Page 394 - Moving the Earth_ The Workbook of Excavation

ROADWAYS

8.56 THE WORK

quantities, and compaction tested after each addition. A moisture-density curve is plotted from
these results.
Moisture content of soil is averaged from a number of spot-check tests, and subtracted from
the optimal moisture. That is, if the soil has 10 percent water by weight, and contains 4, then 6
percent must be added. An arbitrary assumption of combined absorption by the cement, and loss
from evaporation, is made, perhaps 2 percent. The result is that this soil-cement mixture needs 8
percent of its dry weight in additional water.
The weight of material going through the mixer per minute is calculated and corrected to dry
weight. Assuming that this worked out to 9,000 pounds, the water requirement would be .08 9,000,
or 720 pounds. At 8.33 pounds per gallon, this would be 86.4 gallons per minute.
This water is supplied to on-the-road mixers by tank truck. The mixer should have a small tank
to keep it working during truck changes.
When weather is dry, additional water may be added almost constantly to the surface during
processing, as a light or fog spray from a distributor truck. The amount of such addition should
just compensate for evaporation, and depends upon the judgment of the foreman or inspector.

Pavement Design. Soil-cement is used in two principal ways. In one, it is the structural part of
a pavement, but is protected against surface abrasion by a bituminous coating or pavement. In the
other, it is a base (called subbase) for a concrete pavement.
Methods of construction are almost identical. The pavement use will be considered first.
Soil-cement usually has a compacted depth of 6 to 8 inches. On firm subgrades and under light
traffic conditions, this may be reduced to 5 or even 4 inches, but then its advantage over granular
bases becomes doubtful.
Twelve or more inches may be placed under special circumstances, but 8 inches is the most
that should be put down in one layer.
To develop its full strength, the correct proportions of cement, water, and soil must be present,
and compaction must be 95 to 100 percent or (more) of maximum density.

Preparation. If the original road surface is to provide the soil for the new pavement, it must be
carefully shaped to final grade. Very little displacement of material occurs during mixing, and any
inaccuracies in original grade will cause extra work at the end and probably result in too-thin and
too-thick areas in the pavement.
If the roadbed contains oversized stone, or is so hard that the mixing machines do not reach
design depth, it should be scarified or ripped until loose enough to work, and until all stones have
been pulled to the surface and removed. Grading, or regrading, should follow this operation.
Any soft spots that develop during this work should be repaired, as proper compaction cannot be
obtained on a yielding base. It may be necessary to dig out and replace wet material. It may be pos-
sible to stabilize limited areas by mixing in high early cement. Permanent drainage should be installed
where indicated.
Clayey soils that are in dry, hard lumps usually are easier to pulverize if water is added a day or
so in advance. If the lumps are wet, they should be partly dried by scarifying or disking repeatedly
to expose them to air and sun.
If the mix soil is to be hauled in, the road must be shaped and made firm, but scarifying is not
required unless to remove projecting stone.

Cement. On a small job, cement may be placed on the road in bags in a regular pattern. (See Fig.
8.41.) A marked tape or string can be used as a guide in distributing them properly. The bags are
opened by hand and spread in windrows across the work strip. These windrows are then spread length-
wise by a small tractor pulling a spike-tooth harrow or some other drag. (See Fig. 8.42.) Several
trips back and forth should distribute the cement in a fairly even layer.
But in most of this work, the cement is brought in trucks and distributed by a mechanical spreader.
(See Fig. 8.43.) Ordinary dump trucks, fitted with a body cover to prevent blowing, can be used.
There are bulk cement trucks that discharge by means of an auger, and agricultural lime trucks
which might be able to both haul and spread.