Arrangement of stone

The way in which the stones are arranged within a wall, and so transmit forces from stone to stone, is called the ‘opus’. Three factors come into play in the choice of the arrangement of stone for a given wall: its function, its environment and the type of stone available. An aesthetic choice of the customer or builder can also come into play.

The opus is most apparent in the outward appearance of the wall, but the internal organisation of the stones, as well as those visible at the face, must be carefully implemented according to the selected opus.

5.2.1.1.1 Geological influences

Geological characteristics, shape and design of the material will dictate the first choice of arrangement of stone. The two major categories of arrange-ment of stone are coursed masonry (opus assisé) and opus incertum.

1. Coursed masonry: Stones from more or less regular thinly-layered strata are generally used as blocks, with clear top and bottom beds derived from that layering, or bedding. In the case of sedimentary rocks, these surfaces on which the stone naturally splits are called the bedding planes, and they were horizontal when the sediments were deposited, before they turned into rock. Regional metamorphic rocks split on cleavage planes that are defined by their schistosity, and these may not be parallel to any original bedding planes, but as with the sedimentary rocks, they will split into layers on planes that are orthogonal (perpendicular) to the direction of maximum stress they experienced as they developed their present form, and this means that they will have their greatest strength and stiffness in a wall if they are placed with these planes horizontal (Figure 5.1), so that they are again carrying their greatest loads in a direction orthogonal to their bedding or schistosity. Rocks that do not have either characteristic do not usually break down into neat layers that can be used this easily in masonry, which led to the next opus, described in the text that follows.

2. Opus incertum: Some stones come in a form that does not allow them to be placed in layered beds without being resized; this makes the arrangement of stone ‘uncertain’. (Figure 5.2) The stones are set in rela-tion to their form, interlocking against each other and using their own geometry. Each stone must as far as possible be in contact with adjacent stones on every side, whatever the number of sides. This is the case for granite, some sandstone, basalt rock, as well as certain limestone.

Although these are the major types of arrangement of stone, there are others, and some walls can combine different opus types.

• Opus cyclopean: When the walls are built with very large stones, it is called Cyclopean opus – by reference to the work of the mythical giant ‘Cyclops’ (Figure 5.3). This is so whether or not the stones can be placed in clear layers.

• Opus clavé (vertical construction): This is used in areas where it is imperative that the wall is sufficiently free draining to cope with large water forces. These might be flows through the wall, such as in

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Figure 5.1 Coursed masonry. (a) Drawing. (b) Example in schist in St Germain de Calberte, France.

(a)

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Figure 5.2 Opus incertum. (a) Drawing. (b) Example in granite, experimental wall build-ing in process at Pont de Monvert, France.

Figure 5.3 Opus Cyclopean, near Le Collet-de-Dèze, France.

locations where springs are emerging from the ground during heavy rain. The forces may come from flows against the wall, for example, if it is constructed on a creek bed to protect the bank from erosion, where it may have to contend with the maximum flow of the stream (the talweg). The most challenging situation is one in which a wall has to resist sea waves driven by a storm, such as in a harbour wall. In these situations walls are often constructed in the fashion described as ‘clavade’ (Figure 5.4). The stones are placed upright on their edges – contrary to normal practice in masonry, which is to place stones with their bedding planes horizontal. The stones must be packed tight against each other to keep them vertical, but even so, the frequent vertical joints make this form of construction less susceptible to clog-ging by fine soil and resistant to strong flows through the wall. The stones within the wall, as well as those at the face, should be placed carefully on their edges to assist the flow. The stones at the face will often have their longer dimension running into the wall, and some-times stones will be chosen so that they span the entire width of the wall. The stones in this form of construction are not easily dislodged by the impact of waves or by strong currents, because there are no large flat horizontal surfaces to act on, and the stones are more secure because they are pressed together side by side. Such walls are often curved to form a vault effect against surges and current, which con-siderably increases their resistance. This form of construction works well on ground that is relatively unstable, because it is good at resist-ing uneven pressures. Opus clavé is used in some regions for slates and shales, which have comparatively smooth surfaces, whereas their edges are rough; because their weight is resting on their rough edges, the walls have good resistance to failure on internal sliding surfaces.

It is also commonly used to form the coping of a wall (see Section 5.2.4.3).

Rock

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Figure 5.4 Opus clavé. (a) Drawing. (b) Example in schist in Boscastle, United Kingdom.

5.2.1.1.2 Social, cultural or artistic influence

The quality of the arrangement of stone can also attest to the technical skill of the builder and the richness of his creative mind. Some arrangements or decorations, or even playfulness in the form of the construction may express cultural traditions or artistic intentions – these also demonstrate the skill of the builder.

• ‘Opus quadratum and Roman’: When the stones are cut ‘square’, that is, with two right angles to their facing, and are arranged in carefully coursed masonry with excellent bonding, the resulting reg-ular arrangement of stone is called ‘opus quadratum’ (Figure 5.5).

The opus quadratum is built with rectangular stones with horizon-tal courses that can vary in thickness along their length (Figure 5.5).

In the ‘Roman opus’, each bed is of rigorously constant height. This arrangement can be made with granite, sandstone, limestone and sometimes schist as well. Both of these opus types require substantial work in cutting stone and adjusting its size, especially in comparison with the tradition of using the stones as they are found. A substan-tial additional skill is required from the waller, and the process takes much longer than methods that focus on the strength of the construc-tion rather than a rigid protocol for its appearance. The observer of a completed wall might suppose the blocks to be rectilinear throughout the thickness of the wall, making them easy to place level and well supported, but these rectangular blocks may only form the outer face of the wall. It is also possible that only the outer face of a stone is rect-angular, and the stone may even be supported on wedges to ensure the correct alignment of that face. The skilled waller will nevertheless have built a wall that is structurally sound, even though inordinate effort has gone into providing an appearance to please the imperial masters!

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Figure 5.5 Roman opus. (a) Drawing. (b) Example in limestone at Balsiegès, France.

• Herringbone pattern and fishbone pattern, ‘Opus spicatum and pis-catum’: The ‘opus spicatum’ (meaning like ears of corn, also called laying herringbone) and opus piscatum (referring to the fishbone) are two arrangements already in use in Roman times and still tradition-ally found in some areas (Figure 5.6). The stones are inclined, and the direction of inclination of each course is reversed compared to that of the preceding course. In the case of herringbone pattern, the beds are nested by overlapping the upper and lower ends of the stones, whereas in fishbone pattern, the stones rest on the ends of the stones below without overlap, resulting in a well-defined line of contact. This type of arrangement of stone is achieved with flat stones or oblong pebbles.