CHAPTER XIII. EARTH-WORK. 627. Earth-work.—The term "earth-work" is applied to all the operations performed in the making of the excavations and embankments to prepare them for receiving the road-covering. In its widest sense it comprehends work in rock as well as in the looser material of the earth's crust. 628. Equalizing Earth-work is a term applied to the process of so adjusting the formation or sub-grade level of an inter led work that the earth from the cuttings shall be as nearly as possible sufficient to make the embankments and no more. The art of making this adjustment by the eye upon a profile of the ground with sufficient accuracy is soon acquired by practice. In most cases it is essential to economy in the cost of the work. For any surplus of embankment over cutting must be made up by borrowing, and the earth from any surplus of cutting over embankment must be wasted, both of these operations involve additional cost for labor and land. But cases sometimes occur in which it is more economical to make an embankment from borrow-pits close at hand than to bring the necessary material from a far-distant cutting on the line of the works, or in which it is more economical to waste part of the material from a cutting than to send it to a far-distant embankment on the line of the works, and these points must be decided by the engineer to the best of his judgment in each particular case. 629. Transverse Balancing.-When the road lies along the side of a hill, so that it is partly in excavation and partly in embankment, it is necessary to so place its centre line that these two parts of cross-section may balance. When the ground has a uniform slope the desired end would be obtained (if the side slopes were the same for excavation and embankment and if no shrinkage existed), by locating the centre line of the road upon the surface of the ground. In other cases, as when the side of the excavation slopes 1 to 1 and that of the embankment 2 to 1, a formula to determine the position of the centre line may be readily established. 630. If earth be wanted for a neighboring embankment, the amount of excavation may be easily increased by moving the centre of the road farther into the hill, with the additional advantage of lessening its liability to slip. The line may be thus changed on the map according to the notes of the cross-section in the level book, and be subsequently moved by a corresponding quantity on the ground. 631. When the slope of the ground is very steep the transverse balance must be disregarded and the road made chiefly in excavation, to avoid the insecurity of a high embankment. 632. Borrow-pits.-When the excavations on the line of the road do not furnish sufficient material for the embankments, the deficiency is obtained either by widening the excavations, or from an excavation termed a "borrow-pit," made in the vicinity of the embankment. 633. Spoil-banks. If the excavations furnish more material than is required for the embankments, the excess is generally deposited in a convenient place on the land adjoining the excavation, in banks termed "spoil-banks." Both these cases are expensive and objectionable. It is therefore very desirable to make the excavation and embankment "balance" each other. If the calculations show much disparity in the two amounts, the location of the line should be changed in some way so as to effect the desired equality. 634. The equalization must, however, be restrained within certain limits, for it should evidently be abandoned when, in order to form sufficient excavation to make the embankment, it would be necessary to go to such a distance that the cost of transport would exceed the cost of borrowing for the banks and wasting the distant excavation in spoil-banks. 635. The comparison of the price of transport with that of excavation and land will therefore determine the distance within which the balancing must be established. 636. The form to be given to the borrow-pits and spoil-banks will depend in a great degree upon the locality; they should as far as practicable be located so that the cost of removal of the earth shall be the least possible. 637. Staking out Borrow-pits.-Borrow-pits should be staked out by the engineer and their contents calculated, unless the contractor is to be paid by embankment measurements. A number of crossprofiles are taken of the original surface, and (on the same lines) on the bottom of the pit, after it is excavated, which furnish the depth of cutting at each required point. Borrow-pits should be regularly excavated so that they may not present an unsightly appearance when abandoned. 638. Shrinkage.-The equality recommended must be taken with an important qualification, dependent upon the fact that earth transferred from excavation to embankment shrinks, or settles so as to occupy less space in the bank than it did in its natural state. Rock, on the contrary, occupies more space when broken. 639. In estimating the relative amounts of excavation and embankment required, allowance must be made for difference in the spaces occupied by the material before excavation and after it is settled in embankment. The shrinkage of the different materials is about as follows: Rock, on the other hand, increases in value by being broken up, and does not settle again into less than its original bulk. The increase may be taken at fifty per cent. Thus an excavation of loam measuring 1000 cubic yards will form only about 880 cubic yards of embankment, or an embankment of 1000 cubic yards will require about 1120 cubic yards measured in excavation to make it. A rock excavation measuring 1000 yards will make from 1500 to 1700 cubic yards of embankment, depending upon the size of the fragments. 640. The lineal settlement of earth embankments will be about in the ratio given above; therefore either the contractor should be instructed in setting his poles to guide him as to the height of grade on an earth embankment to add the required percentage to the fill marked on the stakes, or the percentage may be included in the fill marked on the stakes. In rock embankments this is not necessary. 641. Failure of Earth-work.-The failure of earth-work is due to the slipping or sliding of its parts on each other, and its stability arises from resistance to the tendency so to slip. In solid rock, that resistance arises from the elastic stress of the material, when subjected to a shearing force; but in the mass of earth, as commonly understood, it arises partly from the friction between the grains, and partly from their mutual adhesion; which latter force is considerable in some kinds of earth, such as clay, especially when moist. But the adhesion of earth is gradually destroyed by the action of air and moisture, and of the changes of the weather, and especially by alternate frost and thaw; so that its friction is the only force which can be relied upon to produce permanent stability. 642. The temporary additional stability, however, which is produced by adhesion, is useful in the execution of earth-work, by enabling the sides of a cutting to stand for a time with a vertical face for a certain depth below its upper edge. That depth is greater, the greater the adhesion of the earth as compared with its heaviness; it is increased by a moderate degree of moisture, but diminished by excessive wetness. The following are some of its values: 643. One of the effects of the temporary stability due to adhesion is seen in the figure of the surface left after a "slip" has taken place in earth-work. That surface is not a uniform slope, inclined at the angle of repose, but is concave in its vertical section, being vertical at its upper edge, and becoming less and less steep downwards. It is not capable, however, of preserving that figure; for the action of the weather, by gradually destroying the adhesion of the earth, causes the steep upper part of the concave face to crumble down, so that the whole tends to assume a uniform curved slope in the end. 644. The Permanent Stability of earth, which is due to friction alone, is sufficient to maintain the side either of an embankment or of a cutting at a uniform slope, whose inclination to the horizon is the angle of repose, or angle whose tangent is the coefficient of friction. This is called the natural slope of the earth. The customary mode of describing the slope of earth-work is to state the ratio of the horizontal breadth to its vertical height, which is the reciprocal of the tangent of the inclination. 645. The angles of repose for different earths are given in Table XLV. But for all practical purposes it may be said that all earths, sand and gravel, stand at a slope of 33 degrees 41 minutes, or 1 to 1. If the slopes of an excavation in sand are to be left unprotected by sodding, they should be given a slope of 21 to 1. The ratio of slopes, their angles and length, are given in Table XLVI. TABLE LXV. NATURAL SLOPES OF EARTHS (WITH HORIZOntal Line). Angle Length. LENGTHS AND ANGLES OF SLOPES. Slope. with Horizon (Height taken as 1.00.) Slope. with Horizon (Height taken as 1.00). Angle Length. |