Anchors in Slate

Bolts test bed1

PPE bolts are tested in either concrete or limestone. Over the years several bolts have been placed in slate, so a group of us felt it would be valuable to create a test bed in a block of slate and test these anchors to see if they were up to the job!

Initially (February 2012) we placed a small batch of commonly used anchors, including some non-PPE anchors (commonly used in DIY and industry) and tested them using a commercially available test rig, which was limited to around 20kN. The following year we were able to borrow the BCA test rig which has a higher range enabling us to pull the anchors to destruction, the observations from these tests are available to download by clicking the link below.

In December 2014 the BCA funded the purchase of a further 66 anchors, with a further 22 donated by Simon Wilson (IC Anchors). 5 of each type were placed in 4 different test beds, representing the range of regionalised metamorphic stages of slate in North Wales mines.

In all 76 anchors were placed in December 2014, and in January 2015 all pulled to destruction, the raw data is available to download here. We’re now working through that data applying some statistical analyse to try and present some findings.

Download the article about our testing here:

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UPDATED: Click here for the 2015 test results


Click here for the 2015 raw data

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The anchors tested were:

*only 16 of these anchors were tested. The IC anchor is not currently commercially available, a batch was specially prepared for this phase of the project at short notice, but unfortunately arrived a little late for inclusion in the Corris test bed.

4 sites were selected for these tests representing different qualities of slate around North Wales, 2 sites in Cwmorthin slate mine (one in the Stripey Vein and another in the Back Vein), one in Cambrian slate mine near Llangollen and another in Briach Goch mine, Corris.

Anchors were placed in the two most commonly used/accessible planes within the slate mines, split between the working face (so into the Cleavage Plane) and pillar (into the Pillaring Plane). Care was taken to select test sites that were off the beaten track within the mines for conservation reasons, which at times made the testing challenging!

Good practice in setting anchors suggest holes should be cleaned out thoroughly, including flushing out with water and drying. During the initial preparations it was noticeably difficult to thoroughly clean the holes with water as the slate seemed to leave a muddy paste in the base of each hole on washing. To investigate this as potential issue I decided to leave one placement “dry” and only brush and pump that hole clean. All anchors were placed over 20cm apart.

Another issue highlighted by the BMC (click here) relate to expansion anchors loose on some climbs in the slate quarries, hypothesising repeated heavy loading as the culprit. It’s worth noting these were 10mm anchors, which although a different type, we’d found to fail at very load loads in our Feb 2013 tests. To try and examine this we decided to pull 2 of each batch of 5 anchors 6 times to 6kN effectively exercising them, one anchor in each plane of rock.

So each test bed had 5 anchors, split as follows

  • 1 into the cleavage plane cleaned by brushing, pumping, flushing with water and dried. Exercised to 6kN, 6 times before the test to destruction
  • 1 into the cleavage plane cleaned by brushing, pumping, flushing with water and dried
  • 1 into the cleavage plane brushed and pumped clean only
  • 1 into the pillaring plane cleaned by brushing, pumping, flushing with water and dried
  • 1 into the pillaring plane cleaned by brushing, pumping, flushing with water and dried. Exercised to 6kN, 6 times before the test to destruction

All anchors have now been tested (January 2015). You can download the raw data by clicking the link above, and watch videos of each of the anchors being extracted in the links from the menu bar. It’s worth highlighting only 1 anchor failed below the 15kN axial pull required for the EN959 standard, and that one was placed in noticeably poor quality rock.  The consistency of some anchors such as the IC and Bolt Products appear to be marginally better than the Collinox and Goujon supporting the theory that longer anchors perform better than shorter anchors. Also the method of failure with the expansion anchor was far more catastrophic than the resin anchors.


We’re currently working through the data and applying appropriate statistical formulas to provide some (hopefully) accessible results!

Click here for the February 2012 tests

On the 1st of February 2012 a large block of slate within Cwmorthin Slate Quarry/Mine was identified as being suitable. In it we placed 2 of several different types of bolts.

On the 2nd of February armed with a Hydrajaws 2000 Medium Duty bolt tester (click here for more details) we pulled all the bolts (directly out) to 6kN which is the test standard for PPE anchors in industry. We then continued to pull them all to 20kN which was the maximum the Hydrajaw unit could manage.

Following testing the bolts we placed we located 2 x 8mm Petzl spits that had been estimated to be over 10 years old and tried to pull them out.


A cautionary note...
The following results relate to the bond between a variety of bolts in a single block of good quality slate. Selecting a good quality piece of rock appropriate to bolt can be very difficult, particularly given that the rock has been quarried.

First round of tests (Feb 2012), tested to 20kN

With the exception of bolt number 19 all bolts where placed on the top of the slate block so through at around a right angle to the cleavage plain (pillering plane) of the slate. Bolt 19 was placed into the cleavage plain. Click on the tab to view the results from each bolt…

10mm Colinox (resin) bolt (1 & 2)12mm Goujon with Couer hanger bolts (3 & 4)Spit Rock Screw8mm Spit with aluminium hanger (11 & 12)



Bolt type: Colinox 10mm resin anchor bonded with SPIT Maxima M10 ampoule

Observations: Bolt started to deform around 10kN. Gauge slipped at 11-12kN. After slipping a number of times at 12kN taken up to 20kN. Clear deformation of hanger section but anchor remained. Identical results on second bolt.


Click here for the full version (not speeded up and with sound)



12mm Goujon

Bolt type: Petzl Goujon 12mm expansion anchor with Coeur hanger

Observations: Noticed deformation of the hanger (plate lifting) around 5-6kN. 12kN hanger noticeably deformed and plate lifting at 15kN. Bolt held at 20kN after some slippage (load cell twitched and dropped).

Once the load was taken off it was obvious that the bolt had shifted a little as the hanger was loose. The bolt was also slightly bent. Very similar results with both bolts.


Click here for the full version (with sound, not sped up)





Bolt Type: Spit Large Diameter Tapcon Concrete Screw 128mm x 8mm (M10). Fitted with Coeur Hanger

Observations: Plate started to distort and lift at 9kN. Some unpleasant “crackling” noises as 12-13kN reached then catastrophic failure at 17kN with one bolt and 18kN with the other.

Other rock screw tested, Multi Monti 100mm x 8mm. It was not possible to fully tighten one of these screws for some reason so the plate hanger was a little loose to start. Plate hangers performed very similarly to that of all the steal plate hangers, lifting and distorting slightly from 9kN. Neither rock screw failed having been taken up to 20kN.

Video (Bolts 5 & 6, Spit Rock Screws)



petzl-spit_1Bolt type: 8mm Petzl Spit with twist hanger. Bolt 11 placed with drill and finished by hand, 12 placed by hand drill

Observations: Bolts 11, Hanger started to deform almost as soon as the load was placed on it. Difficult to get to a 6kN load as the hanger plate kept bending and releasing the load. Hanger deformation settled at around 8kN and continued to 11kN where we couldn’t pull the bolt any further due to the limitations of the machine. Plate clearly loose following the test (possible to unscrew by hand) and the bolt seemed proud of the rock by at least 1mm.

Bolt 12, Again started to deform as soon as a load put on the hanger. At about 9kN the hanger lifted so much we could see the anchor. Taken up to 20kN and held. Hanger was loose to remove but anchor looked fine.


Click here for the full versions (not speeded up and with sound) 


Other bolts tested

Bolts 7 & 8

Screwfix M12 x70, fitted with Coeur Hanger

Observations: Noticed hanger lifting at 9kN. Held at 20kN. Difficult to remove nut (although likely the thread was damaged when installing the bolt). Some of the bolts sleeve extracted and bolt looked bent. Hanger noticed to deform at 9kN. Bolt slipping around 10kN.  Bolt noticeably bent on removing nut and hanger, and sleeve visible.

Bolts 13 & 14

16mm B & Q Expansion bolt, fitted with Coeur Hanger

Similar hanger deformation but noticed sleeve of the anchor lifting at 6kN. Bolt also looking to bend at 7kN. Visibly lifting at 10kN. Taken to 20kN and held. Anchor visibly deformed, and sleeve pulled out.

Second Round of Tests (June 2013)

Full write up of observations, including the tests to destruction conducted in 2013

During this round of tests we were able to test each bolt to destruction, using the BCA’s testing rig. The following videos illustrate some of the findings…

Collinox and GoujionRaumer M10 x 110mm