A Compendium of Useful Information for the Practical Man/Masonry

Wall, Stone Rubble
In rubble masonry the stones are not dressed. In ordinary rubble masonry, the work is brought up in courses, the depth of which depends to some extent on the thickness of the stone to be procured from the quarry. The stones should be laid in mortar and carefully fitted; face stones must tail in or be bonded into the backing; for rubble walling, as many through stones as possible should be introduced; this also, to some extent, will depend on the materials to be procured. The work should be brought up in courses not exceeding fifteen inches in depth, or thereabouts, well flushed in mortar worked into every crevice until the whole is well filled in. The stones in the face work should be hacked square, and the joints carefully pointed. The backing should consist of flat bedded stones, as large as may be procured; they should be well fitted together, obtaining as much bond as possible, made solid in mortar, and brought up level with every course of face work; no scabbling should be wasted, but they should be packed into the mortar between the vertical joints of the work; this saves mortar and makes better work; the hammer should be freely used, but not so as to risk the breaking of any of the bedded stones. Walls built of rubble two or three feet thick require a good deal of attention in the bringing up, and according to their magnitude a proportional quantity of workmanship and hammer dressing and hacking must be introduced. Rubble takes a good deal of mortar and flushing, and the consequent settlement of the work must be provided for, more particularly where the walls are of any height. Good work may be procured without coursing, if proper attention be paid to the bringing up of the work, but there can be no good work, or anything of the kind without bond, through stones, and fitting the work together.

Where the rubble is uncoursed, a tolerably level course should be introduced at about every two or three feet, depending a good deal upon the size of the stone used, and the workmanship introduced into the work, and this course should be thoroughly well grouted.

In random rubble, the stones are often used to some extent as blasted from the quarry; where the work is well fitted, when there are sufficient large stones introduced, well filled in with smaller ones, very good work may be produced with sufficient thickness of walling, and the work well filled in; bond is equally attainable here as in the former work, if some attempt be made to obtain it.

Backing may be of rubble stonework or brickwork, and even concrete, when well made and properly used, answers the purpose very well; when made with dry broken bricks instead of gravel, it is very light, at least as compared with ordinary concrete; the backing should be brought up at the same time as the face work, and the whole always bonded in together. The perfect settlement of the interior mass is the more important, since, from the smaller size of the stones and the rougher work, a greater shrinking will here occur than in the face work. Indeed, unless considerable care be exercised in this respect, the defect will eventually occasion the whole superincumbent weight to be thrust on the face work, and thus produce a bulging out, and probably a rupture of the face work. When the interior as well as the exterior face of a wall, or similar construction, is prepared with a fair face either of stone or brickwork, the interior is, for the sake of economy, occasionally filled in with rubble or other rough work, called hearting. Bonding, grouting, levelling, and consolidation are to be rigorously insisted upon in this mode of construction. Otherwise, if the stone facings are allowed to be too thin, too nearly of equal thickness throughout, or not thoroughly tailed into the interior mass, they will act as mere slabs or coatings, and be very liable to separation and rupture. The levelling of each course of face work, must also he particularly attended to; and bond or through stones, fairly and solidly bedded, well fitted, and of sufficient size, must be abundantly provided.

[http://books.google.com/books?printsec=frontcover&dq=%94useful+information%22&lr=&pg=PA94&id=_rYaAAAAYAAJ#v=onepage&q=&f=false The architect's guide: being a text book of useful information for ... By Frederick Rogers]