the ground; DA, part of the base of a cutting; AEC, an embankment the construction of which is carried forward in the direction AE of its full width and height (including a sufficient allowance for settlement), by running dump-carts on temporary tracks from the cutting along the top of the embankment, and tipping them at E, so that the earth runs down and spreads itself over the sloping end EC of the bank, which is called the "tip." Embankments formed in this manner are deficient in compactness, for the particles of earth which are emptied from the top of the bank will temporarily stop in their descent at the point of the slope at which the friction becomes sufficient to balance their gravity; and when more earth comes upon them, they will give way and slide lower down, causing the portions above them to slip and crack, and thus delay for a long time the complete consolidation.

Tipping or dumping the earth over the sides of banks made in the above manner should not be allowed, for the earth so dumped is liable afterwards to slip off.

The solidity of embankments formed in the above manner may be increased by filling from the sides towards the centre in order that the earth may arrange itself in layers with a dip from the sides inwards.

FIG. 58. CROSS-SECTION OF EARTH EMBANKMENT, SHOWING METHOD OF PLACING THE LAYERS.

As the rapidity with which a bank can be made by this method is dependent upon the number of tipping or dumping points, it is usual to form the bank wider at top and narrower at the bottom than it is finally to be, maintaining of course the requisite area of cross-section; the excess at the top (the angles AB, Fig. 59) being subsequently moved down to the bottom, thus securing the required width of base and inclination of side slopes.

It is mistaken economy to first form embankments narrow and

afterwards widen them by lateral additions, for the new material will never unite perfectly with the old.

(2) In Thick Layers.—This process has been used in some embankments of great height. It consists in completing the construction of the embankment up to a certain height by the process of end-dumping already described; leaving that layer for a time to settle, and then making a second layer in the same way, and so

B

on.

FIG. 59.

It involves much additional time and labor, and is seldom employed. It is, however, useful in making embankments of hard. clay or shale, which, when first dumped, consists of angular lumps that lie with vacant spaces between them and do not form a compact mass until partially softened and broken down by the action. of air and moisture.

(3) In Thin Layers.-This process consists in spreading the earth in horizontal layers of from 9 to 18 inches deep, and rolling or ramming each layer so as to make it compact and firm before laying down the next layer. Being a tedious and costly process, it is used in special cases only, of which the principal are the filling in behind retaining walls, behind wings and abutments of bridges and culverts, and over their arches.

654. Side Slopes of Embankments.-In forming the embankments the side slopes should be made with a greater inclination than that which the earth naturally assumes, for the purpose of giving them greater durability, and to prevent the width of the top surface along which the roadway is made from diminishing by every change in the side slopes, as it would were they made with the natural slope. To protect the side slopes more effectually they should be sodded, or sown in grass-seed, and the surface water of the top should not be allowed to run down them, as it would soon wash them into gulleys and destroy the embankment. In

localities where stone is plentiful a sustaining wall of dry stone may be advantageously substituted for the side slopes.

The toe or foot of embankments has a tendency to spread; this may be resisted by excavating a small trench along the toe, or by buttressing with a low stone wall.

655. Drainage of Embankments.-The only drains required for embankments over good ground are the ordinary side ditches, with occasional culverts to convey the water from them into the natural water-courses. When springs are crossed, stone drains or culverts may be built to carry the water clear of the embankment.

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656. Embankments over Plains. When a roadway is carried across an extensive plain, it is almost always necessary, in order to keep its surface dry, that it should be raised above the general level of the ground; and where inundations occur, the requisite height may be considerable. In Fig. 60, A represents a cross-sec

FIG. 60. SECTION OF EMBANKMENTS OVER PLAINS. tion of an embankment for this purpose, the materials for which are obtained by digging a pair of trenches alongside of it. These trenches, by collecting surface-water and discharging it into the nearest river or other main drainage channel, tend to shorten the duration of floods in the neighborhood of the line.

657. Embankments across Marshes.-When the ground is so soft that an embankment made in the ordinary way would sink in it, different expedients are to be employed according to the kind and degree of difficulty to be overcome. The following list of expedients is arranged in the order of an increasing scale of difficulty:

(1) By digging side drains parallel to the site of the intended. embankment, the firmness of the natural ground may be increased.

(2) If the material of the natural ground has a definite angle of repose, though much flatter than that of the material of the embankment, the slopes of the embankment may be formed to the same angle, thus giving it a broader foundation than it would have with its own natural slope.

(3) A foundation may be made for the embankment by digging a trench and filling it with a stable material.

(4) The ground may be compressed and consolidated by driving short piles.

(5) The embankment may be made of materials light enough. to form a sort of raft, floating on the soft ground, such as hurdles, fascines, timber platforms, or dry peat. Dry peat was the material used by George Stephenson to carry the Liverpool and Manchester Railway across Chat Moss. Its heaviness, when well dried in the air, is about 30 pounds per cubic foot; and when saturated with water, 63 pounds. On the dry-peat embankment was placed a platform of two layers of hurdles to carry the ballast.

(6) Should all other expedients fail, a marsh or bog may still be crossed by throwing in stones or gravel and sand, until an embankment is formed resting on the hard stratum below, and with its top rising to the required level. It is found that the material of the embankment assumes the same natural slope that it would do in the air.

Mr. George W. Waite, C.E., gives the following description of a road constructed by him in 1868 in the village of Hyde Park (now in the city of Chicago):

"The line crossed a marsh about one mile wide which extended from about two miles west, easterly to Lake Michigan, and southeasterly to Calumet River, a distance of two miles, and was at that time all covered with water from a few inches to two feet deep. Wild rice grew all over that portion of the marsh, about 8 feet high, and the stalks were from to inch in diameter at the bottom. Through the central portion of the marsh was an open water-way about 10 feet wide in the channel proper, with no perceptible current, which widened out into small lakes every few hundred feet.

"The channel and lakes had from 3 to 4 feet of water and about the same depth of black slush or decayed vegetable matter.

"Soundings showed the turf to be about 1 foot thick, with from 2 to 6 feet of soft black vegetable mould underneath, then a hard bottom of blue clay.

"The method of construction was as follows: Beginning at the dry ground on the south end, an 18-foot inch board, 1 foot wide, was placed lengthwise on the outside, 9 feet from the centre, then

one in the centre, 6 feet in advance of the first, and then one on the opposite side, 6 feet in advance of the middle one. Then the three pieces laid lengthwise were covered with 18-foot sound boards. 1 inch thick, laid crosswise and nailed as fast as laid to keep them in their places. On these were placed three more, lengthwise as at first, one in the centre and one on each side, and these were nailed through into the under ones. Next all the wild rice for a space of about 75 feet on each side was cut down and pitched with forks onto the floating platform or roadbed. It made a compact covering about 2 feet thick. At the end of the first 500 feet a turnaround for teams on one side was made of boards doubled, 36 feet square, thoroughly nailed.

"Then the whole 500 feet of roadbed was covered 16 feet wide with about 15 inches thick of stone, and on this was placed 3 inches of crushed stone.

"After finishing the first 500 feet the turn-around was removed to the end of the second 500 feet, and so on to completion. Near the middle of the marsh was a lake which the line crossed, some 200 feet wide. This was covered with a bent bridge 50 feet long, and the balance with floats, the same as the marsh but wider. The bridge was placed on the pond-lily roots that everywhere abounded in the bottom of all these small lakes, and left about 2 feet higher than needed to allow for settling, but it has not yet settled more than some 6 inches, although a pole can be run down between the network of roots and into the slush underneath about 3 feet below the bottom of the sills before the hard bottom is reached.

"The road settled on an average about 2 feet, with the exception of two or three short distances where it settled 3 feet, but it did not break through the turf in any place. At a high stage of water some places for a few feet in length would be 1 foot under water.

"The road has stood over 23 years and has been considerably travelled, and is in good condition at the present time (1892). It has had but very little top-dressing during the whole time. Since the road was constructed the marsh has nearly all been drained and has mostly become solid, and the land in it, which at that time. was not worth $25.00 per acre, has just been sold for $2500.00 per acre."

658. Embankments across Bogs.-Undrained moss consists of