Road Location Elements

By the same method of analysis, the load for the same team on a level, muddy earth road having a tractive resistance of 200 lb. per ton is 1.2 tons, and the maximum permissible grade is 7.5 per cent.

For a broken-stone road having a tractive resistance of 33 lb. per ton, the load on the level is 7.3 tons, and the permissible maxi mum grade is 2.2 per cent.

What load can the above team take up a 4 per cent maxi mum grade on a broken-stone road having a tractive resistance of 33 lb. per ton? The grade resistance is 20 X 4 = 80 lb. per ton; and the tractive resistance is 33 lb. per ton ; therefore the total resistance is 80 + 33 = 113 lb. per ton. The maximum tractive power of the team is equal to one quarter of its weight, or 600 lb.; and the grade resistance for the weight of the team = 2400 2000 X 80=96 lb.; therefore the net tractive power of the team is 600— 96= 504 lb. Then the weight of the wagon and the load which the team can draw up this grade is 504 = 113 = 4.4 tons.

The above computations are for two 1200-pound horses, but the conclusions would not materially differ for horses of other weight.

Rise and Fall.

By rise and fall is meant the vertical height through which the load must be lifted in passing over the road in each direction. One foot of rise and fall is a foot of ascent with its corresponding foot of descent. In passing over a ridge 10 feet high standing in the middle of a level plain, there is only 10 feet of rise and fall; and not 10 feet of rise plus 10 feet of fall. If the road is level, Fig. 4, then an elevation or depression of, say, 1 foot produces literally 1 foot of rise and a corresponding foot of fall; but if the road is on a steep grade, Fig. 5, an elevation of 1 foot above the grade line or of a like amount below the grade line, literally speaking, produces no rise and fall, because in either case it is a continuous up grade. However, as far as the operation is concerned, the two cases are exactly alike, and each has a foot of rise and fall.

Rise and fall is measured by the number of vertical feet of rise, as shown by the differences of elevation on the profile.

The introduction of rise and fall is a question either (1) between the increased cost of operation and the increased cost of construction required to fill up the hollow or to cut down the hill, or (2) between the cost of operation of the rise and fall and of the increased distance necessary to go around the obstruction.

The following example is often cited as showing the improve ment that can be made in locating roads. "An old road in Anglesea rose and fell between its extremities, 24 miles apart, a total per pendicular amount of 3,540 feet; while a new road laid out by Telford between the same points, rose and fell only 2,257 feet; so that 1,283 feet of vertical height is now done away with, which every horse passing over the road had previously been obliged to ascend and descend with its load. The new road is, besides, more than two

miles shorter. Such is one of the results of the labors of a skilful road maker." * The road may have been economically re-located, but the citation fails to show whether the increased cost of construc tion to eliminate rise and fall was justified by the decreased cost of operation.

The following example from the same author, also frequently quoted, shows that rise and fall was eliminated by increasing the distance, although no attempt is made to show that the increased distance was more economical than the rise and fall thereby elimi nated. "A plank road, lately laid out under the supervision of Mr. Geddes, between Cazenovia and Chittenango, N. Y., is an ex cellent exemplification of the true principles of road making. Both these villages are situated on the Chittenango Creek, the former being 800 feet higher than the latter. The most level wagon road between these villages rises more than 1,200 feet in going from Chittenango to Cazenovia, and rises more than 400 feet in going from Cazenovia to Chittenango, in spite of this latter place being SOO feet lower. It thus adds one half to the ascent and labor going in one direction; and in the other direction it goes up hill one half the height, which should have been a continuous descent. The line of the plank road by following the creek (crossing it five times) ascends only the necessary 800 feet in one direction, and has no ascents in the other, with two or three trifling exceptions of a few feet in all, admitted in order to save expense. There is a nearly vertical fall in the creek of 140 feet. To overcome this, it was necessary to commence far below the falls, to climb up the steep hillside, following up the sides of the lateral ravines until they were narrow enough to bridge, and then turning and following back the opposite sides till the main valley was again reached. The extreme rise is at the rate of 1 foot to the rod (1 in 16i), and this only for short distances, and in only three instances, with a much less grade or a level intervening." Classes of Rise and Fall. In discussing the effect of rise and fall upon the operation of a road a distinction must be made between three classes of rise and fall, as follows: Class A. Rise and fall on grades at a less slope than the angle of repose (the grade on which a vehicle by its own weight will main tain a uniform speed), and so situated as not to require any addition to the total power required to move a load over the road.