Page 392 - Moving the Earth_ The Workbook of Excavation

ROADWAYS

8.54 THE WORK

Soft spots encountered in underlying soil in building secondary roads are sometimes stabilized
by mixing in small quantities of cement. Modification is applicable to a wide range of soils,
including expansive clays. Mixing is usually done on the road, in the same manner as soil-cement,
but less attention is paid to compaction.

Plastic Soil-Cement. This is a mixture in which sufficient water is included to make it soft, like
plastering mortar. It does not require compaction, and therefore can be used in places inaccessible
to road-building equipment.
Plastic soil-cement is used to line or pave ditches, slopes, canal banks, and other places that are
subject to erosion.
It may also be made by mixing high early cement into the natural material in mudholes, for a
temporary emergency patch. Hand shovels and rakes are the usual tools.

COMPACTED SOIL-CEMENT

Compacted soil-cement is usually referred to just as soil-cement, as the other two kinds of soil-
cement mixtures have comparatively little use.
This soil-cement is a mix of pulverized soil and carefully calculated amounts of portland cement
and water, compacted to a high density. The resulting material is a rigid slab, with moderate com-
pressive strength, which is resistant to the disintegrating effects of wetting and drying and freezing
and thawing.
Soil. Practically all substandard soils can be improved for use as structural material by mixing with
portland cement. However, many of them require excessive quantities of cement or are difficult
to work, and therefore are seldom processed in this manner.
Stabilization is most efficient with sandy or gravelly soils with 10 to 35 percent silt and clay,
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with not over 45 percent of the pieces larger than ⁄ 4 inch.
Sandy soils with few fines, or none, require somewhat more cement, and may create traction
problems for rubber-tire processing equipment.
Silty and clayey soils make satisfactory soil-cement, but clay may have a high cement require-
ment and may be unusable if it cannot be pulverized. Both season and weather are important when
working with these soils.
Figure 8.40 gives average cement requirements, by both volume and weight, for various soil types
and miscellaneous materials.
Stones over an inch or two in diameter are considered highly undesirable. Old blacktop can be
included in the mix if it can be broken into fine enough pieces.
Organic material in the soil usually has a very unfavorable effect on soil-cement, so the use of
topsoil, or soil contaminated with topsoil, should be avoided whenever possible. However, the
effect depends partly on other soil qualities, and satisfactory results have been obtained with
organic content as high as 3 percent, without unreasonable increase in cement.
Mixture Design. Presumably because of its appeal as a low-cost road material, soil-cement is
not mixed to obtain maximum strength or durability, but to use the minimum amount of cement
that will enable it to pass two standard laboratory tests.
The wet-dry test involves samples containing varying proportions of cement that have cured
in high humidity for 7 days. They are weighed, submerged in tap water at room temperature for 5
hours, then placed in an oven at 160°F (71°C) for 42 hours. They are then given two firm strokes
on all sides with a wire brush to remove material loosened by the wetting and drying. Reweighing
and subtracting the new weight from the old indicate the amount of disintegration that occurred
during the cycle.
This process is repeated 12 times. The passing grade ranges from 14 percent loss for sandy or
gravelly soils down to 7 percent for clayey soil.