In this regular column, Consultant Barry Hunt answers reader enquiries. If you have a question, email it to: [email protected] marked for Barry’s attention
Q: Large blocks of a well known stone with many veins that have never caused a problem before have recently started splitting. Can you help?
I have inspected and tested the stone in question on several occasions and I know it has been used on many projects without problems, so your query came as a bit of a surprise.
Then again, after so many years investigating unusual stone performance not much is truly surprising and this is a quite typical example of why you must never be complacent with natural stone.
I first covered the issue of what veins are and whether they are a problem in this column back in April 2007. In many instances they are not a problem, as long as you understand them properly. The problem you are now faced with is whether all the veins have the potential to cause splitting, which could mean condemning half the stone you have purchased, or whether the problem is less serious and thus both controllable and financially bearable.
Inspection of your stone revealed that there were four different types of veins. One set of dark grey siliceous veins had formed at the same time as the stone formed, knitting the stone together. A set of competent greenish veins and light grey veins had formed during later mineralisation events, and there were white veins that comprised calcite growing in fractures apparently related to more recent weathering. The different formation events may be ascertained by trying to spot which veins cut across the others, which is not difficult to do with a little patience and perspicacity.
It is often the latest veins that cause problems. They are more likely to have incomplete formation, while any previous improperly formed veins would have had a new opportunity to be filled and knitted together.
My inspection of the stones in question revealed that every instance of splitting was associated with the white veins, suggesting the problem is affecting a relatively small percentage of potentially more geologically weathered materials.
The simplest course of action would be to inspect all the blocks and remove those with these veins in them.
If you want to be sure the other vein types are sound you should undertake testing, although this can be time consuming and create its own problems with regards to supply schedules. Alternatively, you could try some ad hoc testing to get an immediate idea of the stone’s competency. The photograph above is an example of how useful information can be quickly obtained. Rock splitters and other equipment can be similarly employed to obtain a quick insight and a good hammer blow is also helpful, as the ring will often dull when a problem vein is present.
Veins are one of the great features of stone that make it so appealing. Don’t assume they present a problem because they usually don’t, but remain vigilant and try to understand the veins in the stone you are using so that if the stone quality suddenly changes it is easier to spot the problem and nip it in the bud.
Barry Hunt is a Chartered Geologist and Chartered Surveyor who has spent 20 years investigating issues relating to natural stone and other construction materials. He now runs IBIS, an independent geomaterials consultancy undertaking commissions worldwide to provide consultancy, inspection and testing advice. Tel: 020 8518 8646
The advice offered in answer to readers’ questions is intended to provide helpful insights but should not be regarded as complete or definitive. Professional advice should always be sought in respect of each specific stone-related issue.