Page 294 - Geotechnical Engineering Soil and Foundation Principles and Practice

Compaction
Compaction 289

regardless of the number of additional passes of a compactor. If processing
continues past that point, the result is called overcompaction.

A Conflict over Nomenclature
‘‘Consolidation’’ was first described by a geologist, Sir Charles Lyell, in a
textbook published in 1851. Lyell noted that sediments on the sea bottom are
compressed and consolidate under their own weight and not from the weight of
hundreds or thousands of feet of sea water. Despite this prior definition by an
eminent geologist, geologists in the petroleum industry call the process
‘‘compaction’’ and refer to shale as being ‘‘highly compacted.’’ The Oxford
English Dictionary favors the engineering usage, defining consolidation as
bringing together and compaction as packing together. It is important that
geotechnical engineers not confuse the two terms and their distinctive meanings.

13.5.2 Overcompaction

A continued application of compactive effort after soil reaches near-saturation
point not only wastes energy, but the energy is redirected into shearing and
remolding the compacted and nearly saturated soil. Shearing smears clay particles
so that they are oriented parallel to the shear surfaces and permanently weaken
the soil. This must be carefully guarded against, as the soil then must be dried and
pulverized before it can be recompacted.

The effect of overcompaction on strength of a clay is shown in Fig. 13.8, which
shows stress-strain curves for two samples of the same soil at the same moisture
content and same density, the difference being that the weaker soil was compacted
at a higher moisture content than the stronger one.

Clues to overcompaction are vibrations that can be felt under foot, and an
occasional ‘‘thunky’’ sound when stomped on. A bulge of soil sometimes may be

Figure 13.8
Stress-strain
curves show the
reduction in
strength after a soil
becomes
dispersed through
overcompaction
and shearing.
(After Seed and
Chan 1959, with
permission of the
American Society
of Civil Engineers.)

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