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Surface Drainage - Street Drainage

pavement, crown, center, water, curved and arch

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SURFACE DRAINAGE - STREET DRAINAGE. The drainage of the surface of the pavement is provided for by making the center of the pavement higher than the sides. The principle governing the amount of crown for pavements is somewhat different from that of earth, gravel, or macadam roads. First, a hard, smooth and practically impervi ous pavement needs no crown for the drainage of the surface; and on such a pavement, the only advantage of a transverse slope is to drain shallow depressions due to faulty construction, wear, or a set tlement of the foundation, and to aid the rains in washing the pave ments. Second, the surface of the pavement has no tendency to wash; and hence the crown need not be increased on a grade as in the case of earth roads. The less the crown the better for the traffic, and the more uniformly will the travel be distributed over the pave ment, although a slight crown is inappreciable in either of these respects. Therefore pavements require only crown enough to drain depressions of the surface due to faulty construction, to wear, or to settlement of the foundation; and the crown may decrease as the grade increases.

Crown.

There has been much discussion as to the best form of the surface of a pavement. Some claim that it should be a con tinuous curve,while others contend that it should consist of two planes meeting in the center. The curved profile is defective in that it gives too little inclination near the middle, the result being that the pave ment wears hollow in the center and permits water to stand there. To overcome this objection some engineers raise the center of the pavement or 3/4of an inch above the curved cross section. The objection to the two planes is that the sides wear hollow and hold water. An advantage of the curved profile is that the center of the street, which is the part especially devoted to travel, is nearly flat; while the sides, which have the greater inclination, are occupied by teams standing at the curb. Another advantage of the curved profile is that it gives a deeper gutter, which confines the storm water to a smaller portion of the street and reduces the interfer ence with pedestrian traffic.

It is sometimes claimed that the curved form will support the greater load, because of its arch action; but the arch action of a pavement is entirely inappreciable, owing to the flatness of the arch, to the imperfect fit of the so-called arch stones, and to the insta bility of the abutments or curbs.

The surface is usually a continuous curve—generally a parabola. For the methods of staking out each, see § 310 and § 312, pages 200 and 201.

511. The early pavements in this country and at present those in some cities in Europe and South America, slope from both sides towards the center. In this form the most valuable part of the street is devoted to drainage purposes, and it is difficult to carry the water to an intersecting street. The pavements of alleys usu ally slope to the center. This form is better for alleys than a gutter at each side, since it keeps the storm water from flowing along the side of buildings and possibly interfering with light areas, cellar stairways, etc., and it also carries the water over the sidewalk with less annoyance to pedestrian traffic.

512. When construction begins, it is wise to give the one in charge of the work a drawing somewhat like Fig. 108, showing the relation between the top of the curbs and the grade of the foundation, the top of the concrete, and the top of the finished pavement. Such a drawing prevents misunderstandings and dis putes. Notice that the curves in Fig. 108 are not exact parabolas, the ordinates at 4 and 12 being inch too long; but this is suffi ciently exact, since it is not possible to secure mathematical pre cision in this class of work.

Crown in Various Cities.

The following is the practice in different cities. All use the curved profile, and it is immaterial whether it be called a circular or a parabolic arc.

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