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Earthwork - Preparation of the Subgrade

earth, surface, embankments and finished

EARTHWORK - PREPARATION OF THE SUBGRADE. The machinery employed in making exca vations and embankments for pavements is practically the same as that used in constructing earth roads—see § 135-44.

In making embankments great care should be taken to com pact them solid—see Shrinkage of Earthwork (§ 127), Settlement of Embankments (§ 128), Rolling Embankments (§ 130), and Stability of Embankment (§ 133). For data on the Cost of Earth work, see § 154-76.

The excavation for pavements is made by plowing and then removing the earth either with a drag or a wheel scraper (§ 141), or by loading it into wagons or carts with shovels. It is usual to specify that no plowing shall be allowed within 2 inches of the subgrade, to prevent the soil below the subgrade from being loosened. If the subgrade is thoroughly rolled, as described later, plowing a little below the finished surface is not a serious matter; but if the subgrade is not subsequently well rolled, the loosening of the soil below the finished surface is very objection able, since the foundation will then have an uneven hardness.

The subgrade is often finished with pick and shovel, but the work can be done much more economically with the scraping grader (§ 142) or with the sur face grader, Fig. 114. The former

makes a more uniform surface, and is usually more economical; although the latter is an effective implement. In either case the loosened earth must be hauled away with scrapers or wagons.

A considerable part of the excavation is often done before the curb is set, but the curb is always set before the subgrade is finished. The exact position of the subgrade is determined by stretching a string transversely across the street from curb to curl) and measuring ordinates similar to those shown in the upper dia gram of Fig. 108, page 347. Some contractors pick narrow trenches down to the subgrade at short intervals transversely across the street; while others drive stakes with their tops a specified dis tance, say 4 or 6 inches, above subgrade, and provide the work men with a stick of this length with which to measure down from the top of the stake to the subgrade. The former method must be employed when the scraping grader is used. The passage of the grader fills the trench with loose earth, but it is easy to see the relative position of the surface and the bottom of the trench.