RUBBLE MASONRY. This is the lowest grade of masonry laid with mortar. Rubble is built of unsquared stones, that is, of stones as they come from the quarry without other preparation than the removal of very acute angles and excessive projections from the general figure. The only classes of rubble are coursed and un coursed (see § 553).
Rubble is sometimes designated as or rubble according to the number of men required to handle a stone.
Sometimes, when rubble is built of very large blocks of stone, concrete instead of mortar is employed to fill the vertical spaces between the stones, in which case the masonry is called concrete rubble (see § 579).
Rubble masonry is sometimes laid without any mortar, as in slope walls (§ 556), paving (§ 557), etc., in which case it is called dry rubble; but as such work is much more frequently designated as slope-wall masonry and stone-paving, it is better to reserve the term rubble for undressed stone laid in mortar. Occasionally box culverts are built of the so-called dry rubble; but as such construc tion is not to be commended, there is no need of a term to designate that kind of masonry.
Careful attention should be given to bonding the wall laterally and transversely. It is frequently specified that one fourth or one fifth of the mass shall be headers. The corners and jambs should be laid with hammer-dressed or cut stones.
A very stable wall can be built of rubble masonry without any dressing, except a draft on the quoins by which to plumb the corners and carry them up neatly, and a few strokes of the hammer to spall off any projections or surplus stone. This style of work is not
generally advisable, as very few mechanics can be relied upon to take the proper amount of care in leveling up the beds and filling the joints; and as a consequence, one small stone may jar loose and fall out, resulting probably in the downfall of a considerable part of the wall. Some of the naturally bedded stones are so smooth and uniform as to need no dressing or spalling up; and a wall of such stones is very economical, since there is no expense of cutting and no time is lost in hunting for the right stone, and yet strong, massive work is assured. However, many of the naturally bedded stones have inequalities on their surfaces, and in order to keep them level in the course it becomes necessary to raise one corner by placing spalls or chips of stone under the bed, and to fill the vacant spaces well and full with mortar. It is just here that the disadvantage of this style of work becomes apparent. Unless the mason places these spalls so that the stone rests firmly, i.e., does not rock, it will work loose, particularly if the structure is subject to shock, as the walls of cattle-guards, etc. Unless these spalls are also distributed so as to support all parts of the stone, it is liable to be broken by the weight above it. A few such instances in the same work may occasion considerable disaster.
One of the tricks of masons is to put "nigger-heads" (stones from which the natural rounded surface has not been taken off) into the interior of the wall. In order to secure good rubble, great skill and care are required on the part of the mason, and constant watchful ness on the part of the inspector.
Rubble masonry is employed for the abutments of the smaller highway bridges, for small culverts, for unimportant retain ing walls, for foundations for buildings, etc., and for the backing of ashlar and squared-stone masonry.