Vibro compaction of granular soils 45
depth vibrator, the minimum void ratio in zone 2 extends from 0.5 to 2.2 m
after 100 cycles, this zone reaches only 0.5 to 1.0 m for vibratory hammers,
simply because the depth vibrator compacts the soil by multiaxial shearing,
while the vibratory hammer compaction turns out to be a completely radial
symmetric phenomenon resulting in much less multidirectional shearing;
this explains the known superiority of depth vibrators in the practice of
deep soil compaction.
It is hoped that the findings of newer research work to better understand
the complexity of the compaction of granular soil will eventually help to
further modernize this technology particularly by obtaining information
on the compaction process achieved directly from certain vibrator parameters measured during compaction.
3.2.2 Vibro compaction in practice
With the machine running, penetration of the vibrator and its extension
tubes under their own weight is supported by high volume water flow emanating from the lower water jets. At this stage, the jetting water needs to be
only at moderate pressure albeit relatively high volume, sufficient to transport loosened sand to the surface through the annulus between the vibrator
and the surrounding soil. This material flow allows the vibrator to sink.
It is advantageous to maintain a balanced outflow at the surface. Any temporary excess pore water pressure is quickly reduced, again as a result of the
high permeability of the granular material in which the compaction work is
being carried out. In dry sand, any existing local effective stresses or even
slight consolidations from cementation are quickly released and overcome
by water saturation and by the shear stresses emanating from the vibrator.
For very deep compactions in excess of 20 m (Figure 3.7), it may be necessary to attach additional water lines with jets at intervals along the extension tubes, or to apply additionally compressed air to maintain or to support
the upward water velocity within the annulus against the water losses by
seepage from the hole. Should no flushing medium be used in dry sand
during the penetration phase, compaction of the material surrounding the
vibrator will eventually tighten the soil to such an extent that penetration
is stopped. This phenomenon depends—besides the soil properties—mainly
on the characteristics of the vibrator. Sinking or penetration rates are generally 1–5 m/min. It is during this penetration stage that the vibrator tends to
twist to a certain extent, although fins are generally attached to the vibrator
(Figure 3.1) to prevent or at least reduce this behavior, which is the result
of the rotational movement of the eccentric mass. Twisted cables and water
hoses can be adjusted, if necessary, by removing the vibrator completely out
of the ground or, with certain vibrator types, by changing the rotational
direction of the electric motor upon reaching the final penetration depth.
When the vibrator attains the design depth, the penetration aids (water
and air) are usually switched off or reduced considerably. High pressure,
depth vibrator, the minimum void ratio in zone 2 extends from 0.5 to 2.2 m
after 100 cycles, this zone reaches only 0.5 to 1.0 m for vibratory hammers,
simply because the depth vibrator compacts the soil by multiaxial shearing,
while the vibratory hammer compaction turns out to be a completely radial
symmetric phenomenon resulting in much less multidirectional shearing;
this explains the known superiority of depth vibrators in the practice of
deep soil compaction.
It is hoped that the findings of newer research work to better understand
the complexity of the compaction of granular soil will eventually help to
further modernize this technology particularly by obtaining information
on the compaction process achieved directly from certain vibrator parameters measured during compaction.
3.2.2 Vibro compaction in practice
With the machine running, penetration of the vibrator and its extension
tubes under their own weight is supported by high volume water flow emanating from the lower water jets. At this stage, the jetting water needs to be
only at moderate pressure albeit relatively high volume, sufficient to transport loosened sand to the surface through the annulus between the vibrator
and the surrounding soil. This material flow allows the vibrator to sink.
It is advantageous to maintain a balanced outflow at the surface. Any temporary excess pore water pressure is quickly reduced, again as a result of the
high permeability of the granular material in which the compaction work is
being carried out. In dry sand, any existing local effective stresses or even
slight consolidations from cementation are quickly released and overcome
by water saturation and by the shear stresses emanating from the vibrator.
For very deep compactions in excess of 20 m (Figure 3.7), it may be necessary to attach additional water lines with jets at intervals along the extension tubes, or to apply additionally compressed air to maintain or to support
the upward water velocity within the annulus against the water losses by
seepage from the hole. Should no flushing medium be used in dry sand
during the penetration phase, compaction of the material surrounding the
vibrator will eventually tighten the soil to such an extent that penetration
is stopped. This phenomenon depends—besides the soil properties—mainly
on the characteristics of the vibrator. Sinking or penetration rates are generally 1–5 m/min. It is during this penetration stage that the vibrator tends to
twist to a certain extent, although fins are generally attached to the vibrator
(Figure 3.1) to prevent or at least reduce this behavior, which is the result
of the rotational movement of the eccentric mass. Twisted cables and water
hoses can be adjusted, if necessary, by removing the vibrator completely out
of the ground or, with certain vibrator types, by changing the rotational
direction of the electric motor upon reaching the final penetration depth.
When the vibrator attains the design depth, the penetration aids (water
and air) are usually switched off or reduced considerably. High pressure,