Page 302 - Geotechnical Engineering Soil and Foundation Principles and Practice

Compaction
Compaction 297

Question: Can a modified Proctor density be specified with a standard Proctor
moisture content? What about the reverse situation?

Answer: No and no. Why?

13.8 SPECIAL PROBLEMS WITH COMPACTION OF EXPANSIVE CLAYS

13.8.1 So There’s a Problem; Can We Cut a Deal?
Expansive clays do not give up easily. As discussed in the preceding chapter,
Section 12.8.9, expansion may be reduced or prevented by burying under 1 m (3 ft)
of compacted nonexpansive clay (n.e.c.) that can be manufactured by adding
and mixing several percent hydrated lime. What other methods, if any, are
available?

13.8.2 Adjusting the Compaction Moisture Content
Logically, if expansive clay is compacted wet enough it should be fully expanded,
but unfortunately it then will shrink if it is dried out. On the other hand,
compaction on the dry side of optimum will leave it permeable and subject to
expansion on wetting. The latter is illustrated in Fig. 13.12, so the moisture
content for compaction of an expansive clay often is specified to be between the
OMC and OMC þ 2 percent.

The two options, compacting on the dry side or on the wet side of the optimum
moisture content, are further illustrated by data in Fig. 13.15 for an expansive
sandy clay. From the upper graph it may be seen that expansion is over 2 percent
if the soil is compacted to maximum density at the optimum moisture content, is
reduced about one-half if the soil is compacted at 2 percent above optimum, and
becomes zero if compacted at 4 percent above optimum. However, such a soil will
be greatly weakened by dispersion and possible overconsolidation, and if it dries
out it will shrink excessively. On the other hand, if the soil is compacted at
2 percent below the OMC, shrinkage is reduced one-half compared to that which
will occur if the soil is compacted at the OMC, but shrinkage is not completely
eliminated.

Most problems with expansive clays occur when they are under low pressure
and subject to either an increase or a decrease in the moisture content, so
the compaction moisture content can be selected in anticipation of future
changes. For example, moisture tends to accumulate in soil that is sealed off from
the atmosphere, such as under pavements or floor slabs, so to reduce the
likelihood of volume change, expansive clay may be compacted on the wet side of
the OMC.

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