Page 397 - Moving the Earth_ The Workbook of Excavation

ROADWAYS

ROADWAYS 8.59

If traffic must be maintained, a quite long section of one lane is completed, traffic turned onto
it, and the other lane worked. This method creates several problems, the most serious being the
center joint between the two strips.
When traffic can be detoured, the two strips are worked alternately, leapfrog fashion. First 300
to 500 feet is done on one side, then 600 to 1,000 on the other, so that the second lane can be
placed before the first one has set.
Premix. If mixing is done at a central plant, the blended and moistened material is brought by
trucks and dumped into aggregate spreaders. (See Fig. 8.45.) These should not be allowed to run
empty, but should be stopped during truck changes. If the subgrade is dry, it must be moistened
before the mix is placed on it.
Compaction. A variety of rollers may be used for compaction of soil-cement. The sheepsfoot or
tamping roller, either towed or self-powered, is favored for all except the most granular soils.
Weight of ballast is adjusted so that there will be initial penetration to near the bottom, followed
by walking out to the surface in a few passes. Unit pressure may be as low as 75 pounds per square
inch for friable soils, and as high as 300 for heavy clay.
If the feet will not penetrate and pressure cannot be increased, the material may be loosened
by scarifying or dry running through a mixer.
Pneumatic-tired rollers are favored for coarse, loose mixes. Models that permit varying tire
pressure during work are the most efficient. Three-wheel steel rollers in the 12-ton class may also
be used, for the whole job or just for final smooth-up rolling.
Grid rollers, segmented rollers, and various types of vibratory rollers all have their backers for
soil-cement compaction. The most important factor is that compaction be started immediately
after spreading, and continued steadily until completion. Hydration of cement starts as soon as it
is wet, and best strength will be obtained with least work if all processing is finished quickly.

Finishing. For various reasons, the compacted surface will not be entirely smooth, and will usually
need some reshaping with a motor grader. This involves trimming high spots, ridges, and other
irregularities, and either using the material to fill depressions or wasting it at the road edges. (See
Fig. 8.46.)
Differences from trimming a gravel or dirt road include effects of partial setting or drying of the
surface and the possible presence of compaction planes. These are smooth or dense surfaces, usu-

FIGURE 8.45 Placing premixed soil cement. (Courtesy of Portland Cement Association.)