Excavation and Embankment

When the road lies along the side of a hill, one side of the road is usually in cut and the other in fill; and it is customary so to place the center line that these two parts are at least nearly equal. How ever, where the side slopes are steep, it is better to make the road mostly in cuts on account of the difficulty of forming stable fills on steep slopes.

In railroad work it is the custom to balance cuts and fills on the longitudinal profile of the road, but in wagon-road work the fills as shown by the profile of the center line should be slightly in excess, to provide a place for the earth taken from the side ditches. On account of the expense, wagon-roads follow the surface more nearly than railroads; and consequently the earth from the ditches is proportionally more in wagon-road construction than in railroad construction.

Shrinkage of Earthwork.

With most soils, the act of excavation so breaks it up that it occupies more space after excava tion than before; but when the material has been placed in an em bankment it will usually occupy less space than in its original posi tion. The expansion due to excavation is usually 8 to 12 per cent of the volume, and in extreme cases may be 40 per cent; but in placing the material in the embankment, it is compacted by the weight of the embankment itself, by the pounding of the hoofs, and by the action of the wheels, until usually the final volume is less than the original.

At first thought it seems strange that earth should occupy less space when placed in an embankment than when in its original position, seeing that it is not so hard and firm, and that it will usually settle still farther after the embankment is completed. The following facts account for this phenomenon: 1. The continued action of frost has made the soil in its natural position more or less porous. 2. Earths which have been lying in situ for centuries be come more or less porous through the slow solution of their soluble constituents by percolating water. 3. The surface soil is rendered more or less porous by the penetration of vegetable roots which subsequently decay. 4. There is ordinarily more or less soil lost or wasted in transporting it from the excavation to the embank ment.

The amount of shrinkage depends chiefly upon the character of the material and the means by which it is put into the embankment, and somewhat upon the moisture of the soil, the rainfall conditions while the work is in progress and soon afterwards, and the depth to which frost usually penetrates. If the soil is moist when placed in

the bank, it will become more compact than if it were dry. Rain greatly affects the shrinkage, and embankments put up during a rainy season will be more compact than those built during a dry time. Soil from above the usual frost line is more porous than that not subject to the heaving effect of alternating freezing and thawing, and consequently shrinks more when put into an embankment.

The natural shrinkage of the ordinary soils is in the following order: (1) sand and sandy gravel least, (2) clay and clayey soil intermediate, and (3) loams most. The shrinkage according to the method of handling is in the following order: (1) drag scrapers, (2) wheel scrapers, (3) wagons, (4) cars, (5) wheelbarrows. The usual allowance for shrinkage is as follows: gravel 8 per cent, gravel and sand 9 per cent, clay and clayey earth 10 per cent, loam and light sandy earth 12 per cent, loose vegetable surface-soil 15 per cent. The above results are for ordinary earth, and do not apply to such unusual materials as "buck-shot," gumbo, very fibrous soil, etc., which have a much greater shrinkage. Solid rock will expand 40 to 50 per cent.

The shrinkage of earth should be considered in locating the grade lines to balance the cuts and fills.

Settlement of Embankments.

The shrinkage of earth work referred to above takes place chiefly during construction, but the continued action of the weight of the embankment and the effect of rain and traffic will usually cause a comparatively small settlement after completion. Sand or gravel embankments built with wheel scrapers will usually settle 1 to 2 per cent after comple tion, and clay or loam embankments about 2 to 3 per cent. With drags crapers the settlement will usually be a little less than the above; and with dump carts or wagons, a little more. With wheel barrows the settlement is usually about 10 per cent, but may be as much as 25 per cent, depending upon the moisture in the soil, the rain during construction, and the length of time under construc tion.