Page 295 - Geotechnical Engineering Soil and Foundation Principles and Practice
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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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