Excavation and Embankment

The settlement of the embankment after completion should be taken into account when determining whether the bank has been raised to the proper height. The embankment should be built to such a height that after it has ceased to settle it will be at grade. The length of time required for this settlement depends upon the weather conditions. The proper adjustment of the height of the embankments to compensate for future settlement is an important matter with broken-stone roads and with pavements.

The above remarks about settlement do not apply to em bankments built with the elevating grader (§ 149). The settle ment of earth roads put up by these machines is of no importance, and depends upon the amount of rolling they receive.

Rolling the Embankment.

Many writers on roads rec ommend the rolling of all new earth embankments. In view of the usual settlement of banks built with drag or wheel scrapers, it does not appear that rolling with a farm roller would be very effect ive, and a heavier roller is seldom available. Simply rolling the top of the finished bank is not worth much, since the effect of the roller does not reach very deep; and, besides, no roller will compact loose earth so that wheels and hoofs will not make depressions in it.* Further, it is not practicable to roll the bank during the progress of construction, except when the scraping and elevating graders are used. Finally, those who travel the road most are gen erally the ones who pay for the construction, and almost univer sally they prefer to compact the earth by traffic.

It is customary to roll the foundation of pavements, but the chief object of so doing is to discover soft places rather than to con solidate the surface; and, besides, the foundation of a pavement is protected from rain and the action of wheels, and therefore the effect of the rolling is permanent, while with an earth road it is not.

When earthwork is done by contract, the bid includes the cost of removing excavated material and depositing it in embankments, provided the necessary length of haul does not exceed a specified limit. When the material must be carried be yond this limit the extra distance is paid for at a stipulated price per cubic yard per 100 feet of haul. This extra distance is known by the name of "over-haul" or simply "haul." For an explanation of the method of computing "haul," see treatises on earthwork or books on railroad surveying.

The specified limit, i. e., the distance of free haul, depends upon the conditions. It is sometimes made as low as 100 feet, and is sometimes 2,000 feet—the latter usually only in street work. In railroad work 500 feet is a common limit.

Frequently all allowances for over-haul are disregarded. The profiles, estimates of quantities, and the required disposal of material are shown to bidding contractors; and they must then make their own allowances, and bid accordingly. This method has

the advantage of avoiding possible disputes as to the amount of the over-haul allowance, and is adopted by some railroads on this account.

Stability of Embankments.

The principles to be observed in the formation of an embankment depends somewhat upon the machinery employed in doing the work, but a few general considera tions are not out of place here.

Specifications usually require that "all matter of vegetable nature must be carefully excluded from the embankment." It is impracticable to do this when the road passes through grass land— particularly if the grade is built with a "road machine" or "road grader" (§ 142). It is desirable to remove brush, tall grass, and high weeds from the space to be occupied by the embankment and the borrow pit; but small twigs, leaves, and sod are no material detri ment, and their removal is a needless expense—except at the point where the road passes from cut to fill. It is essential that all vege table matter and loose porous soil should be removed at this point, otherwise there will be a soft place ready to soak up water which will make a mud hole and also weaken the bank just below it. When an embankment is to be made across a swamp, bog, or marsh, the site should first be drainea as thoroughly as possible. After this is done, if any considerable amount of soft oozy matter remains, it should if possible be removed, and the embankment started on the hard bottom. If the soft matter is deep, it may be necessary to lay a foundation of logs or fascines to support the earthwork.

Perfect solidity should be the leading object aimed at, and all necessary precautions should be taken to prevent or lessen the tendency of the bank to slip. To secure stability, embankments should be built in successive layers not more than three or four feet thick, and the vehicles conveying the materials should be required to pass over the bank, so as to consolidate the earth. Specifica tions sometimes state that the layers shall be made concave up wards, but this refinement is scarcely ever necessary, although it is well to see that the layers are never very much convex upward. Embankments are sometimes first built up in the center, and after wards widened by tipping or dumping earth over the side; but this should never be allowed.

When embankments are to be formed on sloping ground, it may be necessary to plow the ground or to cut steps in the rocky surface to hold the filling from sliding down the natural surface. In many cases where roads are to be constructed along steep slopes, it is found cheaper to use retaining walls (I 181)' to sustain the road upon the lower side and the earth-cutting on the upper side than to cut long slopes or form high embankments.