How strong is strong enough?
The authors’ believe that rock anchors should have the following MINIMUM specifications:
- Last as long as possible, at least 50 years *1
- Have a Safe Working Load in excess of 16kN using a Factor of Safety of 3 (shear or tension)
- Not deform at loads up to 16kN (shear or tension)
*1 Consider that there are possibly in excess of 10,000 bolts already placed and that more are being placed all the time. To replace say 1000 bolts (about 100 routes) it could cost over R20,000 and take many man hours. Removing bolts also often scars he rock. For these reasons we would like to see the longest possible life span for bolts.
The argument is that sport climbers could routinely generate this in the following circumstances:
- using a Gri-gri
- a fairly stiff (but still dynamic) rope
- 100kg climber
- Fall Factor 1.0 falls
Note that carabiners (and most hangers) have a failure rating of 20-22 kN: this is not their Safe Working Load. It is the load at which 95 out of 100 hangers will not break (5 percentile). But the other 5?
But since they are METAL parts, and are attached to another METAL part, the safety factor does not have to be 3, it can be between 1 and 2. If one uses the Hilti guide of 1.75 for steel-steel failure safety factor, then their Safe Working Load would be around 13kN. And hangers are replaceable if deformed, whereas glue-in studs, mechanical expansion bolts, and U-Bolts are not easily replaced.
Perhaps these criteria could be reduced for some existing bolts. The MINIMUM would be a load of 100kg climber at Fall Factor 1 on a fairly stretchy 10.2mm rope. This is around 10kN. One must then decide what the minimum safety factor would be.
None of the existing anchors installed at the crag appear to be very satisfactory. Therefore, a new type(s) should be used for new anchors. Using the more exacting initial standard, ALL of the existing Swinburne U-Bolts fails the criteria.
If one does not use ANY Safety Factor (i.e. Safety Factor = 1.0), then the following analysis can be made:
- An unknown number of the SAR U-Bolts might fail. Although their AVERAGE tensile and shear rating is greater than 14kN, they are VERY variable. Thus, some could well fail at much less than 5kN load.
- The Davies U-Bolts (U2) will likely deform under both shear and tensile loads, but will be unlikely to fail. A climber taking a big fall on them will likely deform them, requiring their replacement if they are badly deformed.
- The Fergusson U-Bolts are more problematical to analyse. Based on the limited testing (one U-Bolt) and comparing to the Davies (U2) and Long U-Bolt (U1), it would seem that the deeper version (100mm as opposed to 50mm) might be strong enough. However it is likely that even this deeper version’s performance will be varied due to their installation in “interference fit” holes (8mm leg in 8mm hole). And how does one tell by looking at an installed bolt what length it is embedded in the rock? They both look the same from the outside. Unless one was sure that the bolt was the long variety, then this type is not inherently safe.
The Davies U-Bolts appear (barely) strong enough to remain, but no more should be placed at Swinburne, or in other soft sandstone crags. All other U-Bolts at Swinburne appear to be potentially dangerous and require urgent evaluation on a bolt by bolt, route by route basis. Ideally ALL U-Bolts and expansion bolts at Swinburne should be replaced.
Future bolts at Swinburne (and other sandstone crags)
The following list is not exhaustive, but suggests variants that appear acceptable:
- An M10 glue-in with Upat ampoule, and hanger (this is an “off-the-shelf” solution). This combination is EXTREMELY strong in both tension and shear.
- Mechanical expansion bolts are not recommended. This is because they appear to pull out at fairly low loads. It is perhaps possible that a longer M10 mechanical expansion bolt (Hilti or Upat) with a hanger might work. But it would have to be tested. The authors believe that even if it is deeper than the tested one (M1), it will still probably pull at an unacceptably low load.
- Perhaps a Petzl eyebolt with Upat capsule glue. The problem is that that the eyebolt is expensive, over R35.
- A U-Bolt made with a stronger material than 316 stainless (bigger diameter as well), and using better glue, along the lines of the Upat ampoule glue (although one cannot use an ampoule because a U-Bolt does not allow one to “spin” an ampoule in the hole). It is not recommended to have smaller leg spacing than the tested ones of 50mm.
Bolters must consider very carefully their glue choice and procedure: it is critical. Epidermix 372 does not appear to be a good choice, because its mixing is quite involved, and its long full cure-time (7 days). It is also quite “messy”.