The primary concern with the stone is that it continues to have high com-pressive strength and stiffness. Most stones undergo weathering when
exposed to wind, rain, abrasion by wind-blown particles and freezing and thawing. Vegetation can also contribute to the deterioration of stone, prin-cipally through the physical action of roots. The way in which the exposed stone deteriorates over time may not match what is happening to the stone hidden behind the wall face, which is doing most of the work. Wind and rain, as well as providing mechanical weathering of a kind that will not be experienced by the hidden stone, tends to remove any weakened material, and it may not be obvious that this is happening. Wetting and drying can lead to the progressive deterioration of the strength of some of the types of stone that might be used for drystone retaining walls.
The stone that is exposed at the face may be doing little more for the stability of the structure than adding weight – the actual points of contact between the stone may be some distance back from the face and exposed to the air only through a relatively thin gap. Provided that there is still sufficient weight to resist sliding, the loss of stone at the face that is not contributing to the transmission of vertical load may have very little effect on the resistance to overturning, and might even improve the distribution of bearing pressure on the foundation by allowing the centre of mass of the wall to move backwards.
In limestone walls, precipitation of calcite has been observed where the stones are in contact with each other, so that the long-term effect is a strengthening of the connection between the stones and the formation of a hard layer that protects the parts of the stone that are transmitting the forces. Considerable care is therefore needed in observing and interpreting the deterioration of the stone.
There may be stones that are cracked. Close inspection of the cracks might reveal signs of when the crack took place, if the fresh stone surface on the crack changes colour or weathers with time, or the crack fills with dust or dirt. Occasional cracks may occur during the construction of the wall, as increasing load results in a bending moment in stones that are not adequately supported (Figure 6.1). Normally this is not a problem if the
Figure 6.1 A number of cracks are visible in the stones in this photograph, all of which probably took place as the wall was being built.
crack is visible at the face. Even if such a crack results in a running joint, it will only be very short if the rest of the wall has been well constructed.
A cracked through-stone would have a greater consequence, resulting in loss of connection between the front and the back of the wall, but such a crack would be within the body of the wall, and not visible at the face.
A fresh crack in an old wall could be a matter of concern, as it could be a consequence of stone deterioration, external loading or settlement or sub-sidence causing a redistribution of load within the wall. A series of cracked stones is likely to indicate a serious problem of overloading or foundation movement.
6.2.2 Geometry
In most cases, the first measurements will be taken with a long tape to obtain length and height of the structure, but a reflectorless total station will allow much more detailed information to be obtained, probably more safely. This instrument is an electronic theodolite that incorporates laser distance measurement as well as the recording of angles, so that it can store calculated coordinates of the points being observed. Such instruments require a specially designed reflector to measure to, often attached to the top of a pole that is placed on the point being surveyed. Reflectorless instru-ments dispense with the need for this, using a powerful visible light laser that can measure a distance to most surfaces. This means that a wall may be surveyed from a safe vantage point, without the need to make physical contact with the wall, or even approach it closely; this could be particularly advantageous if the wall is next to a road.
If a reflectorless total station is not available, or there is no one available with the expertise to use a hired instrument, then tape measures and spirit levels are likely to be used.
If the face of the wall is not planar, for example, if the face angle varies over its height and along its length, or if there is bulging, then recording requires much more detail. The reflectorless total station will still work well, but it may be difficult to tell if there are distortions from a distance.
If it is not possible to approach the wall closely enough, then it is best to observe horizontal lines of points at different levels, and vertical lines of points at different sections along the wall, so that distortions might be identified once the data have been processed. Such interpretation requires an understanding of the accuracy to which the wall is likely to have been constructed. Indications of this may come from the overall apparent qual-ity of the construction and the extent to which the visible faces have been worked to give the wall a flat face.
Working with tape measures is easiest with clear access to the top of the wall as well as to the ground in front of it. Then a plumb-bob (a line with a weight on the end) can be hung from a piece of timber projecting from the top of the wall, with the weight set to the toe, or a distance in front of the toe if
a bulge must be accommodated. Then horizontal offsets from the line to the wall face may be measured at measured heights to determine the face profile.
A vertically placed levelling staff (Figure 6.2) held to the projecting timber would assist this process, and for higher structures a ladder may be needed.
An alternative approach is to use a laser scanning system. In some ways the actual operation of the system is easier than using a total station, but the equipment is expensive and the software processing is time consuming. The instrument can be set to scan the wall horizontally and vertically, to produce a ‘point cloud’ of the coordinates of the surfaces that have reflected the laser.
Any object that gets in the way will be measured instead, but careful choice of instrument location and observation time (to avoid traffic and pedestrians) would avoid this problem. The accuracy of such points should be to within a few millimetres, which is more than adequate. The frequency of measure-ments can be set, so that the points are not unreasonably close together; how-ever, because they are simply on a grid, rather than choosing suitable points on the face of each stone, a high density of points is likely to be needed. Then points to be used in defining the geometry are extracted from the point cloud back in the office. The system has the advantage of the reflectorless total sta-tion in measuring from a safe distance, and will probably be quicker in the field, but will give much more information than is really needed for defining the geometry. However, if the point cloud is sufficiently dense there will be no need to measure the dimensions of individual stones for surveying the con-struction, as both overall and detailed geometry will be obtained in one scan.