Page 298 - Geotechnical Engineering Soil and Foundation Principles and Practice

Compaction
Compaction 293

A 100 percent density is difficult to achieve unless the moisture content is on the
dry side of the OMC, so the density requirement in the field normally is set at
95 percent of the maximum density obtained in the laboratory test. A target area
may be defined by a moisture content within 2 percent of the OMC and a
compacted density to equal or exceed 95 percent of the maximum is indicated by
the specification area in Fig. 13.10. This area can be modified to meet special
requirements.

Figure 13.10 also shows ‘‘compaction growth curves’’ for different percentages
of the standard compactive effort. As the compactive effort increases, the OMC
is reduced along a line that roughly parallels the zero air voids curve.

13.7.2 Too Dry for Comfort
Proctor discovered a problem that still sometimes is overlooked or ignored—that
compacting a soil on the dry side of the OMC leaves too much air in the soil. The
soil remains permeable to water, and when saturated may become so weakened
that it densifies or collapses under its own weight—even though the compacted
density can meet a 95 percent specification requirement. Some potential
consequences are illustrated in Fig. 13.11. The problem is most common in
trench backfill where the compaction moisture content often is not monitored or
controlled. As a result a strip of pavement that is over a sewer running down the
middle may be unsupported from one manhole to the next.

A particularly serious consequence of settlement of soil backfill in a trench is if the
soil weight is carried by a pipe crossing through the trench so that the pipe breaks.

Figure 13.10
Compaction
growth curves for a
silty clay. The
shaded area is for
a specification
requirement of
95 percent
minimum density
with the moisture
content within
2 percent of the
OMC. (Data
courtesy of Prof.
J. M. Hoover.)

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