Some points common to all rubble walls will be considered before the different classes of work are described.
Considerable skill is required on the part of the builder, who has to work in stones of irregular shape in the most advantageous manner. Such stones should, where possible, be placed on their widest beds so that they may not be crushed, or act as wedges, and force out the adjacent work.
1 Sc. Ruble.
Headers or thorough bonds should be regularly provided, of sufficient thickness to resist fracture. Their numbers, size, and position will be roughly determined by the considerations mentioned in discussing bond stones at p. 39.
In the inferior classes of rubble the spaces between the stones of irregular shape must be packed in with "spalls," 1 and in all cases the "hearting" or inside of the wall should be carefully filled with as large fragments as possible, well bedded in mortar.
All stones in rubble walling should be placed on their natural beds, and as nearly horizontal as the class of work will allow.
The names given to different classes of rubble work vary-greatly in different parts of the country.
In common random rubble work the beds and joints are not dressed, projecting knobs and corners are knocked off with the hammer, and the stones lie together at random, the interstices being filled in with small spalls and mortar. No attention is paid to courses, though each stone should be approximately horizontal.
This is a most inferior description of walling, unless it is executed with very good mortar, upon which its strength greatly depends. It requires considerable skill to build such a wall properly. The bond should be carefully attended to, and though it is almost impossible in the roughest work to break joint on every stone, yet long vertical straight joints should not be permitted. The external appearance and method of building random rubble depends entirely upon the nature of the material, which may vary in every gradation from rough intractable boulders to stones with a beautiful cleavage and natural beds nearly as smooth and even as if they had been carefully worked.
Fig. 124. Random Rubble Walling with Rubble-on-edge Coping.
In this walling the work is brought to a level throughout its length at about every 12 or 14 inches in height, so as to form courses of that depth.
1 Pieces of broken stone. Sc. Shivers.
The work in each course is built random, and may consist of two, three, or more stones in depth, pinned in with spalls as before described. The better the work the fewer the spalls.
Squared Rubble 1 (uncoursed) has the joints and the angles of the faces neatly squared with the tools locally used. The beds are horizontal, and the side joints vertical.
This description of rubble is peculiarly adapted for such stones as have a fine cleavage, affording bed joints which require little or no working. The thickness and length of the stones and style of work depend greatly upon the material. Some quarries furnish a larger proportion of large stones than is shown in the sketches, and others consist nearly entirely of thin beds.
In this kind of walling the work is sometimes allowed to run for short lengths into courses, these being frequently broken by high stones reaching from one course into the next above. Such work is often called "Irregular coursedrubble."
Squared Hubble built in Courses is squared rubble brought to a level course throughout its length at every 10 or 14 inches in
Fig. 126. Random Rubble Walling built in Courses.
Fig. 127. Squared Hubble Walling with Ashlar Quoins.
Fig. 129. Squared Hubble Walling built in Courses, with Saddle-back Coping with Roll.
1 Sc. "Snecked ruble" - the abrupt breaks in the bond being called "snecks." height: it is sometimes known as "irregular coursed rubble brought up to level courses."
In squared rubble straight vertical joints are often allowed, so long as they are not more than a foot or so in height, for random work, and not more than the height of a course in work built in courses (see Fig. 547).
In one variety of this rubble the side joints are left splayed to save labour.
Coursed Header Work is rubble similar to that shown in Fig. 128, except that the headers or bond stones are each of the full depth of the course in which they occur, the intervals between them being filled in with smaller stones.
Coursed Rubble, or Regular Coursed Rubble, consists of stones laid in courses, every stone in the same course being of the same height; the height of the courses may, however, vary from 4 to 8 inches.
Fig. 130. Coursed Rubble Wall with Coping.
With some kinds of stone found in thin layers and having good natural beds, there is a greater distinction made between the thickness of the courses, three or four courses from 1 1/2 to 3 inches thick, alternating with one or two courses from 4 to 5 inches thick, as shown in Fig. 146.
Dry Rubble is rubble (generally "random") built without any mortar. It is the cheapest form of work, but requires considerable skill on the part of the builder.
Flint Rubble is composed of flints and pebbles - or "popples" - laid in mortar. It forms a kind of concrete depending upon the mortar for cohesion. Great care must be taken to keep it dry and safe from the action of frost.
The interior of the wall is sometimes filled in with chalk, broken bricks, pebbles, etc.
Walls may be built with the "rough" flints just as they are dug out from the chalk, or they may be "random " - that is, with the flints irregularly broken.
The stones are frequently "polled" or split, and the fractured surfaces placed flush with the face of the walling. The beds of the flints must be pinned up with fragments, so that their upper surfaces are level, or wet will be led into the wall, and long flints must be used as through stones.
Small sharp pieces or " gallets" of flint are sometimes stuck into the mortar joints, in which case the work is said to be " galleted."
"When the stones are split and roughly squared the walling is called "snapped flint work."
Rustic or Polygonal Ragwoek is built with Kentish Rag or similar stone, in small pieces, which are knocked into irregular shapes and dressed with the hammer, either roughly to fit one another, the interstices being filled in with spalls, which work is called "rough picked "; or with care and accuracy, the stones being carefully worked to regular polygonal forms, in which case no spalls are allowed, and the work is said to be "close picked." Walling of this material is sometimes backed in with " hassock," a soft stone found in layers with the rag and unfit for external work.
Walls such as those built with flints, or other small stones, having but little bond in themselves, are frequently strengthened by building in with them lacing courses, consisting of horizontal bands either of ashlar, coursed rubble, or brickwork (see C C, Fig. 132).
Block in Course, or Blocked Course, is a name given to a class of masonry which occupies an intermediate place between ashlar and rubble.
The stones are of large size, so that they must be procured in blocks, not as rubble; but the beds and joints are only roughly dressed, and so the work cannot be described as ashlar.
The expense of ashlar masonry prevents it from being used throughout the whole thickness of a wall, except in works of great importance and solidity.
Fig 132. Polygonal Kentish Ragwork.
Fig. 133. Block in Course.
It is therefore frequently used merely as a facing, and is backed in with rubble or brickwork; and by some the term "ashlar" is used to apply only to such a facing, not to a solid wall.
Such a construction is open to objections, which are pointed out in Part II.