It may be necessary to curve a wall in order to follow a boundary line, avoid a tree, pool, hollow or other difficult spot, or to incorporate an immovable boulder into the wall. Gradual curves are preferable to corners where either are possible, since they generally require less specialised stone, and if built correctly, are likely to prove more durable. However, most wallers tend to agree that a curve is possibly the hardest ‘standard feature’ to build. Tight curves can be especially tricky, due to problems judging the batter, and in this case a corner may be more satisfactory.
There are a number of common faults to watch out for when building a curve.
- There is a tendency for the outside of a curve to be built too close to the vertical, and the inside to be too sloping.
- Bulges and flat spots can result if the wall is not a true curve.
- Stones are often not sufficiently tight to their neighbours.
Faults a and b are caused by building the curve by eye, and fault c is a result of unsuitable or incorrectly used stone. These may seem minor faults, but when compounded, can result in an insecure structure. The interior of a curved wall is particularly difficult to build, and extra care needs to be taken to construct it as strongly as possible.
Marking the line
Curves can be marked out on the ground with a length of rope or hosepipe. Mark the inner edge of the curve, ‘nick out’ along it, and then measure the width of the wall outwards, to give the outer edge of the curve. Semi or quarter circles can be made by sticking a crowbar in the ground with a rope loosely tied around it and marked at the required radius. In practice few curves are this regular, but need to be fitted to join two sections of walls or to go around certain features.
Placing stone
Although the basic principles of placing stone apply, there are two main complications to building on a curve, both of which increase in severity as the curve becomes sharper.
- Butting the stones tightly together within the wall is difficult. The outside of the curve may look ‘tight’, with good stone contact, but this may mask poor stone contact within the wall.
Poor stone contact between the stones on the inside of the curve tends to show as a ‘slack’ inner face.
If the inside of the curve looks ‘tight’, it probably means that the stones have been well fitted together, with good stone contact.
For good stone contact, try and select stones with complementary internal edges. Often it will be necessary to dress the stones to fit. - b Setting the stones to the correct curve.
On the outside of the curve it is the corners of the stone which should be set to the curve. On the inside of the curve it is the centre of the face of the stone which should be set to the curve.
To achieve this save stones which have slight curves to their faces for the appropriate side of the wall. Avoid using longer stones on the inside of the curve, and on the outside a small amount of dressing of corners will help get more of the stone in line.
Mistakes in the alignment of stones result in inaccurate batter, flat spots and dips. One misplaced stone almost inevitably results in the stones on the subsequent layer compounding the mistake. Given the difficulty of assessing batter on curves by eye, this is often not obvious until the wall is completed.
Covers and coping
Careful selection of stone and a fair amount of dressing are required to produce a good result. Ideally the covers should fit the curve, but in practice this is difficult, and often they will need to poke a little out of line to allow a good match with their neighbours.
To fit the coping, some of the stones need to be wedge shaped, and of sufficient size to span the wall top.
Less satisfactory are small wedges, fitted to ‘fanned’ cope stones, as the wedges tend to pop out with settlement. If fanning is unavoidable, select wedges which fit as closely as possible to the size and shape of the gaps, to give maximum stone contact so they are less likely to loosen.
