Walls across slopes were traditionally built with a steeper batter on the uphill side than on the downhill.
- a An asymmetrical batter was thought to be structurally stronger than a symmetrical batter on a slope, because it would allow for some settlement. With a symmetrical batter, any settlement on the lower side might cause the wall to topple.
- Larger foundation stones were used on the lower side, so that they would be large and heavy enough to withstand some loss of soil on the lower side, without becoming undermined. They would also have to support a greater height and thus weight of stones than the foundations on the upper side. Here smaller stones could be used, as they would tend to get buried with downhill soil movement.
There is also the argument that the steeper batter would look more intimidating to stock, and be more difficult to scramble up, so compensating for the lower height when viewed from the uphill side.
The logic of these arguments can be questioned. The asymmetrical cross-section may in fact create a shearing force which is more likely to cause the wall to tilt back, in effect pushing the footings down the hill. A symmetrical wall with level foundations should transfer the weight directly downwards, so keeping the wall upright.
The difference between the size of the foundation stones on the upper and lower side of the wall may also create stresses which cause the wall to tilt back and eventually collapse.
The stockproofing argument may have some validity, although in practice the general continuity of a barrier and the soundness of its construction is probably more important in keeping it stockproof, rather than the degree of batter. Stock tend to break out at a weak point where a wall has been damaged or has partially collapsed.
In summary, it seems likely that a wall built with symmetrical batter on solid, even foundations should be just as strong and more durable than a wall with asymmetrical batter.
Foundations
Especially on steeper slopes, it’s a good idea to excavate a foundation platform rather than a trench, as the lip of the trench will anyway get displaced during the building process. Dig the platform to leave a ledge of about 12-18″ (300-450mm) between the slope and the edge of the wall. This gives an even surface to stand on during rebuilding, and reduces the chance of soil eroding from immediately below the wall. Put the soil from the excavation just downslope, and then replace it up against the downside of the wall at the end of the rebuilding process, to give extra protection to the base of the wall. Bank it up to remove any ledge which might encourage sheep to walk along or shelter immediately below the wall.
This amount of excavation does involve additional labour, as well as possibly additional stone in order to bring the wall up to the required height. However, this investment is likely to be worthwhile in producing a wall which will stand for a long time without the need for repair.
