For a general discussion of the relationship between cost of construction and cost of operation as affecting the maximum grade, see Chapter II, Road Location—* 71.
In the Borough of Manhattan, New York City, are some business streets having grades as steep as 6 per cent, and a num ber of residence streets have 10 per cent grades, and some have grades of 12, 15 and 18 per cent. Brooklyn, N. Y., has 4 per cent grades on business streets and 12 on residence ones. A number of cities have maximum grades on paved streets of 20 per example, Worcester, Mass., Syracuse, N. Y., Borough of Rich mond, New York City, and Pittsburg, Pa. Burlington, Iowa, has an 80-foot street with a 24 per cent grade up which is laid a zigzag brick pavement 18 feet wide having a maximum grade of 141 per cent with a minimum radius of the inside curb of 16 feet.
For a discussion of the maximum grade for each kind of pave ment, see the heading Maximum Grades in the chapter treating that particular pavement.
It is usually considered that a grade steeper than 15 per cent is impracticable and dangerous even for light traffic; and there fore if this grade can not be obtained, the street should be divided into two parts separated by a terrace or stone wall, each portion being entered only at its intersection with the cross street. A 10 per cent grade is usually considered prohibitive for heavy loads; and 5 or 6 per cent is considered the limit on business streets.
The selection of the proper pavement for the maximum grade is a matter of great importance. It is usually held that sheet asphalt should not be laid on grades steeper than 2 to 3 per cent, although it has often been laid on 6 or 7 per cent grades, and in one instance on a 17 per cent grade (see § 676). Brick, or hard sandstone, or granite may be used upon the maximum grade. The sandstone and the granite blocks should be narrow and should be of a quality that does not wear smooth. It has been recom mended to chamfer the corners of rectangular stone or wood blocks when laid upon steep grades, to give the horses a good foot-hold; but it is at least doubtful whether the benefit of a good footing is not neutralized by the increased tractive resistance. The joints should be filled with tar or hydraulic cement.
Minimum Grade. The street surface should have enough longitudinal slope to drain its surface well. For a discussion of the minimum grade permissible with macadam—a material much used for city pavements as well as for country roads,—see § 86. With a smooth and impenetrable pavement no ruts will be formed, and hence the determination of the minimum permissible grade is mainly a question of the grade of the gutter. If the drainage is carried away by under-ground storm-water sewers, the street may be perfectly level longitudinally, since the necessary grade for the gutters may be obtained by making them deeper as they approach the inlet to the sewer. For a further discussion of this phase of the subject, see Grade of Gutter—§ 505.
If it is inexpedient to vary the depth of the gutter (§ 504) or to increase the grade by constructing additional inlets and catch basins, it is necessary to secure the proper slope for the gutter by inserting a summit in the street solely for drainage purposes— usually referred to as an accommodation summit. However, it is undesirable that there should be frequent changes in the grade, as they give the pavement an unpleasant wavy appearance when one looks along the street.
Grades at Street Intersections. One of the most impor tant parts of the establishment of a system of street grades is the arrangement of the grades at street intersections. It is a common practice to establish only the grade of the intersection of the center lines of the streets; but this has often resulted in much confusion in determining the grade for the corners of the curbs, particularly where the two streets have considerably different grades. For example, in Fig. 84 assuming (for the present at least) that the curb is to be at the same elevation as the center of the street opposite, the elevation of the corner of the curb, D, as computed from the grade of C B is 90 20 ft.; while the elevation of the same point as computed from the grade of B .1 is 91.20 ft.—a difference of 1.0 foot. To obviate this source of confusion, the elevation of each corner of the curb and also of the intersections of the center lines should be established.