Page 389 - Moving the Earth_ The Workbook of Excavation

ROADWAYS

ROADWAYS 8.51

The cut is divided into slope triangles; and a road section, which in turn is divided by line EF into a
rectangle and two triangles. The data given by the engineer are labeled given, and those measured off
the diagram as scaled. The areas of the triangles and the rectangle are readily computed, their measure-
ments in feet being used for convenience, and the result is converted to square yards by dividing by 9.
The areas of a succession of cross sections are obtained in this manner and averaged by adding
together and dividing by the number of sections added. The result is multiplied by the length in
yards of the area in which the sections were taken, giving the number of cubic yards of excavation
required. Figures from in-place measurements are in bank yards.
Where the ground slopes irregularly, the ground surface is simplified by drawing straight lines,
and the cut and fill areas are divided into triangles.
The road and gutter cuts could be figured by averaging the width and average depth at each
cross section, then multiplying the product by the length of the sectioned area. The slope sections
cannot be averaged, as their areas vary with the square of the depth of cut, and use of average
depth would indicate a much smaller yardage than is actually required.
The most convenient way to measure the areas of cut and fill is by counting squares and fractions
of squares. If a lot of work is to be done, areas can be measured by means of a planimeter.

Fill Shrinkage. When fills are rolled to the compaction required in modern roadways, the material
is often compressed into a smaller space than it occupied in the bank. This shrinkage should be
allowed for in figuring cross sections. Loam soils often shrink 10 percent, clean sand 5 percent or
less, and blasted rock, not mixed with other dirt, will show a minus shrinkage, or swell.
Compaction by hauling equipment without rolling is variable and will seldom cause shrinkage.
The examples in Fig. 8.37 use a shrinkage factor of 10 percent, but the figure selected should
depend on job conditions.

Net Cut or Fill. On side hills, one station is likely to include both cut and fill. The smaller
amount is subtracted from the larger, giving net cut or net corrected fill.
Converting to Cubic Yards. The net square yards of the cross section is converted to cubic yards
by multiplying by the length of the road it represents. If sections are taken at 100-foot intervals,
each will represent a piece 100 feet long, that is, halfway to the next section, on each side. If a
special section is taken 40 feet from a 100-foot station, it will cover 20 feet on one side and 30
feet on the other—a total of 50 feet. The adjoining sections will be reduced proportionately.
1
When the 100-foot interval is used, it represents 33 ⁄ 3 yards. It is easier to multiply the section
1
in square yards by 100, then divide by 3, than to multiply by 33 ⁄ 3 .
The net cut and net fill figures, when converted to cubic yards, are used in making a mass pro-
file. The gross cut figures are converted to cubic yards in the same manner to determine the total
excavation, exclusive of topsoil.
Topsoil volume is figured by multiplying the length of the road, the average width to be
stripped as indicated by the cross sections, and the average depth.
Cubic yards of net cut are added and compared with the total of net fill yards, to determine
whether extra fill will have to be obtained from pits or whether fill will have to be wasted outside
the road area.

Mass Profile. A mass profile is prepared by drawing on cross-section paper a straight line to
indicate the road grade, dividing it into stations, and posting cubic yards of net cut above it and
net corrected fill below it, on any convenient scale. It is sometimes helpful to draw in blocks rep-
resenting the fill at each station, as in Fig. 8.39(A).
A curved line, the mass profile, is drawn connecting the station points. The amount of net cut
or net fill at any point along the road can now be scaled off, as well as the haul distance between
cuts and fills.
The haul distance is measured between the centers of mass, or centers of gravity, of the cut and
fill. The longer and shorter hauls should average out.