GUTTERS. The Material. Ordinarily the surface of the pavement adjacent to the curb serves as a channel to convey the drainage to the nearest inlet, i. e., the gutter is formed of the same material as the pavement. With an asphalt or macadam pave ment, it is customary to lay brick or stone blocks in the gutters— with asphalt to prevent its deterioration from being continually covered with mud and water, and with macadam to prevent flow ing water from disintegrating it.
A combined concrete curb and gutter (§ 522) is frequently used, particularly with asphalt, brick, or macadam on residence streets.
A concrete gutter is objectionable on a macadamized street, on account of the crushed stone's wearing below the edge of the gutter, a condition which interferes with the drainage; but if the macadam surface is reasonably well cared for, this objection is not serious. A concrete gutter has been objected to for any pavement owing to the liability of a rut to form along its outer edge. In practice neither of these objections has proved to be serious. A concrete gutter is more efficient and looks better than any other material except asphalt.
Usually the gutter is formed by continuing the ordinary slope of the pavement until it intersects the curb; but occasionally the outer edge of the pavement is given an upward inclination, thus form ing a flat V-shaped channel a little way from the curb. This con struction makes an excellent channel for the water, but prevents the driving of a carriage close enough to the curb to allow people to step in or out easily.
In some cases the curb is set and the gutter formed before the pavement is laid, in which case the curb and gutter are constructed as they would have been if the street were to be paved,—the gutter being composed of stone blocks, brick, or concrete (§ 520). Some times a street is macadamized or graveled when it is not desired to incur the expense of setting a curb, in which case the gutter is built of cobble stones, or stone blocks, or brick, in the form of a very flat V with the side next the property much the steeper.
This can be done with a stone curb, but not with a combination con crete curb and gutter (§ 522), since the latter is made in moulds and hence must have a uniform cross section; and therefore with a con crete curb and gutter, it may be necessary to put a summit in the pavement to secure proper drainage of the gutters. Except in extreme cases, the gutter should not be deeper than 9 inches nor shallower than 3 inches; and ordinarily it should not be more than 8 nor less than 4 inches—usually it is 5 or 6 inches.
It may be necessary to modify the preceding rules when one side of the street is higher than the other (see § 488). In localities where there is a good deal of snow, the gutter must be deeper than stated above, for shallow gutters readily become clogged with snow and slush. In some northern cities, the snow is habitually allowed to pack upon the surface of the street to a depth of 6 or more inches, in which places the depth of the curb must be extremely deep to prevent the melting snow and water from filling the gutter and flowing over the sidewalk into the basements.
The minimum grade permissible in the gutter will depend chiefly upon the material with which it is paved, but somewhat upon the cost of catch basins. Almost any grade can be obtained by estab lishing catch basins close together and raising the gutter half way between them. In a number of cities the minimum grade of gutters paved with granite blocks, brick, rectangular wood blocks, or mac adam is 1 in 300 or 400. Except under very favorable circumstances, a slope of 1 in 200, of 1 per cent, should be regarded as the minimum.
Asphalt decays if continually wet, and therefore the condition governing the minimum permissible grade is different for that than that for other materials. With a slope of less than 1 per cent, the gutter will not keep itself clean, consequently the asphalt will decay owing to the action of mud and water; and hence asphalt should not be laid in a gutter having a fall of less than 1 in 100. If this fall can not be obtained, a concrete gutter should be used, or the gutter should be paved with vitrified brick or carefully dressed granite blocks.