are not under any load, unless the main system fails. Different backups can catch (counter) different main system failures. A stopper knot behind the brake catches a ’slipping brake failure‘, however, it is not able to catch an ‚exploding brake failure‘. A system is only backed up in it’s entirety if every failure mode is covered by at least one backup, given that all backups are substantial (able to catch their specified failure(s)).
Additionally, I’d like to point out, that everything in this article is my personal opinion. It is not to be understood as a recommendation, guideline or tutorial to safe rigging. The entire rig is in fact a ‚fictional‘ one. I would never rig a slackline in this specific way, it is only meant to explain certain aspects of rigging.
After that had been said, let us begin by first describing what is about to happen. I am going to illustrate (with words and pictures!) how I rig a ’normal‘ slackline in the park. The process will be described shortly, as this is not a how-to, and then I shall, in more detail, illuminate some of the ‚key‘ problems one encounters with slacklining whilst trying to prevent disintegrating your or someone else’s face.
1 – Hand Tension
Firstly, I set up everything. This process will vary from line to line, length to length, day to day. The order in which the single steps are done can vary, but regardless of what kind of slackline you rig, these few things are happening now:
- Spot assessment (Do I obstruct or endanger anyone with my rig, am I allowed to be here?)
- Anchor assessment (Do these trees look and feel solid?)
- Ground assessment (Can I fall, do I need shoes or even a crashpad?)
- Install tree protection (Yes, always!)
- Set up static side anchor
- Set up tensioning side anchor
- Hand tension your main system
- Defining rigging* and playing** area
- Final assessment (Is everyone out of the rigging area, can I start tensioning?)
* Rigging area is the area in which the main system failures occur before being caught by their respective backups. This area is off-limits to everyone who doesn’t understand the rig and should only be entered when absolutely necessary. The area is usually defined by a circle around the anchor point with the radius of the fully extended backup system. A tree as an anchor point can create a ’shadow‘, which is safe to walk in and should be your location during tensioning if possible.
** Playing area is the area in which it is safe to move about for anyone, as long as there is noone on the slackline. If there is a person on the slackline, noone should be close enough to interfere with that person or to be caught by the line.
2 – Tensioning Process
- Install a backup to ensure the rigging area is well defined (so that nothing can fly outside that area)
- Tension your line (if you test the line’s sag in between, make sure to install further backups that you might deem necessary/useful (i.e. a stopper knot after the brake))
- Complete your backup system (try to reduce the size of the rigging area as much as possible)
3 – Play Time!
- Noone is to enter the rigging area
- Be aware of your playing area (don’t have people run into the line, etc.)
- Check up on the backup system every now and then
4 – Detensioning Process
- Redefine your rigging area (if no one is around and you can detension from a safe place, you can deinstall all backups if you desire)
- Detension from a safe place if possible
- Clean up
This shouldn’t be news to anyone slacklining, so let us proceed and check out some of the aforementioned steps in more detail!
This is where we stand after completing the Hand Tension process. Our backup consists of a simple tieback with the free end of the webbing around the tree. The rigging area is indicated by the crutches.
Nearing the end of our Tensioning Process. The brake (Sla’Knot and small pulley on steel carabiner) is backed up against slipping by the chest ascender jamming into the front pulley. A slippery hitch with 2 additional half hitches around a solid point will take that function, so we can deinstall the multiplier.
After Tensioning Process is finished. Two individual backups are installed to catch various main system failures. The rigging area is now described by the length of the longest backup plus the length of the pulley system (the static side pulley could whip around, if anything would fail at the back end). I am not entirely sure, if the crutches indicate that area correctly. One should move them a little further outward from the tree, but I couldn’t be bothered to limp around again to take yet another picture.
The static rope backup is installed in a way, that it can catch the carabiner breaking failure without letting the ring fly. At the same time it acts as a backup for the entire system by connecting the front of the pulley system to the anchor point. This is achieved with double-collared bowlines. Bowlines aren’t the most robust knots, which is why I tied an additional overhand inside the load eye. Bowlines are also susceptible to ring loading, but I choose this knot, because it is easy to tie and untie.
The static side anchor. The backup is very much alike the one on the tensioning side. The only difference is the position of the backup knot for the bowline. In this case it consists of 3 half hitches around the shackle to give it a second connection point.
Now we rigged a line, that is very possibly safe to use in the environment it was put in. I assume, that most of what has been described so far is not very controversial. Going onward, I predict, that this will change. I will now look into the choice of equipment for every single point and try to describe my thoughts on it. Again, this line was rigged specifically to illustrate some of these opinions, so it is not a representation of how I usually rig or what I consider reasonable.
We need to make a distinction between safe and reasonable here. Safe is a system that is very very unlikely hurt anyone, even if it fails. Reasonable is a system that is very unlikely to fail in a certain environment whilst considering things like weight, time spent rigging, price and so forth.
Anchor point and area assessment:
We are allowed to slackline here whenever we want. Today, very few people were out (early hours of a cloudy week day) and the slackline doesn’t cross anyone’s way and is pretty visible. Both trees are solid. Roberto (the tensioning side tree) has even been addressed as „one fine tree“ by the city’s official responsible for our park. The ground is nice and soft, which doesn’t matter, since I don’t intend to walk a slackline 13 days after my knee surgery.
Static side anchor:
This is the part which I am the least happy with. Backing up weblocks (against slipping and braking) is not an easy task. I must admit, that I haven’t found a satisfying way to achieve it. Thomas Buckingham pointed out, that it is important that the free end of the webbing (exiting the weblock) should be redirected around the diverter, because he encountered weblocks in which the free end walked sideways around the the tensioned end. If that happened, your weblock would slip (it might even uninstall the webbing completely if it isn’t tied back). I have seen that issue in Monte Piana on a relatively short highline. Low tension to high tension transitions seem to an important factor in this. My line doesn’t see any transitions (other than wind fluctuations), but I still decided to keep this in mind. Therefore the webbing is redirected around the lower side of the diverter, through the shackle and then terminated with a bowline around the tree. Another thing to keep in mind is the angles. A larger angle increases the force on the anchor. Shackles take tri-loading reasonably well, but they aren’t meant to be loaded fully horizontal either.
A little side note:
Bold numbers indicate the MBS indicated by the manufacturer. It is a reliable number. However, the MBS is tested in a very specific way. If you deviate from the recommended use, it no longer has any value.
Italic number indicate the EBS entirely made up by me. It is not reliable. However, it is necessary to estimate certain things, because some of our equipment is used in ‚incorrect‘ ways.
Our choice of gear is as follows:
- Conventional tree protection against abrasion
- 70kN 2m PES round sling
- 60kN 12mm stainless steel shackle
- ??kN weblock prototype (Yang?)
- 26kN Sigma N polyamide tubular
There is little to say about this. The round sling is very very strong and 2m seems about the right length. It can’t be any shorter, but a 3m one would be too long. The shackle is plenty for our rig (28m @ 4kN). Stainless steel shackles bend easily if loaded on a small surface. Keep that in mind when using a lineGrip or placing your pulleys on the pin rather than the bow. I indicated the MBS of the weblock as ?? although I know it’s strength. I decided to do this, because very often, you don’t know the MBS of all the equipment. Especially when you walk lines that aren’t rigged by yourself and use gear you’re not acquainted with. Now we need to formulate an EBS for this piece of equipment. EBSs should be defensive, so I am going to say 30kN. The tube is a good choice. All in all, the static side anchor is made up of reasonable choices. We have high safety margins and not too much excessive weight. A lighter and weaker sling would do the job as well.
Tensioning side anchor:
This should be a little more interesting. Starting with the 12mm delta quicklink (56kN), we can see one of the upsides of the right-angle-reeve. No tri-loading in the connection point. From all the equipment I own, this quicklink is the best possible choice. The carabiner connecting the roundsling to itself not so much. The sling is too wide for the carabiner to be properly loaded (they are tested with 10mm wide round connection points). Although the MBS is 30kN my EBS is 20kN. This is based on a test, slacktivity has performed on their carabiners, which indicates a 10% strength loss for 25mm wide connections in oval carabiners. Therefore, this carabiner is on the verge of being an unreasonable choice. On this short line, I am not particularly terrified, but I wouldn’t mind bringing a shackle instead.
The sling for the brake consists of 20kN polyester webbing. It is tied to a loop with a 1.5 waterknot. The EBS for the sling should be 20kN. We triload the carabiner pretty badly. However, it should not see more than 1.5kN during use, so I would consider this a reasonable choice. The brake apparatus itself relies on the friction the aramid-sheathed prusik lanyard can generate. The lanyard is rated to 22kN eye-to-eye. The gripping power is sufficient for this line. Reasonable choice again.
CT double pulleys are rated to 50kN. They aren’t very heavy. The static rope takes 33kN straight, a minimum of 18kN with a figure of eight knot. We use a double fisherman’s as a termination. Shorter knot, equally strong. The entire system is a little overkill for 28m, but still within the ‚reasonable‘ category. The little quicklink does not need to exceed the 22kN MBS of the pulley’s becket. A larger diameter increases the efficiency of the knot, but that is not necessary here. I would be fine to subject this pulley system to 10kN of tension. However, I would switch to a different brake if I were to go any higher than 6kN. An Eddy or a MPD make reasonable choices, depending on the length and the stretch of your line.
Congratulations, you’re almost there. This conclusion doesn’t conclude anything, since we haven’t gained any actual information from this little trip to the park. Nothing failed. I could just as well have stayed at home. However, I want you to realize something and that is the difference between a safe system and a reasonable one. Consider the ways your equipment can fail and make sure, that if it does, it can NOT hurt anyone. Carabiners have been breaking right and left these days, this doesn’t mean anything changed about carabiners. They do, what they were intended to do. Misuse them, if you want, I surely will (they are so much sexier than shackles, aren’t they?), but never ever not back up.
How do I make this clear? Let me show you these ‚calculations‘:
I rigged my slackline in a specific way 1000 times. It never failed. Now, I don’t know the probability of it failing, but I know it could fail the next time I rig it. So, let us assume it does fail the next time. Now the probability of my rig failing is 1 : 1001. That is not terrible, but still not great. But, I also install a backup every single time. This has been loaded 5 times (because I forgot to undo it before detensioning) and it held every single time. It also held, when the main system failed on the 1001st rig. Again, let us assume it fails the next time. This means the probability of my backup failing is 1 : 7.
By using a backup every time I now have a failure rate of 1 : 7007 (this is not true, because for our imaginary backup failure, we would need another imaginary main failure, so it should be about double of that value). If I were to install a second identical backup, this probability would shrink by a factor of 7 again. And a failure rate of 1 : 7 for my backup is very defensive.
This is why backups are such a powerful tool to rig a safe line.