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Why are sleeve bolts better than wedge bolts in soft rock?
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Conventional wisdom seems to be that in soft rock, sleeve bolts (like 5 piece bolts) are better than wedge bolts. Why is that? I've heard people mention that the surface area in contact with the rock is higher with sleeve bolts, but this doesn't check out. The angle of the cone is generally the same on 5 piece bolts and wedge bolts, so the actual portion of the sleeve/clip that is being pressed against the rock by the cone should be the same for sleeve and wedge bolts. So why do people prefer sleeve bolts to wedge bolts in soft rock? |
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You answered your post in your post. The sleeve has more area than the wedge. |
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...but it doesn't actually have more area that is pressed into contact with the rock by the wedge? That extra area doesn't actually get pressed into the rock. |
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This is why:  The sleeve expands massively, digging into the (super soft) rock, and locking it in nicely. A wedge bolt sleeve simply won't catch or expand in rock this soft (so you wouldn't be able to achieve proper torque when tightening the wedge bolt). The only reason that we can see what this bolt looked like under the rock is that a big chunk of rock cracked off while removing the old bolt and the entire bolt came out - which is a good reason to never bolt near an edge in softer rock, this bolt was only about 4-5" from an arete. We were replacing everything with 6" long Fixe glue ins - this was at Castlewood Canyon, Colorado. |
Alex Fischer wrote: Guess I’ll be asking my alma mater for a refund on that mechanical engineering degree then |
Alex Fischer wrote: The wedge bolt only allows a very small amount of compression to happen in a much smaller area with in the hole and that is where the collar meets the cone. That is it.. The sleeve of the sleeve anchor actually fills up, makes more surface area contact and is a much tighter fit over the entire hole, unlike the single collar point of the wedge bolt. |
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Part of it is also that sleeve bolts don't rely on the bolt rock interface to tighten. Wedge bolts in my experience generally pull right out when tightening in soft rock as the rock must 'hold onto' the clip on the wedge against the tension you create tightening it order to expand the clip over the end of the wedge. On a sleeve bolt these forces are basically contained within the bolt/sleeve. The bolt rock interface isn't really needed much for the tightening of the bolt Try expanding the clip on a wedge bolts over the cone vs expanding a sleeve bolt over it's cone while the bolts are not in the rock. |
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Hi, I did an analysis on a fatality caused by wedge bolts in soft rock and can send you a copy if you give me an email (miklclaw at that Gmail thing). :Examination of a rock-climbing fatality caused by equipment failure", Michael Law, Stephen Hawkshaw. Engineering Failure Analysis 22 (2012) 21–27 We tested a bunch and had outwards failure loads of 1.3, 0.6, 0.8, and 1.6 kN (min was about 110 lbs). Video of removing some of the bolts. https://youtu.be/xQt4AbQmJjE Three problems - the wedge needs the correct sized hole but the same drill makes a bigger hole in soft rock than hard. -The wedge reams the hole even bigger when you tap it in. - Rock debris reamed from the side of the hole packs under the wedge and stops it working. (But wedge bolts are great on hard rock, and very comforting when drilling on lead from poxy little edges- reliving the 70s glory days) |
Mikl Law wrote: That was an absolutely horrifying video and a nightmare come true. |
Mikl Law wrote: Just curious, did y'all ever measure the ID of those holes/were they 3/8s wedges? One of the YouTube comments mentions people "waltzing in from overseas" and putting those up. Obviously rock quality is critical, but just made me wonder if they drilled with 10mm bits and installed smaller hardware |
Mitch Monty wrote: It was an 8mm (roughly 5/16", generally discouraged for climbing use even on hard rock) double wedge bolt. |
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From the paper "To test the effect of rock strength on finished hole size, holes were drilled with a masonry drill bit of 8 mm nominal size (measured 7.8 mm). Five holes were drilled in strong volcanic rock, the holes ranged from 7.9 to 8.1 mm. Six holes were drilled into blocks of Sydney building sandstone (stronger than the rock at the accident site); the holes ranged from 8.3 to 9.1 mm. The sleeves on the Raumer bolts measured from 8.4 to 8.7 mm (average of 8.6 mm) indicating that in the larger holes found on the weaker rock there would be no interference between the sleeves and the inside of the hole." They were never going to work! I also recently found a forest of 3/8" wedge bolts on top of a local cliff (of much better rock than the big cliff where the fatality occurred), maybe to make a highline anchor? Average tension strength 1.2 kN, Minimum 0.3Kn (270 and 70 lb). These had double wedges but holding strength seemed rubbish  |
Mikl Law wrote: Those double wedge anchors are an antiquated construction design and were fairly quickly replaced with a single large expansion clip. They were designed for anchoring in old concrete where the quality of the aggregate was suspect or unknown. The thought was that if a piece of aggregate left a pocket after drilling there was double the chance of a clip contacting solid concrete. Now most anchor manufacturers use an expansion clip longer than the average aggregate size to avoid the pocket issue. Plus, testing shows that one clip is all that is ever fully engaged anyway. It’s not surprising those had very low pull strength, the expansion clips on the Raumer double wedges are tiny! |
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I use the standard single cone 10mm (3/8") ones for ground up adventures on hard rock, and have tested a few on soft rock. Very similar results to the double cone ones, just not the right thing in poor rock. |
