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Rock Anchors Using Resins

By Kangy King

With Reference to BMC Equipment Investigations

"BMC Summit" issue 4 page 12 has a report on placing staples in rock with resin.  Three incidents of staple failure were discussed in the report.  Two caused injury and all of them involved staples which were pulled out easily.

Hedbury Quarry, Swanage.

The climber lowered her weight onto the staple which although seemingly secure pulled out.

Tram Station Crae, Pen Trwvn

The brand new looking staple was pulled out preparing to abseil.

Lone Wall. Cheesedale.

The staple came out preparing to abseil.  Five others on the crag were pulled out easily.  It was found that the hardener had not mixed with the resin.

Each incident was investigated and advice given.

The report should cause some concern because some of the advice given may not be safe.

R.S. King and G. Bevan had a telephone conversation with the BMC editor and made the following points which are given here in the interests of safety. The comments were made from a perspective of many years engineering use and practical experience of resin systems, metal to metal bonding and composites and apply both to industrial use and direct life support systems involving resins.

Strength of resin bond.

A reasonable strength resin would not be expected to cure at less than room temperature.  All work should done on a dry day in summer.  (And not at all in a cave!)  Setting times to cure to full strength will be extremely variable under these conditions.

The recommendation that a blob of processed resin should be checked is excellent.  But not by taking to a warm dry home!  It must be left in the same environment as the work. Putting the test piece onto a piece of metal or at least on paper put into a plastic bag and tying it to the staple would be better.  Please note that hardening of the resin is a good indication that it has been mixed correctly but it is not an indication of attainment of full strength.  Even a week or more may not be enough to achieve this and at low temperatures it may never happen.

Cleanliness of the hole is indeed vital.

It was suggested that rather than flush the dust out with water, which will be difficult to remove and will prevent a good bond, the dust should be blown out with a tube.  There will still be moisture from the breath but this is not so serious as a wet surface. An intimate contact can be achieved, see below Reason i).

Mixing is vital.  The best strength is obtained by stirring the correct proportions both clockwise and anticlockwise. Better still use a commercial mixer.  Discard the first 50mm of resin expelled.

BMC Reasons

Reason i)

Seems to be a feeling that a rough surface will give a better joint than a smooth one.  A smooth surface will in fact bond to another surface if the correct resin and process is used.  The essential is that both surfaces should be dry and clean.  In particular the surfaces must not be contaminated by grease. If a staple has been handled with unclean or bare hands this could be enough to destroy any chance of a good bond. Mechanical abrasion of the surfaces increases the surface area and removes some contamination, however it could make complete decontamination more difficult.  The best DIY way to clean a smooth staple might be to lightly abrade with Scotchbrite and alcohol and wipe with a clean dry cotton cloth.

Reason ii)

The process of spreading the resin so that it makes intimate contact with the bonding surface is called "wetting".  It is an essential part of a good bond.  Rotating a rod in a hole is not good enough; using a rod to rub resin into the surface of the hole would be better.  Both contacting surfaces should be completely wetted.

Reason iii)

"Not advisable to drill two holes close together."  Agreed.  A minimum distance between holes depends on the state of the rock.  Our experience in drilling rock is that a near second hole can damage the rock between the two and another site is needed.  For a hard rock free of flaws a minimum distance of 6 times the diameter of the hole is a reasonable working rule.

BMC Conclusions.

"The legs should be not be smooth and should ideally be bent"

Bent legs should not be necessary.  Presumably they are intended to introduce a mechanical resistance to removal.  They would prevent a tight fit between the metal and rock.  Cleanliness is more important than roughness.

Commonly people make mistakes in mixing (Hardener with hardener! wrong proportions, insufficient mixing; great care must be taken.)

"Remove all dust from the hole, if necessary flush out all dust with water".  Cleanliness and freedom from grease is essential. The surface must be dry.  Flushing with water may do more harm than good for the reasons given above.  Note that some sealants are intended for use in water; adhesives are usually not.

"Place a blob of resin on paper take it with you (see above) and check after 24 hours". Always check that the resin and hardener have been mixed properly by making a test piece.  A mechanical test specimen is used in industry to give confidence in the quality of the process.

Resins both uncured and cured deteriorate with age and have limited lives.

BMC Broad Conclusions

Cheedale; agreed that the hardener was not mixed properly.  The staple at Swanage was probably greasy, perhaps through handling rather than "smooth and straight".

Specialist Advice

Pay attention to care with: -

The correct materials stored in sealed packages in a cool dry place and not used past the sell by date.

Cleanliness, particularly no grease.

A good fit between components and a good joint geometry for maximum expected load direction, usually this is at right angles to the axis of the hole.

A correct cure with regard to mixing, temperature, pressure, low humidity and time.

To cure: -

Following the manufacturer’s instructions closely.

Sacrificial test samples, if the anchor itself moves, destroy it!

My own recommendations are not to use resins for this purpose.  They are not foolproof.  It is insisted that the metal must be a good fit in the hole.  This will make a great difference to the shear strength of the joint.

For large holes with a poor fit, set lightly corroded steel in high strength cement (with PVA) and clean sand.

Better still use corrosion resistant Mechanical fasteners which may be removed for inspection and replaced.

All anchors must be inspected regularly.  Give them a bloody great yank along the hole axis (out of the hole) before use.  This will be far less than failure load and should do no harm, except to an unsafe anchor.  Think about third party claims if that helps and go for it.  If the load is normal to the anchor - and you don't leap about - you may not need glue because the staple acts like a hook - but I wouldn't recommend it!

Comments by

L.G. Bevan, International Aerospace Composite Committee (Diver) R.S. King, M.Sc. M.Phil. M.R.Ae.S. C.Eng. ( Bristol Exploration Club).