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Street Grades

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STREET GRADES. The fixing of street grades is one of the most important functions of municipal engineering, since the grade system of the streets is the foundation of all municipal engi neering matters. The grades should be established before the sewer system is planned; and if they are established before the property is improved. the problem is comparatively simple, since they may be laid chiefly with reference to obtaining desirable gradients for the street within proper limits of cost. But when buildings have been erected, sidewalks constructed, and trees planted, it is frequently extremely difficult to secure grades which will harmonize the various and conflicting interests.

Elements Governing Grades.

The grades necessarily depend mainly upon the topography of the site: but in general the determination of the proper grade for a street requires the consid eration of the following elements: (1) the drainage, (2) the cost of earthwork (3) the acccommodation of the traffic, (4) the effect upon the abutting property, and (5) the general appearance of the street.

Drainage.The streets are the natural drainage channels of the city; the lots must drain into them, and the house must drain into the sewers placed in the streets. When no storm water sewers are to be constructed, the grades become very im portant, since the streets must provide for the surface drainage of the city, and particular consideration must be given to relative grades and gutter capacities in order to prevent the excessive concentration of storm water at the lower levels and to provide for its proper distribution and disposal.

Cost

of Earthwork.Not infrequently the cost of making the excavations and embankments is given undue weight. The balancing of cuts and fills is often properly a controlling element in country road construction, but it should have relatively little weight in determining the grades of city streets. The expense for earthwork is incurred once for all, and a few hundred dollars more or less is usually unimportant in comparison with the ex pense of maintaining the street surface and the drainage system. and the cost of conducting traffic over the grades, and also in comparison with a better general appearance of the street.

Accommodation of Traffic.

The question often is whether or not to secure ease of traction at the expense of increased cost of construction. The discussion in Chapter II, 62-86, sheds a little

light, and only a little, as to the proper method of answering this question. Apparently engineers are inclined to overestimate the disadvantage to traffic of a slight grade. Practical experience has demonstrated that there is not much difference in effect upon the cost of transportation between level roads and those having grades of 2 or 3 per cent unless such grades are very long or have an unusually smooth and well-kept surface, Effect upon Abutting Property.The private interests of the property holder should be carefully considered; although it is frequently impossible to establish proper grades without injury to the adjoining property. The general question is how far private interests should be sacrificed to the general good. It is better that the city or the other residents on the street should pay the owner damages than that lasting detriment should be done to the appear ance of the street or to the traffic.

General Appearance. Some attention should be paid to the appearance of a longitudinal view of the pavement. It is de sirable that the longitudinal grade be not changed so frequently as to give the street a wavy appearance. Further, the transverse grades at street intersections and on side hills should be so arranged as not to produce a confused appearance in looking along the street. The grades of the streets, both longitudinal and transverse, have a material effect upon the general appearance and beauty of the city.

Maximum Grade.

In a general way the principles gov erning the determination of the permissible maximum grade of a city street are the same as for a country road, i. e., it is a question between the cost of operation on the one hand and the cost of construction and maintenance on the other, except that for a country road the cost of construction is chiefly the cost of moving the earth, while for a city street the cost of construction should also include the effect upon abutting property of high embank ments or deep excavations, and except further that usually in the city heavy loads can take a circuitous route and avoid the maximum grade entirely. In determining the maximum grade for a street, the fact should not be overlooked that the smoother the pavement the more serious is a steep grade.

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