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Compaction
290 Geotechnical Engineering
seen moving as a wave ahead of tires on heavy equipment, increasing energy
consumption because the equipment always is moving up out of a hole of its own
making. A relatively dry crust can develop that if broken through causes miring
of the equipment. Overcompaction commonly occurs on construction haul roads
on wet, fine-grained soils after repeated passes of heavy construction and earth-
moving equipment.
The remedy for overcompaction obviously is not more compaction. Instead
the soil must be ripped out and aerated or kiln-dried. If the soil contains active
clay minerals, in particular smectite, an alternative procedure that may be less
expensive and saves time is to add several percent hydrated lime or lime-rich fly
ash, which raise the plastic limit and the optimum moisture content.
13.6 COMPACTION OF GRANULAR SOILS
13.6.1 Free-Draining Soils
Water content is not critical for compaction of free-draining soils such as gravel
and crushed rock. Vibratory compaction is most effective with granular soils
including sands, as vibrations create oscillations of intergranular contact stresses
and friction.
13.6.2 Bulking and Flooding
Intergranular friction caused by capillary tension causes a granular soil to ‘‘bulk
up’’ or decrease in density after it is disturbed. Thus, a bucket filled with dry sand
will weigh more than a bucket filled with wet (not saturated) sand, even though
the latter includes some additional weight from water. This effect is called bulking,
and must be taken into account during batching operations for materials used in
concrete.
Bulking also occurs when sand or gravel is spread as a base for a floor or
foundation. Since the cause is capillary action, the density can be increased
by saturating the soil with water. This has led to a somewhat haphazard practice
of ‘‘compacting’’ granular soils by flooding with water, but it should be
emphasized that the result is equilibrium with the self-weight of the soil, and not
to any additional load. Flooding induces consolidation, not compaction. A soil
that is consolidated to equilibrium with its self-weight is still compressible under
an additional load. It therefore is said to be normally consolidated. The low
densities achieved by flooding may go unnoticed with light structures as
settlement is almost immediate, hopefully occurring before the concrete has set.
Although flooding is better than no treatment at all, it is not a substitute for
compaction.
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