The curbs used in different places vary considerably in form and dimensions. Stone curbs vary from 4 to 12 inches in width and from 8 to 24 inches in depth. They are usually employed from 3 to 6 feet in length and set with close joints.
The depth must be sufficient to admit of their being firmly bedded, and to prevent overturning into the gutter. The front of the curb should be hammer dressed to a depth greater than its exposure above the gutter, and the back deep enough to permit the side walk pavement to fit close against it where the side walk adjoins the curb. The ends of the blocks should also be dressed to the depth of exposure, and the part below the ground trimmed off so as to permit the dressed ends to come in contact when laid.
Granite is usually considered the best material for curbs, although both sandstones and limestones are used in many places. In the vicinity of New York the North River Bluestone has proved a good material for the purpose.
There are various ways of setting the curb. The object should be to bed it firmly on a.solid foundation.
The best method is to place a bed of concrete under it. This construction is shown in Fig. 39, which sents the method used in setting granite curb in Wash ington, D. C. The curb is held firmly in place by the concrete foundation, which joins it rigidly to the road way pavement.
Where the concrete foundation is not used under the curb a deeper curbstone is necessary, usually from 18 to 24 inches in good work. Curbs are very com monly set in the natural ground, the pavement coming against it on one side; but it is usually found advan tageous to lay them upon a bed of gravel or broken stone, with gravel filled in the trench about them. The ordinary method of setting curbs is shown in Fig. 40.
The Washington specifications for ordinary work require that a bed of gravel 4 inches deep be used under the curb, and that the trench be filled with gravel placed in layers 3 or 4 inches deep, each layer being thoroughly rammed before adding the next.