Soft Sandstone Rock Anchor Testing Summary

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A number of rock anchors were installed and tested to destruction at Swinburne crag Free State, South Africa on June 10th and 11th 2000. The rock is fairly soft sandstone, not unlike that at the nearby “Mnt Everest” crags, at Harrismith. To date, routes at Swinburne have been bolted using mainly U-Bolts, from various sources. There was concern about the integrity of the anchors due to the soft/friable nature of the rock.

There had been some concern that due to the nature of the rock, soft sandstone, that some of the existing rock anchors would be weaker than what was originally thought to be the case at the time of bolting.

Based on these tests, the following conclusions can be drawn:

Safety of Existing Bolts:
It appears that ALL the anchors fall short of what would seem to be the desired safe working load and should be considered on a bolt-by-bolt, route-by-route basis for replacement. This was not due to negligence on the part of the bolters, who generally followed the currently accepted bolting standards and in general co-operated with and encouraged this testing program. Some bolters even placed test anchors specifically for anchor validation.

Anchor Design:
The Davies U-Bolt, and all other U-bolts, at least those placed in this type of rock, have interference from their close leg spacing (close in terms of normal civil engineering practice for rock and concrete bolts). This interference tends to reduce their potential pullout load. They are also shorter than what is required for this type of rock.

Installation and Glueing Procedure:
The glue-in procedure and type of glue used is of crucial importance. The practice of placing a glue-in bolt in the same size hole results in an under-strength anchor. The anchor will be much stronger, as well as provide more consistent results, if placed in a larger hole.

The Upat glass ampoule provides excellent bonding to the soft sandstone rock and none of the other glues tested gave suitable results. It is possible that other Hilti or Upat cartridge glues will prove better for this type of sandstone.

Anchor Manufacture:
Both the installed rock anchors at Swinburne as well as a large number of the rock anchors available to South Africans are not made according to any standard, in particular some of the locally made ones. Not all local ones are variable although, those of Vektor, Upat, and Alpha-Vertical, are of a high quality and are well designed.

Most of the bolt manufacturers’ work has been done on concrete/cement and under controlled laboratory environments. However, the principles can be related to rock. In general, the following is true for both rock and concrete anchor strength:

  • Strength depends on the loading mode: either tension (pullout) or shear
  • Tensile strength is strongly affected by rock type, and depth of embedment
  • Shear strength is less affected by these
  • Both tensile and shear strengths are reduced by the proximity of another anchor, or to an edge, up to a certain critical distance, greater than this there is no effect
  • Anchors tend to fail in one of the following manners: rock/concrete breakage, bond failure (or expansion pullout), or anchor material fracture (metal breaks)
  • Selection of appropriate glue can allow the same or greater strength as the rock/concrete the anchor is placed in. Bonding is improved by roughening the anchor legs, either by grooves, knurling, or threads. This gives the glue something to key onto
  • The strength of anchors, in either shear or tension, is quite variable. Therefore an overall strength rating can only be gauged by performing a certain minimum number of tests, at least three, but ideally more

The most common anchor used at Swinburne is a U-Bolt (U2) made by Andy Davies of Cape Town. It is made of 316 stainless steel, of 7mm diameter, 50mm leg-leg spacing, and is embedded 60mm into the rock. The embedded part of the leg has an M8 thread rolled onto it to give the glue something to key onto.

There are a few other types of U-Bolts, either made by local climbers or from old South African Railways stock.

Standardisation:
The international climbing community in North America, Europe, Africa, Austral-Asia and elsewhere does not have any standard bolting practice or follow a particular standard. The Cape Town Section of the Mountain Club of South Africa has drawn up an initial set of “Guidelines for The Western Cape”. This is a start for the region, although an excellent beginning, it is not yet complete, as will be seen later on.

Safe Working Load:
Rock anchors should be designed with a Safe Working Load of 16kN. This is based on a fall of Fall Factor 1.0, use of a Gri-gri, the border-line 11mm UIAA dynamic rope (not very stretchy), and a 100kg climber.

This is what the SABS EN795 uses, as well as what the rock anchors could well take. The maximum load could be 22-23kN, but anchors would not normally see this (a fall above Fall Factor 1.0 is quite unusual in bolted routes, and would tend to be possible only on multi-pitch sport routes on anchor/stance/belay points which are doubled-up).

Rock anchors are quite different from other climbing gear (like ropes, harnesses, slings and carabiners):

  • Are fixed and exposed to the elements year round
  • Do not tend to be inspected (are difficult to be inspected) and tend not to be replaced unless they are obviously damaged, and even then sometimes not. In contrast, ropes, carabiners, harnesses and other gear tends to be regularly replaced and maintained. And if not, the risk is borne by the climber
  • Are installed by relatively untrained, unsupervised people
  • Are in a very brittle, variable base material
  • Are installed by one party and used by others. Thus the bolter is responsible for more than his own safety. Anchors also tend to be subsidised by organisations, whereas an individual’s gear is not
  • Most climbing gear is designed to be as light as possible, whereas anchors do not have to be

Therefore, rock anchors should probably be tested to destruction, and have much bigger safety factors than other climbing gear.

Testing Procedure:
The authors decided to load the anchors to failure in tension as well as in shear, thus allowing one to calculate their safe working loads. This means that enough tests (three minimum) must be done for each case to get an idea of the spread/range of results. An average (arithmetic mean) of each testing case as well as the standard deviation can then be calculated. The “three sigma” approach (the arithmetic mean minus three times the standard deviation) can then be performed to give a quantified result for the strength of the anchor in both shear and tension.

Then, bearing in mind that anchor installation is carried out under adverse conditions by relatively unskilled people, and in brittle base materials (rock), a safety factor should be applied to these failure loads to arrive at a Safe Working Load, both in tension and shear.

In the case where the anchor is loaded at an angle, involving a combination of tension and shear, there are recognized methods for calculating the combined loading

The anchors were tested by slowly applying the load, rather than under impact. This was partly due to practicalities (it would be much more difficult and would require enormous amounts of tests to determine impact failure loads).

A hydraulic testing machine from UPat was used for the testing. It is equipped with a hydraulic piston and pressure gauge. It was calibrated against an electronic loadcell; itself calibrated by the SABS. A jig was made to enable the hydraulic rig to be used to test anchors in shear, as well as in tension/pullout.

The anchors were placed on the horizontal flatter parts of some large boulders (top was 5m by 10m at least) at the base of the crag. This was for more convenient testing. The possibility that this rock was weakened due to more frequent water exposure was not investigated. It was “bone dry” at the time of testing.

Summary of Rock Anchor Specifications
 

Manufacturer
Anchor
Code
Bolt
Diameter(mm) 

Embedded
Depth(mm) 

U-Bolt
Leg
Spacing
(mm)
Hole
Size(mm)
Glue
Type
Comments
Upat S1 10 100 n/a 12
Upat Ampoule UKA 3
carbon steel M10 threaded rod
n/a S2 10 100 n/a 12
Hilti HIT-HY 150 Injection
carbon steel, cut threads
n/a S3 8 100 n/a 10
Hilti HIT-HY 150 Injection
carbon steel, cut threads
Hilti M1 10 90 n/a 10
n/a
Test Variant U1 8 120 50 10
Hilti HIT-HY 150 Injection
carbon steel M8 threaded rod
Davies U2 7 60 50 10
Hilti HIT-HY 150 Injection
stainless steel rolled threads, quite high uniform quality
Test Variant U3 10 60 80 10
Hilti HIT-HY 150 Injection
carbon steel, cut threads
SA Railways U4 12 50 37 12
Epidermix 372
carbon steel, cut threads
Test Variant U5 8 30 50 10
Hilti HIT-HY 150 Injection
carbon steel, cut threads, chromed
Fergusson U6 8 100 37 8
Epidermix 372
stainless steel, cut grooves, very variable quality
SA Railways U8 9 35 37 10
Epidermix 372

An attempt was made to perform at least three tests on each variety in both tension and shear mode in order to get statistically significant test data. Due to time, battery, and glue restraints this was not accomplished for all variants, but the results allow analysis of both the existing bolts on routes of the crag, as well as other potential anchors.

Results:
The bond is VERY critical in the Swinburne sandstone. The Upat capsule (UKA 3) glue appears to infiltrate the sandstone very well. The Hilti HY-150 and Epidermix 372 did not appear to bond very well to this rock ype though. The Epidermix was however messy to use and requires care on the operator’s part to mix it properly. It is likely that poor mixing could lead to bond failure.

The possibility that the Epidermix was not fully cured after 24 hours is possible. However, bolters DO use anchors within 24 hours. Therefore, the worst/conservative case is to test rock anchors after 24 hours.

The diameter of the hole is sometimes varied for the same diameter glue-in anchor. Although this effect was not investigated in detail, it does not seem to be a good practice to insert, for example, a 12mm U-Bolt (U5) into a (nominal) 12mm hole with a small amount of glue squeezed in/out (an “interference fit”). This can be seen by comparing the U-Bolt (U5) to all the other U-Bolts of 50-60mm embeddment. The U5 failure load is much more variable and the mean is lower.

The problem here is that attempting to force a 12mm piece of steel into a 12mm hole in rock will compress the steel, but not do much to the rock. It will only require a small amount of material movement/relaxation to reduce this confinement force. And as seen with the glue failiures,  the bonding is CRUCIAL to bolt failure. This sort of force fit does not allow much bonding to occur.

Hilti states that strength is a linear function of depth up to a certain maximum for each diameter of anchor. It follows that anchors installed in less than the nominal depth are weaker.

Best Anchor
Best by far is the M10 glue-in stud with Upat capsule (S1). Although there were no shear tests done with it, it should behave just like the M10 glue-in stud with Hilti glue (S2). This only deformed under shear due to the HANGER, the bolt itself was fine. It will thus likely hold any factor 2 fall, in shear or tension.

Mechanical Expansion
The Hilti M10 mechanical expansion bolts (M1) were good in shear (will behave like a glue-in stud in shear). In a big tensile load, they will start to pullout, making them less able to survive further falls.

Davies U-Bolts (U2)
These were quite variable in pullout/tension. They will probably not fail (and thus not result in a death or injury) but will suffer deformation under a big fall. In shear, they are strong, but will also likely deform under a big fall.

SAR U-Bolts (U4 & U8)
These were very variable. The U-Bolt itself is very strong (12mm diameter galvanized high tensile steel). However, they were placed in tight holes (12mm leg in 12mm hole). This tended to reduce their potential strength, based on their embedment depth and leg spacing. Also note that they used Epidermix 372 glue, which is not fully cured at 24 hours (requires 7 days for full cure).