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Preparing the Subgrade for Broken-Stone

surface, road and earth

PREPARING THE SUBGRADE FOR BROKEN--STONE The broken stone is de signed to take the wear of hoofs and wheels, but the earth founda tion must support the load, and therefore any road which is con structed without giving due attention to the earth road-bed is wrong from the start, and will never be a good road until the defect is remedied.

For instructions concerning the construction of embankments and excavations, see § 119-22. In building an embankment upon which broken stone is to be laid, every reasonable care should be taken to prevent uneven settlement. It is sometimes advisable to delay the laying of macadam for at least a year in order to give the embankment time to settle, for it is impossible to construct an embankment of earth more than a few feet in height without hav ing subsequent settlement. If this settling took place evenly all along the embankment, no particular harm would be done to the macadam laid upon it; but owing to the difference in the soils posing embankments, and also in the way the earth is dumped, there is always a tendency for some parts to settle more than others.

Sometimes the road surface is placed so low that it forms a gutter to drain the adjacent fields, which of course is very objec tionable. Occasionally the earth from the side ditches and from the trench in which the stone is placed, is deposited at the side of the right-of-way instead of being used to raise the road surface. In this connection, see § 126.

After the subgrade has been brought to the proper form (§ 305), it should be rolled thoroughly—both to consolidate it and to discover soft spots. For a discussion of road rollers, see § 336-40.

In rolling, quicksand spots are sometimes discovered, in which case the troublesome material should be excavated and suitable material be substituted. If the road-bed be of sand or of material of such a nature as to push along in a wave in front of the roller, a thin layer of broken stone or gravel strewn over the surface will enable the roller to consolidate the road-bed. If the surface is clay that sticks to the roller, sprinkle a thin layer of sand or cinders over the surface.