As a bit of a local I have been through Croesor Rhosydd more times than I care to recount and have enjoyed watching the fixed aids change over time and have, where appropriate, adjusted and tidied things up where abuse has caused failure.

I was in there on Saturday just gone and found the first steel zipline had snapped, I tensioned the old rope hand line which can now be used with care to avoid getting wet but this won't last long.

That steel cable did not last long (2/3 years). For two reasons I suspect. Firstly it was far too thin and secondly people are turning up with no pulley and dragging malions and crabs across the cable which chews through the top leading to a snap...

I am looking at replacing the steel cable but before I do, is there any individual or group that takes responsibility for the last round of rigging that happened in there? I am aware some funding was found previously?

My thought would be to install a decent heavy gauge cable along with a fixed pulley which can not be removed. This along with a braking damper on the other side should last a good while. An expensive solution though but maybe worth it?!

My worry is, given how popular this trip is with "non technical" people, if someone abseils down the two pitches and gets to the zipline to find it missing they either have to swim across and try and get out on the other side or attempt the climb back up the ropes, both are potentially hazardous for the ill-equipped!

I have a length of galvanised cable sat in my office that should be sufficient.

Any idea what length is required?

There's also a bag of various fittings that may be of use as well - I'll try and drag the bag out and see what's in it...

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I'm very alarmed that a zipline in there actually snapped. It must have been worn down to almost nothing and people still were using it? Our 12mm lines will hold even with only 1 of the 7 strands intact, so this line must have been in terrible condition indeed.

That's a great way to get our first CRTT fatality.

Unless, as a more likely cause, a rock fell on it like what killed the bridge. In which case the anchors, which were way undersized last time I saw them, will be good for scrap now after a shock like that.

I might be able to get a small team of helpers together? we're just working through the Go Below ziplines at the moment replacing them all, so quite experienced at this stuff. Might be able to get the guys out there to put a new thick line in with massive anchors etc?

Or happy to donate some money towards new gear if you'd rather do it yourself.

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@ Vanoored....

Amazing, thank you. What diameter is it? Some of the existing fittings may be useful if the threads are not totally rusted.

I need to go back with a measuring stick and double check but I would say its around 15-20m. The equalized anchors look ok for the time being so the new cable can go straight into the maillons as per the snapped cable.

@miles...
The cable appears to have worn due to miss-use rather than collapse. It was a 6mm or possibly an 8mm... far to thin! The cable was very thin and I suspect not good quality, the same cable appears to have been used further on in the mine where a consequence of failure would be a lot worse. On inspection this weekend the others do appear to be ok at the moment but worth keeping an eye on.

Some pretty dubious rigging of safety lines as well using the same anchors and I even spotted a death triangle which I need to re-tie next time I am in there...

I am not set on doing this myself, if its just a case of pulling in a new cable and re-using existing anchors I can do that by myself. You are right to suggest what is really needed are some proper anchors and a serious piece of cable as per the lines in Cwmorthin... The time and cost in doing that may be prohibitive for hobby users but il happily help make it happen if there is enough interest in doing so.

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I think 12mm or 1/2" - that being the maximum diameter that will fit between the cheeks of a Petzl Tandem pulley (IIRC that's also the size of what was there a while back).

The trick with rigging it is not to bring it too taut, otherwise the forces on the wire and the anchors will be massively increased.

IIRC something like 5 degrees off horizontal, the force of an 18 stone individual exerts a force or around 3T on the anchors.

Miles should have some familiarity with the calculation but it is necessary to have a bit of sag built into the cable, as this will significantly reduce the loadings - particularly as a steel cable is not dynamic and will not stretch and sag.

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Yes I remember the original 12mm cable and large industrial pulley block. It was there for atleast 10 years if memory serves before one of the strands snapped and someone replaced it with the current 6/8mm thing....

Agree on the required sag, it would also help prevent excessivly enthusiastic travellers from hitting the wall on the other side with too much force 😛 :surrender:

Okay, basically this is the deal with Zipline loadings.

There are two important numbers for a zipline. The first is the Breaking Load (what it snaps at) and secondly the Safe Working Load, which for ziplines is usually 1/5 of the BL for most types of cable.

If you load a cable to the BL, it snaps instantly of course. If you load it to the SWL or below, nothing happens, there is no stress or fatigue and you can basically load and unload it repeatedly forever (wear and corrosion permitting).

Now, if you load it to a figure between the SWL and the BL then it does not snap, but it will slightly fatigue and the BL will come down slightly from before. Load it the same again, the BL comes down a bit more. Eventually the BL reaches the load and it fails. It might take 5 goes. It might take 1000 goes. But it *will* fail as the number of loads it will take between SWL and BL is finite.

6mm cable has SWL of 470kg and BL of 2350kg.
8mm cable has SWL of 836kg and BL of 4180kg.
12mm cable has SWL of 1881kg and BL of 9405kg.

We load test our cables with a sag of about 10%, and with a single 100kg person on the line it can go to about 900kg cable force. Two people, which we test for in case we need to go help somebody stuck, can see it rise to about 1700kg. If the sag is less, around 5%, these figures double.

So if the CRTT lines are 6mm, or even 8mm, assuming a sag also of about 10% would see the SWL exceeded. Greatly so if it is 6mm.

So you can put a 6mm line in fine, and test it over and over and it will probably be OK for a while but it is guaranteed to fail at some point, because every time you go on it the BL comes down a few percent. It's just a waiting game.

This is why we use 12mm cables at GB, nothing smaller stays comfortably within the SWL (although 10mm does if you don't need to rescue people). Nor can you generally go bigger as virtually every pulley/trolley on the market has a maximum cable size of 12mm-13mm.

We have a special machine that goes around a zipline and tells us to the KG what forces are currently on it. Very useful to ensure that your lines are safe.

All this talk of cable says nothing of the anchors. Last time I looked that first zipline was held in by a couple of Petzl M10 expansion bolts. The SWL of those? Less than 500kg.

We use M30 resin studs on ours, 50cm deep. You get a SWL of about 7,000kg on a pair of them. Fit and forget!

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I would be more than happy to help replacing the wire and new anchors if you need some help Miles.
:)

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Miles' explanation of how to specify zip wire cables is basically correct but it's important to realise that it only applies to plain steel cable and fittings. Fittings and cable made of fancier materials such as high strength steel alloys, stainless steels and aluminium must be specified in a different way.

The reason for this is that plain steel can be repeatedly loaded at a small factor of its yield strength indefinitely without undergoing fatigue. Many fancier materials do not have this property and will undergo fatigue if loaded repeatedly, no matter how small the load.

Generally speaking this does not matter because the ultimate strength of the metalwork is huge in comparison to the actual loading (whch is why the BCA 'P' anchors are fine - it's the resin which fails and not the metal) but when you start to look at the loads imposed by zip lines, fatigue strength becomes an important factor.

The factor of 5 mentioned by Miles is well established in industry and (based on many years experience in many, many applications) takes account of wear, corrosion and manufacturing tolerances, but it does not necessarily take account of fatigue so for materials other than plain steels, a different factor may be required.

It can be tempting to think that stainless cables and anchors will provide better service than plain steels in a damp environment such as underground, but for zip wires at least, this may not be the case.

Thanks Miles and Nick, Thats really interesting. As Meaty says, I too would be more than happy to give my time to help lug stuff in to do the job properly.

Thanks for the input Nick.

Yes I should have said that applies to normal galvanised steel cable, not anything exotic. But then this application doesn't warrant anything exotic.

As a rule we don't use Stainless cables, except in one place where the cable is subject to daily submersion.

Wear tends to be the main reason for zipline retirement. We generally use 7 strand 19 wire, independent wire core, right hand ordinary lay which is very bog standard. About 5,000 crossings with adults on Petzl Tandems is enough to start seeing the wear on the top start to cause individual wires to sever.

Having the cable able to rotate with swivels greatly extends the life as the wear can evenly go all over the cable body.

Would like to try 7x7 cable as it can take much more wear than 7x19 but I believe it is quite stiff stuff to work with. Our usual supplier doesn't do it in 12mm.

A nice lock-coil cable would be super smooth but it's very expensive.

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I would be more than happy to help replacing the wire and new anchors if you need some help Miles.
:)

:thumbup:

Might be able to get a working party together then, maybe PK/MC/ML would be up for a raid on it.

Been a while since you broke one of my drills? Must be overdue :lol:

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Glad to hear nobody was hurt with this zip line failing!

It's surprising to hear it's failed without an external influence. Cable zip lines unloaded have fairly low loads/forces, really just reflecting the weight of the cables. The trigonometry/theory of the forces involved are often misleading in practice. I wouldn't be surprised that something fell onto the cable and damage it given the absence of a corpse attached to the cable lighting up the pool!!

I'd be happy to help out with some replacing of the cable and anchors in particular. Would be nice to see those expansion anchors replaced with some larger resin anchors places a little further apart.

Stainless steel wire can also be a bit more brittle than galvanised so under some circumstances - ie where there's regular movement concentrated on a point - that factor should be included alongside considerations about corrosion resistance, abrasion resistance and general flexibility.

My feeling is that the potential for issues with stainless wire cracking would be sufficient to rule it out.

Anyway - I've got 60m of 12mm galvanised wire sat in the corner of my office, waiting to be used.

I'll get a photo of the various rigging bits that are with it and upload it later today or (more likely) tomorrow.

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given the absence of a corpse attached to the cable

That water is deep and people often do the trip on their own.

Still, an abandoned car would ring the usual alarm bells.

Thanks for offering to help Detrus. Pete K and some of the other fellas have offered a day of their time so if you're game too that's quite a good team to go to a raid on it.

Vanoord - yep if you got 60m of good 12mm cable lets use that.

We have load of ferrules and eyes and stuff, and a hydraulic crimper.

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Isn't stainless wire more usually used for less critical applications as it is weaker (~8t for 12mm as opposed to ~9.5t for galvanised steel).
I suspect that greasing the rope would make it less susceptible to wear from people using the wrong gear as well as making it horrible to grab hold of! :devil:

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the absence of a corpse attached to the cable

IIRC there was a rope line in addition to the cable, so potentially if the primary cable did break under someone's weight, then they should have been caught by their backup.

At least, I think that's what's meant to happen... :o

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Maybe post up when the work will be done? I'm currently a bit swamped in work but that should get better at some point. Would be fun to lend a hand.

the absence of a corpse attached to the cable

IIRC there was a rope line in addition to the cable, so potentially if the primary cable did break under someone's weight, then they should have been caught by their backup.

At least, I think that's what's meant to happen... :o

Yeah, not seen the more recent one but the one that was there last time I did the trip was so saggy I'd have been 30 feet under the drink before it would have come tight. So I didn't even bother clipping to it.

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