When a curve occurs on an ascent, the grade at that place must be diminished in order to compensate for the additional resistance of the curve.
The width of the wheelway on curves must be increased. This increase should be one-quarter of the width for central angles between 90 and 120 degrees, and one-half for angles between 60 and 00 degrees. Excessive crookedness of alignment is to be avoided, for any unneces sary length causes a constant threefold waste; first, of the interest of the capital expended in making that unnecessary portion; secondly, of the ever recurring expense of repairing it; and thirdly, of the time and labor employed on travelling over it.
• The curving road around a hill may be often no longer than the straight one over it, for the latter is straight only with reference to the horizontal plane, while it is curved as to the vertical plane; .the former is curved as to the horizontal plane, but straight as to the vertical plane. Both lines curve, and we call the one passing over the hill straight only because its vertical curvature is less apparent to our eyes.
The difference in length between a straight road and one which is slightly curved is very small. If a road between two places ten miles apart were made to curve so that the eye could nowhere see farther than one-quarter of a mile of it at once, its length would exceed that of a straight road between the same points by only about four hundred and fifty feet.
Zigzags. The method of surmounting a height by a series of zigzags or by a series of reaches with practicable curves at the turns, is objectionable.
(1) An acute-angled zigzag obliges the traffic to reverse its direction without affording it convenient room for The consequence is that with slow traffic a single train of vehicles is brought to a stand, while if two trains of vehicles travelling in opposite directions meet at the zigzag a block ensues.
(2) With zigzags little progress is made towaros the ultimate destination of the road; height is surmounted, hut horizontal distance is increased for which there is no necessity or compensation.
(3) Zigzags are dangerous. Li case of a runaway clown hill the zigzag must prove fatal.
(4) If the drainage cannot be carried clear of the road at the end of each reach, it must be carried under the road in one reach only to appear again at the next, when a second bridge, culvert, or drain will be required, and so on at the other reaches. If the drainage can
be carried clear at the termination of each reach, the lengths between the curves will be very short, entailing numerous zigzag curves, which are expensive to construct and maintain.
Final Location. The route being finally determined upon, it requires to be located. This consists in tracing the line, placing a stake at every 100 feet on the straight portions and at every 30 or 25 feet on the curves. At the tangent point of curves, and at points of compound and reverse curves, a larger and more permanent stake should be placed. Lest those stakes should be disturbed in the process of construction, their exact distance from several points outside of the ground to be occupied by the road should be carefully measured and recorded in the notebook, so that they may be replaced. The stakes above referred to show the position of the center line of the road, and form the base line from which all operations of con struction are carried on. Levels are taken at each stake, and cross levels are taken at every change of longitudinal slope.
Construction Profile. The construction or working profile is made from the levels obtained on location. It should be drawn to a horizontal scale of 400 feet to the inch and a vertical scale of 20 feet to the inch. Fig. 7 represents a portion of such a profile.. The figures in column A represent the elevation of the ground at every 100 feet, or where a stake has been driven, above datum. The figures in column B are the elevations of the grade above datum. The figures in column C indicate the depth of cutting or height of fill; they are obtained by taking the difference between the level of the road and the level of the surface of the ground. The straight line at the top represents the grade of the road; the upper surface of the road when finished would he somewhat higher than this, while the given line represents what is termed the sub-grade or formation level. All the dimensions refer to the formation level, to which the surface of the ground is to be formed to receive the road covering.