Page 352 - Moving the Earth_ The Workbook of Excavation

ROADWAYS

8.14 THE WORK

is good practice to place a blanket of clean, coarse sand to interrupt capillary flow and add to road
stability.
The subgrade should be compacted if it is practical to do so. However, temporary gravel roads
are often put across wet spots that are not workable. Rock fill or extra depth of gravel is used to
make up for lack of subgrade preparation.
The use of soil cement is a means for making a subgrade of granular material with some fines into
a stabilized surface. The soil and cement are blended in a portable mixer and conveyed over the
surface like any paving material to make the improved subgrade. More about the uses and methods
for soil cement will be found in the coverage of Soil and Cement Mixtures at the end of this chapter.

Cross Sections. Three cross sections in common use are shown in Fig. 8.8. The feather-edge
construction in (A) calls for a flat subgrade. Its advantage is ease of construction. Disadvantages
include poor drainage of water out of the center gravel, deficient strength at the edges, and the neces-
sity of blading fill from gutters or shoulders into the road during maintenance.
The trench section (B) provides center drainage and strength to the outer edge of the gravel.
However, frequent bleeder drains through the shoulders may be needed to prevent water from
ponding in the edges, soft shoulders may be a hazard in wet weather, and maintenance work will
put dirt over the gravel.
The full-width surfacing in (C) is the best construction, and is to be recommended wherever
the price of gravel is not a controlling factor. It saves the trouble and expense of edging, provides
hard shoulders and good drainage throughout the surface, and minimizes maintenance difficulties.

Placing Gravel. On good subgrades, gravel may be very thin, but it is the best practice to use 6
to 8 inches compacted depth, and to spread it in two layers. On soft ground, the depth may be 12
inches or more. The greater part of deep gravel is usually in the bottom layer.
The best gravel available should be in the top layer. It should not contain many stones larger
than 1 inch, or at the most, 2 inches in diameter. It should be coarse enough to resist the action of
tire suction, water, and wind, and should have enough binder to hold it in dry weather but not
enough to make it sloppy when wet or thawing.
In the bottom, stones up to two-thirds of the layer thickness can be tolerated. Clean sand with-
out stone may serve, if the top layer is thick and well bound enough to hold it together.
Gravel is ordinarily trucked in and spread by a dozer or grader. Occasionally, hauling and
spreading can be done by scrapers.

FIGURE 8.8 Cross sections of gravel roads.