Bedrock
Bedrock in the line of the wall should be treated as foundations, which is not a problem if the bedrock is relatively flat and regular. However where it is sloped, stepped, or has awkward lumps it is more difficult to build on.
A wall normally has to be continued even over awkward bedrock, as fencing to stockproof the gap is also difficult. Route the wall over the flattest part of the bedrock.
One advantage to building on bedrock is that the wall is less likely to settle, and provided the wall itself is well built, can last a very long time. Often walls on bedrock form the only more or less intact sections of older upland walls. Walls on bedrock tend either to collapse overnight if there is a problem, or last indefinitely if well built, rather than degrading over a period of time.
Slopes
The only feasible way of building on bedrock which slopes up or down from the face of the wall is to use stones with a triangular profile.
This is far from ideal as the angle of the slope will tend to force the stone out of the wall, a problem which becomes more severe as the slope becomes steeper.
To minimise problems select stones which meet the two following criteria:
- The angle of the base of the stone should match the angle of slope of the bedrock, giving good stone to stone contact without the need for any wedging.
- When in position, the top of the stone should be as close as possible to horizontal to facilitate sound subsequent building.
These criteria are more important than choosing a stone which matches exactly the correct line or batter of the wall.
The next step is to ensure that these stones are well tied back into the wall so that they are held as securely as possible.
Select a suitable stone, even if it has to be fairly thin, to level up with the top of the triangular stone.- Place a tie stone on top of these two stones.
- Level off with a smaller stone, and then place another tiestone as shown.
When building a length of wall in this way with triangular foundation stones, use as many tiestones as possible. Choose the longest stones for the first layer of tiestones, which should be at least three-quarter length and preferably full through stones. Subsequent layers of tiestones which lap onto these should at least reach the centre of the wall. The tiestones not only help secure the foundation stones, but distribute the weight of the wall evenly across the foundation, so that none of the triangular stones is taking more weight that it needs to.
Lips
Building over lips has similarities to building on slopes.
Start the wall with a three-quarter or full through, continuing as shown in the diagram. Always build up first the side of the wall away from the lip, to prevent the wall overbalancing. The weight of the wall on the back of the upper tie helps hold everything in place, and the result can be surprisingly secure. Whilst it is best to use full through stones, in practice it is sufficient if at least half the length of the first tie is sitting on bedrock, and is held by a tie running more than half way into the wall.
Where there is a shelf below the lip which would take a layer of stones, this can be utilised, but these stones are merely to improve the appearance of the wall, and should not be considered as load-bearing.
Mini-smoots
Small smoots can be constructed to bridge lumps, lips and some slopes. Normally these only need to be one sided, with the ‘back’ of the wall built as normal.
One interesting use of this technique can be found in Llanberis, North Wales, where a series of one sided smoots form the footings of a wall on bedrock. It’s not certain why this technique has been used in this particular situation, but it could be a useful approach when you do not have sufficient triangular stones for the footing. In this case use what you have got and bridge between them, rather than relying on poorer stones to support the wall.
It is possible that this will place more weight on the triangular footings that are used, and the footings cannot be tied back until after lintels have been placed. Its viability as a technique will depend on the specific situation and the stone available.
Boulders
If there are boulders within the line of the wall which need to be incorporated into it, plan the exact line in advance to make best use of the boulder, and to avoid any sudden kinks. Avoid a line which will leave the boulder sticking out on both sides of the wall, as this may give access to stock from both sides. Take the line that leaves the boulder protruding on one side only, to minimise the amount of extra stockproofing needed.
It is not essential to build wall heads at the boulder, although it is advisable to take extra care in ensuring that the stones alongside the boulder have their lengths running into the wall, and include at least as many throughs butting up against the boulder as there are courses of throughs in the wall.
Ensure the wall fits tightly against the boulder by shortening or lengthening each layer to conform to the boulder’s contours. As a last resort use pinnings to ensure a tight, close fit.
If the boulder is lower than the top course, continue the courses straight over it as if you were building on bedrock.
Stockproofing
If the boulder is higher than the top course it can be used as a base for the coping, which is continued over it uninterrupted. The coping acts as a visual deterrent to stock, by increasing the height of the boulder.
Alternatively, where the boulder is sufficiently steep-sided to be stockproof as it is, the coping can be stopped where the wall abuts the boulder, finishing with a blocky stone.
Boulders which are lower than the finished height of the wall may still need to be stockproofed, as a lip or bulge as small as 1″ (25mm) can give a foothold to agile hill sheep. Stockproofing will either require adapting the coping, or building the wall higher.
Where the projection is only a few inches it’s normally sufficient to project the coping or the coverstone by about 4-6″ (100-150mm).
Smaller coverstones can also be used, as long as at least half their length is on the wall and anchored by the cope. The arrangement of the coverstone and coping can be adapted as shown, to provide an overhang where the boulder projects on both sides of the wall.
Another option is to create a double cope, projecting from both sides. This tends to be the least stable of the methods as the weight of stone on the wall may barely counterbalance the projection unless the shape of the copestones are suitable for overlapping.
Where the boulder projects too far to be stockproof with an overhanging cope or cover, then the height of the wall must be increased.
In this example at Blaen y Nant, Nant Ffrancon, North Wales, the height of the wall has been raised over the boulder and then gently curved back to normal. By using larger coping stones at the apex, grading down as wall height diminishes, a smooth curve has been made.
Wet ground
Where there are patches of ground which are sufficiently wet at all times of year to be a barrier to stock, the wall can be brought to a head at either side and the wet ground left unwalled. More often though the wet ground will have to be walled or fenced off in some way.
In wet ground it may not be possible to reach ground firm enough to make foundations, even by digging quite deep. In order to support the wall and to try and prevent uneven settlement, use large slab-shaped foundation stones which will help spread the weight of the wall.
These should be laid to form a scarcement at ground level, the wall itself then being stepped in and built to its normal dimensions. Where the supply of stone is limited, you may need to sacrifice height in order to build stronger foundations. A lower, but more stable wall is usually preferable to a normal height wall which is likely to collapse.
The recommended gap of 1-2″ (25-50mm) between the two rows of foundation stones should be increased to 4-6″ (100-150mm), and water smoots constructed in hollows or wherever water may try to drain across the line of the wall, remembering that the wall itself becomes a channel for water. Digging short lengths of drainage ditch from the smoots can help them drain the wall more effectively. Eventually any drainage measures within the wall are likely to become blocked, but in the short term they should lessen any uneven settling, so helping to ensure the wall’s long term stability.
In some situations there may be no alternative but to fence across the wet ground. Use high tensile wire which minimises the number of fence posts needed, as these are difficult to anchor securely in wet ground. See Fencing – Constructing a post and wire fence for details.
Trees
Trees and walls make poor neighbours. A tree immediately next to a wall can push it over as the trunk thickens, and surface roots displace the footings. Even at a distance, ‘root-heave’ can destabilise walls as the root plates of large trees shift in the wind. The most common problem is of root growth which gradually works into any point of weakness, such as a running joint, initially displacing one or two stones which then causes collapse of the stones above.
If the line of the wall can be moved, choose a line which is at least 6′ (2m) clear of any trees. Where the line cannot be moved, some felling and removal of tree stumps may be required. If it is necessary to preserve both the trees and the wall line, the following tactics should lessen the inevitable damage and uneven settling.
Bridging roots
Bridge robust roots with small smoots.
Ideally, at least 12″ (300mm) should be left on either side of the root, and about 6″ (150mm) above the root, to give room for growth, and to give sufficient clearance from the effect of root heave. In practice the gap will be dictated by the length of the lintel available. Use the longest stone available, even if it is quite thin. Techniques for strengthening thin lintels are described here. Clearance from the root is probably more important in the long term life of the wall than the strength of the lintel. Smoots are usually constructed as shown above, but some examples occur where the root or tree buttress is bridged with a low arch of mortared stone.
If there are problems with smoots being large enough to allow lambs through, gaps can be blocked temporarily with stones or similar during the lambing season. Alternatively, a short wooden or metal stake can be driven in at the smoot, sufficient to block the passage of lambs in the short term, but which will become redundant in the long term as root growth fills the gap.
Trees in the line of the wall
Given that walls and trees do not mix, the best solution is to leave a gap for the tree. This gap should be as wide as is acceptable to the parties involved. Normally allow at least 3′ (1m) either side of the tree, or at least 12″ (300mm) clearance from any exposed roots. Given that standard treated softwood rails (‘motorway rails’) are 12′ (3.65m) long, this may be a convenient size for the gap.
Build the wall to a head at either side, and erect the section of post and rail fence in front of the tree.
If leaving a gap is not acceptable, then the wall should be built with only once face where it abuts the tree. As the trunk thickens the fillings should compact up to a point, without pushing out the face. In the longer term rebuilding may be necessary.
A rare alternative to this method can occasionally be found where wall heads are built 12-24″ (300-600mm) from the tree, and the runners extended to help stockproof the gap between the head and the tree. This method is likely to last longer than building against the tree, but is dependent on having suitable runners.
