This section contains part of a report I compiled for Gwynedd Council Environment Directorate acting as Department's Representative for the Welsh Assembly Government Transport Directorate. It covers the most common aspects of poor workmanship, with plenty of photos. The photos are black and white to speed loading, clicking on any image will give a larger colour photo (almost full screen) of about 150K.
Wherever possible, stones should be placed with their longest axis into the wall. This greatly reduces their potential to become displaced during settlement. Stones placed with their long axis along the line of the wall are known as "traced" stones and tend to be easily dislodged. Narrow traced stones are particularly easily dislodged; traced stones lower in the wall tend to be more of a weakness than those higher up as there are more forces trying to displace them.
With some stone types, most notably laminates such as slate, tracing can be unavoidable and consequently requires a specialised structure.
Within most walls the tracing of occasional stones is acceptable, although the grouping of traced stones alongside each other or on top of each other can create a weakness as can a proliferation of traced stones sprinkled liberally throughout the wall.
Tracing stones can be a fast method of walling (i.e. fewer stones are required).
How well the stones fit together in the face of a wall is referred to as "tightness" with "slackness" as the self explanatory extreme [Compare photos left (slack)and right (tight)]. A "slack" face with more gaps has more potential for movement during settlement, not only because the stones could move into the gaps, but there is also less friction between stones to hold them in place.
At a more technical level, the effective degree of tightness that can be achieved can vary with stone size and type. In all cases stones, should be butting to their neighbours, but, for example, a wall built of regular/flat bedded stone should be tighter than one built of irregular stone, and rounded stone is likely to appear to be slacker than squarer stone. Smaller stone should result in a tighter build than larger stone - a 5cm2 `gap` is not a problem where the butted stones have 200cm2 faces, where they only have 100cm2 faces it is of far more concern.
Where a stone fails to sit on one below it a `letterbox` results, often the result of a traced stone bridging three stones, which do not quite provide a level surface to build on. Consequently the stone below the letterbox is not securely held in place and could relatively easily drop out during settlement.
In some instances small gaps are filled with small stones, generally known as pinning giving the appearance of tightness, this process is discussed in more detail below [stone placement/structure- pinning].
Stone contact within the wall is of particular importance. If there is good contact between the edges of adjacent stones there is far less scope for movement during settlement, a key aspect of good wall building that can only be effectively assessed during construction. It is quite easy to create a "tight" looking wall from the outside whilst creating a "slack" wall on the inside. It is easier to butt `points` than to get good fits in every plane.
The way stones are placed affects subsequent building. It is no good having a stone that meets all the other criteria but cannot be readily built on. Stones with badly sloping tops can initially look good but tend to create major problems as they try to shed the next stone placed on them. Small steps between stones usually necessitate the use of inappropriate undersized thin stones or slivers to provide a level for the next stone, or result in a stone placed at an angle with only one or two points of contact and gaps. The way stones butt to neighbours is also important since acute/obtuse angles can result in inappropriate gaps, or poorly placed stone to counteract the problem.
Most of these problems can only be observed during the construction process itself.
Walls are `A` shaped, tapering in from a wider base to a narrower top. This slope is known as the batter and is integral to the `stability of the wall.
Walls stay standing when the weight of the stone is transferred as evenly as possible throughout the structure onto the foundation. The weight of the stone forms a thrust line and as long as this thrust line remains within the wall, it stands up. Uneven settlement (particularly loose or badly placed stone), forces on the wall (stock, people climbing it), the walls own weight, can act to move this force towards the outside face. This in turn can act to try to tip stones rather than transferring the forces directly downwards, moving the thrust line outside the line of the wall and/or displacing stones, eventually causing a collapse.
The more vertical the wall, the nearer it is potentially to failing to contain the thrust line, hence the need for a batter, - a compromise being reached with needing to have the wall vertical enough to contain stock. Bulges and dips displace the thrust line and hence potentially cause catastrophic failures. The ability of the wall to withstand the displaced thrust line will be reliant on other factors such as length of stone into the wall, crossing of joints, stone contact (all dealt with elsewhere). All other things being equal bulges and dips are far weaker than an even, well battered, face.
As in brickwork stones should have a good bond to distribute forces and tie stones together, that is one stone should sit on two, and two on one (sometimes referred to as the waller`s prayer). The better the wall the more evenly spaced the joints, ideally (as with bricks) half on one, one on half. During settlement the stones either side of a joint have less holding them in place than do stones which overlap.
2 stone joints are not overly frowned upon, unless they proliferate (see Photo left, note also tracing and pinning ) , and tend to be more common/acceptable for more regular types of stone.
3 or more stone joints are referred to as "running joints". (These 2 Photos show complete foundation to coping joints, something of an achievement. Also note how "slack" the one on the right is). These represent a severe weakness creating a seam in the wall, which is likely to widen as the wall settles. "This weakness increases geometrically for each additional uncrossed joint in vertical line" (Brooks et al 1999. P.54)1.
Where several vertical joints are only slightly crossed, i.e. where a stone is only just lipped onto one below it, it can be almost as bad a weakness as a joint. This often shows itself up in the form of "stacking", (Photos below show stacking and poor jointing in general) where a series of stones are effectively just piled on top of each other, creating a piece of wall a potential lack of integral strength.
Stones should be placed so that they sit relatively securely with a minimum of wedging. Any wedging should be at the back or sides (within the wall, not face) only, not as shown left.
Basically a stone should not be rocking when you try to place a stone on top of it. Normally this can only be assessed during construction.
Generally stones should be set to the horizontal rather than sloping. In keeping the stones flat the forces are better transferred onto the stones below helping to bind stones. Sloping stones can exert sheer forces on stones below them, this can serve to open joints or force stones out of line. Building the wall`s layers or courses to follow a slope rather than the true horizontal can mean that the weight of each stone is trying to force it downhill. This can create severe problems. However where the wall is regularly coursed it might be the only possible method of construction.
Hearting is the small fill used to fill voids in the centre of a wall. By filling the voids it reduces the potential for the wall to fall in on itself during settlement. It is particularly important in preventing the movement of any wedges stabilising the face stones. It should be carefully placed rather than poured/shovelled, angular stone is best as it binds better than rounded pebbles. Small granular fill is generally a bad idea since if it gets under a face stone it can act like ball-bearings making it easier for the stone to be displaced.
These are single stones that connect the two faces of the wall. This helps to prevent any bulging during settlement by tying the faces. They also maintain "the wall`s equilibrium by distributing the weight of the upper layers equally onto both faces below" (Brooks et al, 1999. P.55)1. The style and spacing of through-stones varies from region to region. In North Wales they are frequently sporadic or no present at all. Where they are present they are frequently undetectable since they are almost always set with their faces flush with the building stone, rather than projecting by a few inches, the more normal practice in most of Britain.
The basic reasons for placing the largest stones in the base of the wall has already been outlined. Unless a wall is coursed it is likely to be necessary to use some smaller stone lower in the wall, but the general effect should be that of size diminishing with height. If smaller/larger stones are used out of sequence then they should not be grouped since this tends to create a pocket within the wall that will react differently to other parts of the wall during settlement creating a bulge either at the point of grouping or immediately alongside it.
As a general rule stones are set flat rather than on edge, that is their largest surface forming their base. This facilitates their sitting securely and distributes weight/forces efficiently. A stone set on edge is relatively easy to displace as it is relatively top heavy, and can have little holding it in the wall, with traced stones set on edge being particularly unstable. The greater the height of the stone relative to the width of the base into the wall the more unstable the stone.
Setting stones this way is a common practice in mortared walling and cladding where the mortar holds the stone in place, but as a practice is not readily transferable to dry stone walling. It is a regional practice on Skye where heavy basalt blocks (about 8% heavier than granite and 16% heavier than limestone) are frequently set on edge but this is a specialised structure compared to `normal` walling not applicable to smaller, and less dense stone.
Occasionally relatively thin stones are set on edge to fill a narrow gap between two stones. Whilst not a generally accepted practice, provided the stone is tight with its long axis into the wall it is not entirely unacceptable. However if its use is widespread (as in the 2 photos here) it would tend to suggest a generally poor technique, as the waller should not let such gaps keep developing.
Where any cross sectional part of a stone is triangular this should be set as the stone`s face.
If the triangular cross-section is set within the wall weight from above will work on the wedge shape of the stone to force the stone out of the wall.
Pinning can mean several slightly different things, all variations on a theme. The strictest interpretation is the use of small stones inserted into the face of the wall (rather than built in) to secure larger stones (Photos left and also photo above under
Crossing Joints . This can be a highly dubious practice as the pin(s) often pop out during settlement, since they were securing what was probably an ill fitting or loose stone in the first place this creates a serious weakness. In much of Scotland pinning is an accepted practice. The practice here varies from the strict interpretation in that the larger face stones are not reliant on the pins for their stability; the pins only fill small voids in the face once the wall has been built. Theoretically they are hammered in with care so as not to force stones apart, if they fall out the wall is no weaker than it was, and if they stay in place the wall has less potential for settlement. This is not a generally accepted practice in Wales.
Pinning can also be used to describe small stones in the face and tends to be applied where small stones are sprinkled liberally (and hence inappropriately) throughout the structure, or where a small stone(s) are ill fitting (square peg in a round hole) or loose.
The coping stones serve to seal the top of the wall holding the final course of each skin in place, binding them together. They can also perform a highly technical function in that they help to keep lines of thrust (see batter) within the wall, in much the same way that gargoyles do on the roofs of buildings. This is a role that is often neglected in favour of local practices which often (especially in North Wales) call for the coping to be offset to one side of the (wide topped) wall, and its true value has not been widely considered or investigated.
Many styles and regional variations exist with coping (a reasonably comprehensive description of which can be found in Brooks et al pp.132-139.)1, however the following principles would apply to most types.
Provided the top of the wall is narrow enough and the coping stones wide enough (which should not be too much of a problem with a new wall of suitable stone such as that on the A5/A55) then each stone should sit securely on top of both faces. Each stone should be placed to maximise contact with its neighbours (irregular stones make poor coping) in order that they lock together. Gaps between the tops of stones are wedged/pinned to help lock the wall, with care being taken not to actually force the stones apart (this should not be able to happen if the stones are well placed in the first instance). Additionally any gaps on either side of the coping are wedged to help secure the stones, reducing the potential for movement during settlement, again taking care not to force the stones apart. In some instances a lack of wedges results in this being neglected, this is only really acceptable where the stones have good complementary fits, and should not really be the case with new walls where since wedging is normal practice and stone ought to be provided for it.
1 "Dry Stone Walling." Brooks, Adcock, Agate. BTCV, 1999.