Page 391 - Moving the Earth_ The Workbook of Excavation

ROADWAYS

ROADWAYS 8.53

Mass Diagram. Many engineers prefer to use the mass diagram shown in (B). A straight base-
line or zero line is drawn on cross-section paper and marked off for road stations, and plus and
minus yardages in the same manner as for the mass profile.
Points are plotted for cumulative or total yardages, starting at zero station. Points are placed
one-half an interval farther to the right than the station they represent, as the full yardage figured
for each station is not accumulated until the end of that station block.
At station 1 50, the minus yards of fill for station 1 0 is entered. At station 2 50, the
total of the fill for stations 1 0 and 2 0 is posted; and at 3 50, the total fills for stations 1,
2, and 3.
When a cut is reached, at 4 0, the cut yardage is subtracted from the accumulated fill so that
the line turns up. This line, called the mass curve, crosses the baseline when the accumulated cut
equals the accumulated fill, and continued cuts raise it above that line until a fill is reached and
pulls it down.
In short, wherever accumulated fill, starting at zero station, exceeds accumulated cut, the mass
curve will be below the base. If there is an excess of cut, it will be above.
The mass curve line does not show total yardages of either cut or fill.
The points of loops which are farthest from the baseline indicate changes from cut to fill, or
fill to cut. They also represent the total net yardage to be moved from cut to fill along the road
line, but disregard sidecast material.
Any horizontal line drawn on the diagram is called a balance line. The yardages between any
two places at which it intersects the curve have a balance of cut and fill. The baseline will often
serve as a balance line, as in the illustration.
The centers of mass of a cut or fill can be found by drawing a vertical line from the outermost
point of a loop to the balance line. A horizontal line is drawn through the center point of this vertical.
Its points of intersection with the sides of the loop are approximately at the center of gravity of
the cut and the fill for that balance line.
A single balance line may be used for the whole road. Any part of the mass curve which
extends beyond the last balance point to the first or last station of the road will represent yardage
to be borrowed, if it is below the base; or to be wasted, if it is above.
A loop above the balance line indicates fill movement to the right in the diagram, and below it to
the left.
Any number of balance lines can be used as long as they end in points on the mass curve and
do not overlap. The vertical distance between two balance lines represents borrow or waste in the
part of the curve connecting them.
The mass diagram is a very flexible and useful aid in studying yardage distribution. However, it
is so confusing to persons not accustomed to this type of computation that the average contractor
working out such a problem may do better to use a mass profile.

SOIL AND CEMENT MIXTURES

Types. There are three general types of soil and cement mixtures: compacted soil-cement,
cement-modified soil, and plastic soil-cement. The first of these is by far the most important, and
will be discussed in some detail in a following section.
In all of these, portland cement is mixed with native (in-place) soil or with borrow from nearby
sources, to produce a low-cost, consolidated material. For reasons of economy, cement content is
usually held to the minimum to produce a specific result.
Cement-Modified Soil. This is the most economical and casual of these cement mixtures. The
cement used may be only 1 or 2 percent of the soil volume.
The purpose is usually to improve a soil that is unfit for a pavement base or subgrade. The
modified product may be caked or slightly hardened, but the principal object is to reduce plasticity,
water-holding and volume-change capacities, and increase its load-bearing strength.