BCA Regional Anchor installers technical forum - Last call for interested participants.

[Ian P]

Looks like we have another topic for next week.

Name the anchors

 

[Bob Mehew]

I would expect an anchor to be permanently deformed from a trial test with its full intended system design load applied (IIRC that is acceptable as long as there is not anchor failure under BS 7883), but not for a proof test of the service load (anticipated real world use load). 6kN is a pretty standard periodical proof test load for industry, and any anchor made for that use should not be affected by that kind of force.

It is worth bearing in mind that peak forces seen by breaking ropes vary from 6 to 16 kN. The standards usually link back to a value of 6 kN based on an estimate / guess that that is what most people will walk away from. (Though not if you already have a 'bad back'.) Parachutes are reckon to open with a force of around 12 kN but it should be remembered that the parachute harness is a hell of a lot better at spreading the load into the whole of the body than most SRT harnesses that cavers use. The late Paul Seddon wrote a decent report on the topic entitled "Harness suspension: review and evaluation of existing information" in 2002. It looks like the HSE web site has lost the page so if anyone wants a copy, PM me.

I have not specifically checked if IC anchors flex at 6 kN but some FMEA calculations I had done indicated they would. My memory is that this flexing in BP anchors becomes a permanent set at around 10 kN, that is the head is left permanently distorted. (Though I may not have saved the evidence of this.)

For the uninitiated, EN 795 is for anchors being used with PPE and BS 7883 is the UK code of practice on them. The anchors used in caves under the BCA set up, conform to EN 959 which is for mountaineering anchors. A key feature of EN 795 anchors is that they are removable where as EN 959 are placed by gluing into the rock and thus are not. Further complicating matters EN 795 anchors fall into various types but EN 795 Type A anchors (which are similar to EN 895 anchors) use fixing methods to attach them to part of the structure (such as a cast in threaded sleeve). EN 795 Type anchors are required to hold a 12 kN static axial load. BS 7883 goes onto propose that each EN 795 Type A anchor after installation should be subject to a 6 kN axial load. Where as EN 895 anchors are only required to hold a 15 kN axial load and no comment is made about testing installed anchors. Hence the push to adopt routine pull testing.

 

[andrewmcleod]

Another point which arose during that time was routine pull testing of anchors in line with that recommended for EN 795 PPE anchors. What we did not realise is that the heads of the DMM P anchor, the PECO anchor, the BP anchor and the IC anchor all flex at or below 6 kN.

Which is not terribly surprising given that none of the above are rated for EN795 and proof-loading is outside of the manufacturer's instructions for use...

 

[andrewmcleod]

EN959 (which BP anchors are, and IC anchors are tested to the standard of) requires a higher test load than the industrial standards. EN959 is 15kN axially, 25kN radially (UIAA requires 20kN axially). EN795 (industrial anchors) only require 12kN (unless there is non-metal involved).

I suspect/have previously heard claims that the extra strength for mountaineering anchors is to compensate for the lack of regular inspection and/or account for the much greater variation in substrate. The standard testing is done in a block of concrete of a certain compressive strength; rock can be weaker than that so you want some safety margin. The BP anchors and IC anchors, I think, get greater than 25kN both axially and radially.

 

[Bob Mehew]

I suspect/have previously heard claims that the extra strength for mountaineering anchors is to compensate for the lack of regular inspection and/or account for the much greater variation in substrate. The standard testing is done in a block of concrete of a certain compressive strength; rock can be weaker than that so you want some safety margin. The BP anchors and IC anchors, I think, get greater than 25kN both axially and radially.

Testing is not simple and as you say, mountaineering anchors are required under EN 959 to be tested in concrete. The strength of concrete is much less than a range of rocks. So the high axial pull out values found for BP anchors of 30 plus kN is reflective of the strength of the limestone we tested them in, but that is not always the case. The first round of PECO anchors were tested in one quarry with a mean pull out value of 27 kN due to the quarry rock being thinly bedded and relatively weak bituminous limestone strata. In a different quarry the anchors reached 34 kN. If you read Bolt Product's web site, you will see he manufactures BP anchors in different lengths for different rocks (sandstone for example being weak). But as a warning, my memory is that an anchor placed in an oolitic limestone (as I recall in a mine) came out at 12 kN due to rock failure. It also helps if one places the anchor in a solid bed of limestone; hearsay has it that one installed anchor was found the following week at the bottom of a pitch after the slab had peeled off the wall.

The topic also gets complicated in how you apply statistics to it. Having taken the advice of a chartered civil engineer BCA E&T eventual decided to adopt the following statement in its meeting of 5 April 2014:

The standard for acceptance of an anchor type on the basis of an axial load is based on the 15kN axial load value as cited in Section 4.3.1 of the Mountaineering Equipment – Rock Anchors – Safety requirements and test methods BS EN 959 : 2007, as computed as the 5% fractile value as specified in Section 4.2 (3) of the Euro Code Basis of Structural Design Standard BS EN 1990 : 2002 from the results of a batch test of a minimum of 5 anchors provided there is supplementary information showing the distribution of results follows a normal distribution, else the minimum size of the batch test should be 32.

The 5% fractile value is less than the mean value usually quoted. See BCA E&T minutes of 15/3/15 for more detail.

 

[Pete K]

My sincere thanks to Ian P for organising the forum yesterday. It was a brilliant day and was the perfect balance of discussion, demonstration and practical activity. It was also great fun. Thanks to everyone who came along.

 

[CNCC]

Our Training Officer has put together a very comprehensive report (and a selection of photos) of this event here:

Anchor Installers Forum 2023

BCA Anchor Installers' Technical Forum On 14th October 2023, the CNCC hosted a technical forum for BCA regional anchor installers and other interested people. An incredible amount of hard work by numerous...

cncc.org.uk

A quick glance down the diversity of topics that were covered...

• Route to becoming an anchor installer
• Record keeping
• Reporting loose anchors
• Re-using holes from pulled anchors
• New placements permission requirements
• Drawing rigging topos
• Extracting anchors and patching holes
• Rigging standards
• Testing of anchors
• BCA Anchor Coordinator role?
• BCA E&T Committee now part of T&E working group; Relevance?
• Drilled threads
• Chain linking anchors
• Pull through rings/ anchors
• Temporary anchor options
• Fixed aids / hand-lines
• Ground stakes
• Insurance

... should emphasise the importance of events such as this for knowledge sharing. We would like to thank Ian for organising this, and opening this out nationally, and to everyone who attended