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X. FENCES,-Full details of those shown on sketch which it is proposed to utilize for defense.

XI. OBSTRUCTIONS, DEMOLITIONS, TRENCHES, AND OTHER WORKS.-Details of these to be submitted on a separate report if required, showing kinds, time required, number of reliefs, number of men in each relief, tools required, etc. (Note example of position sketch and report.)

CHAPTER XXV.

LAYING OUT ROADS.

SELECTION OF SITE.-Use of maps. If a contoured map of the country is available, that which is probably the best line may be selected from it. An idea of a possible site may be obtained from any existing map. On account of drainage, water-sheds are more advantageous than either water-courses or hill-sides.

Ruling points. The road should be so located as to require the least amount of labor and expense, due regard being paid to direction and gradients. There will usually be some feature whose passage will be difficult. The part of this feature that can be passed most easily will constitute a ruling point, provided it is not too far out of the direct line of the road. A col in a chain of mountains would be such a point.

Secondary points. Between ruling points other points of less importance are chosen. These are called secondary points and in their selection there is much latitude. Ruling and secondary points are usually determined by an inspection of the ground. After this inspection, the possible lines would be compared and the best one selected.

LAYING OUT THE GRADE. This may be done with a pair of dividers on a contoured map, or with the clinometer, slope board, or transit on the ground.

On a contoured map the maximum gradient is specified in the instructions. Take off on the dividers, from the scale of horizontal equivalents, the distance corresponding to the maximum slope. Should the distance between contours be greater than this, the road can have any desired direction. Should

the distance be less, the road must be run obliquely between contours so as to make the distance between them equal to or greater than that corresponding to the maximum slope.

On the ground. Standing at one point of the road, direct an assistant to move up or down the hill until he comes to a point on the desired slope, as indicated by the instrument used. A picket is driven into the ground at this point and, if required, its bearing from the preceding point taken. The maximum gradient should be used only where absolutely necessary, a smaller one being chosen when practicable, so as to make the gradient as uniform as possible throughout. The work of marking the center grade line is continued until a ruling point is reached. Should the line surveyed run above or below the ruling point, the pickets will have to be shifted in order to equalize the gradient throughout, or, if this be not possible, a new line between ruling points will have to be run.

Leveling to grade between pickets. This is done with the clinometer, slope board, or transit. The instrument is taken to one picket and set to read the slope to the next picket. An assistant then moves along the line between pickets and at the points of change of slope drives in stakes and marks on them the amount of cut or fill as indicated by a rod or staff carried for this purpose. a and b (Fig. 215) are the positions of the pickets at points of grade. The position of the target on the

Eig 215

rod held at the intermediate points shows the amount of cut or fill.

LAYING OUT THE SECTION.—In rough work this may be done with any instrument that can be used as a level; for accuracy the wye level is necessary. This operation is sometimes called cross-section leveling.

The required width of the road is measured off horizontally from the center line and the exact edges marked by means of stakes or by scoring the ground.

The required slope of the road bed from center to sides is usually specified in the instructions. If not specified, it must be assumed and the difference of level between the center and side stakes computed. The amount of cut or fill at any point

a

Fig 216.

along the central grade line being known, the amount at the sides corresponding to it can be computed and marked on the side stakes.

Side slopes. To locate the slope stakes. Having the readings on the center and side stakes, a rough profile of the ground should be constructed on cross-section paper carried for the purpose. The location of the stakes for the side slopes is then determined as follows: Suppose ab (Fig. 216) to be the profile showing the natural surface of the ground as determined by the level, o being the position of a central picket. Plot d and d' on the diagram, their position having been computed as explained. Draw dd; then from d and d' draw db and da, having an inclination corresponding to the required side slopes. The points where these last lines intersect the line ab will be the plotted positions of the bottom and top of the required slopes respectively.

The horizontal distances corresponding to eb and ea on the diagram will be the required distances to lay off from the stakes at e and e' to determine the bottom and top of the side slopes. Stakes are driven at the points and b.

Sections should be made about every thirty yards, or as required by the conformation of the ground.

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