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Placing the Line

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PLACING THE LINE. The controlling points of a line are certain points at which the position of the road is restricted within narrow limits and is not subject to change. These may be points where the location is governed by the necessity of providing an out let for the traffic, or points where the position of the line is restricted by topographical considerations—such as a summit over which the road must pass, or a suitable location for a bridge.

After the reconnoissance of the locality is completed and the position and elevation of the controlling points are known, the line must be marked upon the ground. For example, assume that it is desired to run a road from A to D, Fig. 7, page 68, D being a pass over the ridge. If the road follows the line A BC D, it will have the profile shown near the bottom of Fig. 7. The average grade from A to B is 1 per cent, and from B to C 5 per cent. If it is de sired to locate a road that shall have a grade no steeper than 5 per cent, we may begin at D and locate a line having an uniform 5 per cent grade. If is best to commence the location from D, since usually the slopes nearer the foot of the hills are flatter than those at the summit, and consequently there is more choice of position of the line there than at the summit. Frequently in rough country, the only controlling point fixed before beginning the location survey is the lowest pass over a ridge or mountain range.

Beginning at D, a line may be located either (1) by setting off the angle of the gradient on the vertical circle of a transit or on a gradienter,* and sighting upon a rod which is moved until the line of sight strikes it at the same height from the ground that the instru ment is above grade; or (2) the points for the line may be found by running a line of levels ahead of the transit, and measuring the distances by which to reckon the rate of the grade. The line D E C, Fig. 7, has a uniform gradient of 5 per cent.

If a contour map is at hand, the line can be located approxi mately by opening a pair of dividers until the distance between the points corresponds to 100 feet, setting one point on the place of beginning and the other on the next lower contour, which gives a line 100 feet long with a grade equal to the distance between con tours—in Fig. 7, five feet.

The line D F G has a uniform grade of 5 per cent. From H to A the road will have considerably less grade than 5 per cent, and can have a comparatively wide range of position.

The average grade from A to D is a little less than 5 per cent, but the slopes are so steep between D and C that it is impossible, within the limits of the map, to locate such a line. If such a gradient is located from D toward A, it will necessarily make a number of short turns on itself, which, although undesirable, are sometimes un avoidable. These short turns seriously impede traffic, since vehi cles can not easily pass each other on such short curves—particu larly if each is drawn by a long team. Short turns are also dangerous in descending, in case control of the vehicle is lost or the team runs away.

The line A BCD may be considered as an old road which it is proposed to improve by reducing the grades. Substituting the line C E D for C D changes the maximum grade from 10 to 5 per cent.

In placing the line attention should be given to the nature of the soil on alternative lines, since on one side of the valley the surface may be clay, upon the opposite gravel; in the bottom of the valley the soil is usually alluvial, while higher up it is generally better for road purposes. It should be remembered that in almost all steep slopes covered with loose material, the debris is either slowly moving down the slope or has attained a state of repose so deli cately adjusted that an excavation for a road-bed on the inclined surface will again set the mass in motion. Such movements are particularly common in loose materials in countries where the frost penetrates deeply and the ground becomes very soft when thawing, and frequently entail long-continued and serious expense in main tenance.

If the road is to have a surface of gravel or broken stone, the relative proximity of the materials for the original construction as well as for repairs should be considered in deciding between possible locations. However, it should be remembered that after the road is completed, the amount of hauling required to supply materials for maintenance must of necessity be small in comparison with the ordinary traffic over the road; and hence this consideration should not have undue weight.

Attention should also be given to the disposal of the drainage water, and to the question of danger from high water in the streams. For example, in Fig. 7 it is possible to locate a line on the upper side of the map with an uniform grade of 4 per cent, but such a line will lie so near the branch entering the main stream at B as to be in danger from floods. The matter of crossing streams should receive the most careful study. Bridges are comparatively expensive to build and to maintain.

It may be cheaper to carry the road across the gully on an em bankment or a trestle than to make a detour around the head of the valley. This question can be determined by comparing the greater cost of construction of the shorter line with the capitalized value of the greater cost of operating the longer line.

In some localities the protection of the road against snow is an important matter. Deep cuts almost always catch snow; and for this reason it is sometimes better to go around a point by a sup ported grade than to cut through it. In a snow country roads should be located on slopes facing south and east in preference to slopes facing north and west, as the sun has greater power on the former to melt the snow.

" Nothing pays like first cost in road building," i. e., money ex pended in intelligent study of the location is the most economical expenditure in the construction of a road.