Page 289 - Geotechnical Engineering Soil and Foundation Principles and Practice

Compaction
284 Geotechnical Engineering

have negative pressure that draws the grains together, increasing sliding friction.
Unless augmented by the presence of clay minerals, capillary tension and negative
pressure disappear at zero moisture content. Capillary tension also disappears as
the soil moisture content approaches saturation, giving the two highest compacted
densities in a granular soil.

13.3 ZERO AIR VOIDS LINE

The zero air voids line normally is included on moisture-density (or moisture-unit
weight) plots because it defines the edge of prohibited territory, which is the
saturation moisture content for a particular density. If voids are completely filled
with water there is no air left to be expelled. The saturated unit weight can be
calculated from a block diagram or from the following formula:

max ¼ G s
w = 1 þ G s m=100ð½ ފ ð13:1Þ
where
max is the zero air voids unit weight,
w is the unit weight of water, G s is
specific gravity of the soil grains, and m is the moisture content in percent.
Solutions for several values of G s and m are shown in Table 13.1 for plotting on
compaction curves.

13.4 COMPACTING IN LAYERS

13.4.1 A Critical Requirement
Compaction requires time and energy, and it may seem a huge bother to have
to spread and compact a soil in a series of layers. Occasionally an inexperi-
enced operator will decide that it is not worth the bother, with grim and
predictable consequences. The reason for this requirement is because pressure
applied over a limited area at the ground surface spreads and dissipates with
depth. This is illustrated in Fig. 13.4, where stress contours are drawn on the
basis of elastic theory and decrease as the square of the distance from the
contact area.

13.4.2 A Virtual Compactor
As a soil is compacted layer on layer, an important function of a previously
compacted lower layer is to in effect cause stresses to ‘‘bounce up’’ and intensify.
This is illustrated in Fig. 13.5. The argument is as follows: if the previously
compacted soil layer is sufficiently rigid that no deflection occurs, its influence can
be exactly duplicated by replacing the lower layer with vertical uplift pressures
in the same pattern as the downward pressures caused by the weight of the
compactor. This ‘‘virtual compactor’’ therefore doubles vertical pressure in the
lower part of the soil layer that is being compacted.

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