106 Ground improvement by deep vibratory methods
of into available recipients. The water can contain significant amounts of
suspended silt and clay which require proper handling in settling ponds and
standing pools before the water can be reused for the stone column production or before it is released. These procedures need to be well organized
to avoid disruption of the work or slowing down of production. In this
context it is, however, necessary to remember that sufficient water volume
is an integral part of the wet vibro replacement stone column method to
guarantee stability of the hole and its diameter.
On reaching the desired depth, the vibrator is often completely withdrawn from the bore before it is allowed to repenetrate rapidly— sometimes
a few times—to full depth, in this way allowing the bore to be cleaned from
loosened soil material and its diameter to increase. In most cases the bore
created is at least temporarily stable, and coarse backfill can now be filled,
preferably in small doses, into the bore hole. The vibrator is then lowered
again to full depth and the vibrations, together with a slight upward and
downward motion with amplitude of generally not more than a meter,
cause the backfill to be compacted and rammed into the sides of the bore.
With increasing density of the backfill, vibrations are also transmitted to
the surrounding soil. The resulting shear stresses may cause the soil to
collapse leading to a further increased bore diameter as the continuing
water flow, movement of the vibrator and of the backfill, transports this
fine material to the surface. When equilibrium is reached, column building begins as further stone is added as described or through the annulus
around the vibrator remaining in the bore hole. Rising resistance indicated by slowing down the sinking rate of the machine, accompanied by
increased power consumption of the vibrator motor, is a sign that column
building is completed at this level and that repetition of this procedure
should start at the next higher level.
In this manner, an stone column is formed up to ground surface in a
self-compensating way with diameters of about 0.8–1.2 m, depending on
the soil resistance and the shearing and flushing action over the time of the
build-up (Figure 4.1a).
It is evident that this replacement process causes only relatively little disturbance to the surrounding native soil, without apparent smear damage
(Greenwood, 1976), allowing, under loading conditions, the pore water to
freely drain into the columns, resulting in a considerably accelerated consolidation process (see Section 4.3.4).
In more stable insensitive cohesive soils with strength values c u = 30–50
kN/m 2 , the dry vibro displacement method is applied whereby the depth
vibrator penetrates by vibratory impact and by its own weight, sometimes
increased by that of the heavy extension tubes, and always helped by compressed air emanating through the bottom jets of the machine. When the
design depth is reached, it is always necessary to extract the vibrator completely from the ground to allow coarse backfill material to be introduced in
small quantities into the bore. The compressed air is used primarily to prevent
of into available recipients. The water can contain significant amounts of
suspended silt and clay which require proper handling in settling ponds and
standing pools before the water can be reused for the stone column production or before it is released. These procedures need to be well organized
to avoid disruption of the work or slowing down of production. In this
context it is, however, necessary to remember that sufficient water volume
is an integral part of the wet vibro replacement stone column method to
guarantee stability of the hole and its diameter.
On reaching the desired depth, the vibrator is often completely withdrawn from the bore before it is allowed to repenetrate rapidly— sometimes
a few times—to full depth, in this way allowing the bore to be cleaned from
loosened soil material and its diameter to increase. In most cases the bore
created is at least temporarily stable, and coarse backfill can now be filled,
preferably in small doses, into the bore hole. The vibrator is then lowered
again to full depth and the vibrations, together with a slight upward and
downward motion with amplitude of generally not more than a meter,
cause the backfill to be compacted and rammed into the sides of the bore.
With increasing density of the backfill, vibrations are also transmitted to
the surrounding soil. The resulting shear stresses may cause the soil to
collapse leading to a further increased bore diameter as the continuing
water flow, movement of the vibrator and of the backfill, transports this
fine material to the surface. When equilibrium is reached, column building begins as further stone is added as described or through the annulus
around the vibrator remaining in the bore hole. Rising resistance indicated by slowing down the sinking rate of the machine, accompanied by
increased power consumption of the vibrator motor, is a sign that column
building is completed at this level and that repetition of this procedure
should start at the next higher level.
In this manner, an stone column is formed up to ground surface in a
self-compensating way with diameters of about 0.8–1.2 m, depending on
the soil resistance and the shearing and flushing action over the time of the
build-up (Figure 4.1a).
It is evident that this replacement process causes only relatively little disturbance to the surrounding native soil, without apparent smear damage
(Greenwood, 1976), allowing, under loading conditions, the pore water to
freely drain into the columns, resulting in a considerably accelerated consolidation process (see Section 4.3.4).
In more stable insensitive cohesive soils with strength values c u = 30–50
kN/m 2 , the dry vibro displacement method is applied whereby the depth
vibrator penetrates by vibratory impact and by its own weight, sometimes
increased by that of the heavy extension tubes, and always helped by compressed air emanating through the bottom jets of the machine. When the
design depth is reached, it is always necessary to extract the vibrator completely from the ground to allow coarse backfill material to be introduced in
small quantities into the bore. The compressed air is used primarily to prevent