110 Ground improvement by deep vibratory methods
natural or artificial recipients. If this shortcoming is environmentally
acceptable, the construction of stone columns with diameters in excess of
0.6 m can best be achieved by employing vibrators with larger diameters
(see Table 3.1) and high-volume, low-pressure water pumps. Very often
when soil conditions prevail where layers of sand alternate with cohesive
soils, the use of high-amplitude, low-frequency vibrators is advisable as
these machines allow both efficient compaction of the sand layer and
forming of stone columns in one operation.
With the dry vibro displacement method, vibrators with higher frequencies and smaller diameters are used which generally penetrate more effectively into the ground. The water pipes are replaced by smaller diameter
air pipes also leading to the vibrator point. Compressed air generated by
standard compressors (9 m 3 /min, 5 bar), often mounted together with the
electric generator or hydraulic power pack on the rear of the crawler crane,
produces a moderate airflow, just strong enough to avoid suction developing when the vibrator is withdrawn from the ground, and sufficient to keep
the stone backfill flowing in the stone supply system when the bottom feed
method is used. Strong airflow at high pressure should be avoided as it
often does more damage to the fabric of soft soils than helping to remove
the vibrator from the bore hole (Greenwood and Kirsch, 1984). It can also
cause unwanted heave of the ground.
Today, we mainly use purpose-built machines with vertical leader and
on-board generator and compressor. Since these machines are generally
mounted on crawlers they are often called vibrocats. A special gravel container at its front allows the stone backfill to be funneled into the bore
hole, generally in small quantities just enough to form about 1 m of stone
column length. As effective penetration aids, these machines develop a pulldown force of up to about 300 kN. When obstructions or stiff layers prevent the vibrator from reaching the design depth, a more powerful vibrator
can resolve the problem or preboring with continuous flight augers might
become necessary, preventing uneconomical slow production and unnecessary wear and tear on the depth vibrator.
The special bottom feed equipment both for crane-hung and vibrocat
operation is today the standard setup for dry stone column construction.
Figure 4.2 shows a vibrocat rig with a special depth vibrator whose extension
tubes have been transformed into gravel containers, feeding the stone backfill
through a half-moon-shaped gravel pipe to the point of the depth vibrator.
The construction details of a bottom feed vibrator are shown in
Figure 4.3 in a schematic mode. Since its cross section is substantially
increased as a result of the attached gravel pipes, penetration resistance of
the soil increases considerably, requiring much more pull-down capability
of the vibrocat—hence the need for the strong activating forces available in
modern machines. As can also be seen from Figure 4.3, the cross section
through the vibrator and gravel pipe is of oval shape, no longer circular.
The stone columns so constructed are also of an oval shape and need to be
natural or artificial recipients. If this shortcoming is environmentally
acceptable, the construction of stone columns with diameters in excess of
0.6 m can best be achieved by employing vibrators with larger diameters
(see Table 3.1) and high-volume, low-pressure water pumps. Very often
when soil conditions prevail where layers of sand alternate with cohesive
soils, the use of high-amplitude, low-frequency vibrators is advisable as
these machines allow both efficient compaction of the sand layer and
forming of stone columns in one operation.
With the dry vibro displacement method, vibrators with higher frequencies and smaller diameters are used which generally penetrate more effectively into the ground. The water pipes are replaced by smaller diameter
air pipes also leading to the vibrator point. Compressed air generated by
standard compressors (9 m 3 /min, 5 bar), often mounted together with the
electric generator or hydraulic power pack on the rear of the crawler crane,
produces a moderate airflow, just strong enough to avoid suction developing when the vibrator is withdrawn from the ground, and sufficient to keep
the stone backfill flowing in the stone supply system when the bottom feed
method is used. Strong airflow at high pressure should be avoided as it
often does more damage to the fabric of soft soils than helping to remove
the vibrator from the bore hole (Greenwood and Kirsch, 1984). It can also
cause unwanted heave of the ground.
Today, we mainly use purpose-built machines with vertical leader and
on-board generator and compressor. Since these machines are generally
mounted on crawlers they are often called vibrocats. A special gravel container at its front allows the stone backfill to be funneled into the bore
hole, generally in small quantities just enough to form about 1 m of stone
column length. As effective penetration aids, these machines develop a pulldown force of up to about 300 kN. When obstructions or stiff layers prevent the vibrator from reaching the design depth, a more powerful vibrator
can resolve the problem or preboring with continuous flight augers might
become necessary, preventing uneconomical slow production and unnecessary wear and tear on the depth vibrator.
The special bottom feed equipment both for crane-hung and vibrocat
operation is today the standard setup for dry stone column construction.
Figure 4.2 shows a vibrocat rig with a special depth vibrator whose extension
tubes have been transformed into gravel containers, feeding the stone backfill
through a half-moon-shaped gravel pipe to the point of the depth vibrator.
The construction details of a bottom feed vibrator are shown in
Figure 4.3 in a schematic mode. Since its cross section is substantially
increased as a result of the attached gravel pipes, penetration resistance of
the soil increases considerably, requiring much more pull-down capability
of the vibrocat—hence the need for the strong activating forces available in
modern machines. As can also be seen from Figure 4.3, the cross section
through the vibrator and gravel pipe is of oval shape, no longer circular.
The stone columns so constructed are also of an oval shape and need to be