CORNISH HEDGES A DIFFERENT SLANT
This article appeared as the "Professional Column" in the Spring 2004 edition of "The Waller and Dyker" The Official Magazine of the Dry Stone Walling Association of Great Britain.
This edition, I am indebted to Sean Adcock for responding to Robin Meneer’s article in the last issue of the Waller & Dyker.
I read Robin Meneer`s article with interest, but I am not convinced by his analogy comparing the concave batter to an arch.
An arch is an immensely strong structure that functions by basically transferring any loads placed on it to its solidly placed, fixed ends. The arch stones (voussoirs) also tighten together under compression, in a way it gets stronger as a load is put on it. A Cornish hedge has only one fixed end so it cannot act as if it were an arch. Any load placed on the back of the stone face (i.e. on the "arch") will in effect try to straighten the curve and will force the top of the wall up, and the face will straighten. I think that it might be possible that in certain circumstances it works similarly to an arch in that the weight of the top courses would act like a less rigid base. Provided that the thrust on any point is less than the thrust provided by the weight of stone acting down on that point, the integrity of the face is maintained. You would have to ask an engineer to be sure, but assuming it is the case then really it is only the same basic principle that applies to a ‘normal’ straight batter. Vertical weight keeps stones in place.
Walls are normally "A" shaped partly because this keeps the thrust line created by the wal1`s weight within its faces, essentially that is why they stand up. The weight of stones, hearting, and uneven settlement, coupled with outside factors such as stock, wind, or people climbing, try to push the face stones out. This forces the thrust line towards the outside face of the wall and once the thrust line moves into the outer third of a stone it will try to pivot, and if movement continues the wall will collapse. Bulges and depressions in the face will also move the thrust line outside of the line of the wall and collapse is likely.
A simplified extension of this reasoning is that the more even the batter the better, and the more vertical the batter the less stable the wall. Hence the greater the batter the more stable it becomes but ultimately less stock-proof, and so we compromise.
The North Welsh Clawdd is earth filled similar to the Cornish hedge, but is much more battered than a dry stone wall. It predominates in areas that were originally mostly cattle, and consequently its batter was less important. They are also rarely more than a metre high and so not too unstable, but all things considered not much use against sheep. This might be where the shape of the taller Comish hedge comes into play.
Whilst in Cornwall researching the 1999 revision of the BTCV’s "Dry Stone Walling" I noticed variation on the concave batter in some areas, with the wall built to three differing straight batters. This started me thinking.
To be strong a lot of hatter is best, here the bottom of the wall has the most batter as it is under the most pressure from the weight of the wall and fill. A typical wall is likely to have a batter in the region of 1 in 8, a Clawdd would typically be around I in 5, Robin Meneer’s 5 inches in 12 equates to 1 in 2.5, immensely strong but virtually flat in terms of stockproofing!
To be stock-proof little batter is required. The final 1 inch in 12 of the Cornish Hedge is as near to vertical as matters, incredibly stock-proof, but not very stable, but then does it really need to be as it has relatively little weight working on it.Of course we have to get from a batter for strength to a batter for stock proofing, and hence the transitional batter(s). A Cornish hedge does this remarkably well with the strength aspect decreasing in line with the decrease of pressure on it.
Getting back to straight batters and thrust lines, nothing is of course that straightforward. The ability of the wall to withstand the displaced thrust line will be reliant on other factors such as crossing of joints, good stone contact, and length of stone into the wall et cetera. In fact the longer the stone the more vertical th~ batter can be without overly compromising structural integrity.
The use of the relieving triangle was well known to the Mycenean Greeks over 3000 years ago, which was developed into corbelling to form ‘domed’ dry stone roofs.
These techniques can also be found in dry stone walls which, by comparison, make Cornish Hedges look positively flat and stable!
The walls around Rhosilli on the Gower Peninsula, South Wales are concave on one side (left).
Here the concavity can actually extend to the coping overhanging the base. This is achieved through using long stones, frequently 'upside down' in that their angled faces are inverted compared to the water-shedding norm. The most spectacular variation on this theme I have come across is a 4m high retaining wall in a disused quarry at the head of Cwm Ystradllyn in North Wales (right).
Well over 100 years old the wall overhangs its base by around 1.5m, without a stone being out of place. This has been achieved through the use of massive (up to 2.5 m long) corbelled slate slabs. The wall 'supports' a waste slate heap, with its original function having been to prevent waste
cascading onto the horse drawn tramway that operated below the overhang. What is the point of these examples, apart from the fact that they appear to defy gravity? Well they too are not arches; they are just basic principles applied with a high degree of skill, which is what at the end of the day makes any stone structure stand up.
Some of the technical information and diagrams have been borrowed from an article, by lain Richardson, published in the North Wales DSWA Branch Magazine in 1996