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Street Grades

streets, blocks, cent, grade, pavement and desirable

STREET GRADES.

The grades of city streets necessarily depend mainly upon the topography of the site. Wherever possible, it is desirable that grades be uniform between cross streets.

In establishing grades for new streets through unim proved property, they may usually be laid with refer ence only to obtaining the most desirable gradients for the street within a proper limit of cost. But where improvements have already been made, and located with reference to the natural surface of the ground, it is frequently a matter of extreme difficulty to give a desirable grade to the streets without injury to adjoin ing properties. In such cases it becomes a question of how far individual interests shall be sacrificed to the general good. It may be said in this connection that adjustments to new grades are usually accomplished much more easily than would be anticipated, and when accomplished the possession of a desirable grade is of very considerable value to adjoining property.. Too great timidity should not, therefore, be felt in regard to making necessary changes because of the fear of in juring property in the locality.

Where a grade if made continuous between inter secting streets would be nearly level, it is frequently necessary to put a summit in the middle of the block and give a light gradient downward in each direction to the cross streets in order to provide for surface drain age. The amount of slope necessary to provide for proper drainage depends upon the character of the sur face and smoothness of the gutter. For a surface of earth or macadam the slope should not be less than about i in ioo, and for paved streets from i in 200 to z in 250.

In some cases it may be possible to give sufficient slope to gutters to carry off the surface-water by mak ing the gutter deeper at the ends than in the middle of the block without making a summit in the crown of the street. The curb in such case would be made

level or of uniform gradient.

It may frequently be necessary to consider the effect of grade in determining the character of pave ment to be employed upon a street. Asphalt is com monly limited to grades of 4 or 5 per cent, although some engineers use it on 6 or 7 per cent grades. Brick is commonly used on grades up to about 8 per cent, and in some places has given satisfactory service on io per cent grades. Wide joints, about I inch, are some times used in brick pavements on steep streets, in order to afford a better foothold for horses. This, however, in other instances appears to be unnecessary, provided the pavement is kept clean and in good condition.

Wood blocks may safely be used on grades of 5 or 6 per cent, while smooth stone blocks are employed in about the same manner as bricks, being if anything a little more slippery than bricks. Stone blocks of some what rough character are successfully used in some instances on grades of 12 or 13 per cent.

In a report on the streets of Duluth in 1890, Messrs. Rudolph Hering and Andrew Rosewater recommend for steep streets, in addition to the above, that brick may be used in which the tops are rounded, and that wood blocks for such use have their upper edges cham fered on each side, or if round blocks be used, around the blocks. Subsequent experience has, however, seemed to indicate that, except in extreme cases, such special construction is not necessary.

On the streets too steep for smooth pavement it is not unusual to pave part of the street width with a smooth pavement, like asphalt, and the remainder with stone blocks or some rough pavement for use in slippery weather.