These should not be used in soft rock, but were tested for comparison. Trubolt type anchors have 1 or 2 short expansion sleeves. The cone is held in place initially by friction forces between the sleeve and the rock. When the nut is tightened the cone is drawn through it and generates outwards force. Tightening the 2 bolts that went in properly, the bolt pulled out showing that the cone was sliding in the rock.
Raumer 8mm units (with double expansion cones) which had been used and removed from a climb near Pierce’s Pass of a type which failed and caused a fatality were tested. In shear testing, one jammed in the hole, leaving 20 mm sticking out, it failed in a similar manner and load to the correctly placed one.
I also tested 4 Raumer 8mm units in a block of building sandstone which seememd more solid than the rock at Barden Ridge, results in worse rock would probably be lower. All came out with the cones in the unexpanded position.
Table t1 Trubolt results (all tests in shear)
|Test #||Unit||Comments||Mode||Load (kN)|
|T1||10mm Trubolt||Claw home-made hanger broke (figure T1)||shear||15.3|
|T2||Raumer 8mm Figure T2||shear||9.3|
|T3||bad Raumer 8mm bolt sticking out 20 mm (figure T3)||shear||10.3|
Reasons for reduced holding power
For a wedge anchor in hard rock, the cone generates a large friction force and has to be hammered in. When the nut is tightened the cone pulls back inside the sleeve and expands it, giving a very solid unit with typical pull out strengths of >20 kN. When wedge anchors are tapped into very soft rock the sleeve may ream the hole out to a greater diameter. This has 2 effects;
- It makes the hole bigger (which reduces the available frictional force).
- It fills the area ahead of the sleeves with rock dust which stops them from interacting with the expansion cones.
Additionally, the hole size tends to be larger when drilling into soft rock. Holes were drilled with an 8 mm SDS rock drill bit. In hard volcanic rock the holes ranged from 7.9 to 8.1 mm while in Sydney building sandstone (harder than the rock at the accident site) the holes ranged from 8.3 to 9.1mm. The sleeves measured an average of 8.6 mm on an 8 mm shaft)
When tightening a wedge anchor in soft rock, the sleeve may not be expanded by the cone for 2 reasons:
- The sleeve slides back out towards the surface as less friction force is available in the larger hole. An average value of this pullout load was about 0.3 kN in the chopping operation.
- The rock dust caught between the cone and the sleeve pushes the sleeve straight back out of the hole under tightening, rather than forcing it to expand.
Figure T1 homemade hanger on 10mm Trubolt (T1)
Figure T2 8mm Raumer bolt at 9kN, showing first expansion cone free from the rock (T2)
Figure T3 Jammed Raumer bolt (T3)
Figure T4 (Left) Wedge bolt in pull test, note floating sleeve is in the unexpanded position, and cone is packed with debris. Right; un-used and tested wedge bolts, note packed debris and unmoved sleeve on tested bolt.