Page 288 - Geotechnical Engineering Soil and Foundation Principles and Practice

Compaction
Compaction 283

Figure 13.2
Compacted
moisture-density
relationship for an
SM soil and a
standard Proctor
compactive effort.
Modified Proctor
compaction moves
the curve upward
and to the left to a
higher density and
lower OMC.

13.2.4 Not Lubrication
It once was assumed that the optimum moisture content was optimum because it
provides just enough water to lubricate the soil grains. However, the effect of
water on non-clay minerals is to increase instead of reduce sliding friction. As
previously discussed, water weakens ionic bonding in a mineral surface so that it
scratches more readily, increasing sliding friction.

The anti-lubricating effect of water also is shown by compaction curves such as
Fig. 13.3, which is the same as in the previous figure but with additional data on
the dry side of the OMC. Compacting the dry soil resulted in a higher density
when the soil was dry than was obtained at the OMC.

The explanation for this behavior can be seen in Figs. 11.6 and 11.7 and the
accompanying discussion in Chapter 11: annular rings of water linking soil grains

Figure 13.3
Additional test data
for the soil
in Fig. 13.2
demonstrates that
friction and
capillary tension
define creates
optimum moisture
contents for
compaction.

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