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Chapter 3
Vibro compaction of granular soils
3.1 THE DEPTH VIBRATOR
The design principles of modern depth vibrators have changed little since
the time they were invented and then further developed to suit the needs of
practical use on-site. The vibrator is essentially a cylindrical steel tube with
external diameters ranging between 300 and almost 500 mm, containing
internally as its main feature an eccentric weight at the bottom, mounted
on a vertical shaft that is linked to a motor in the body of the machine
above. The length of the vibrator is typically between 3 and 4.5 m and its
weight ranges from 1500 to about 4500 kg. Figure 3.1 displays these features in a cross-sectional presentation.
When set in motion, the eccentric weight rotates around its vertical axis
and causes horizontal vibrations that are needed for the vibro compaction
method. The dynamic horizontal forces are thus applied directly to the surrounding soil through the tubular casing of the vibrator, with the machine
output remaining constant regardless of the depth of penetration; this is the
key factor distinguishing vibro compaction from other methods employing
vibratory hammers with their vertical vibrations.
A flexible, vibration-dampening device or coupling connects the vibrator with follower tubes of the same or slightly smaller diameter, providing
extension for deep penetration into the ground. These tubes contain water
and power lines for the motor, occasionally also air pipes for jets located
at the nose of the vibrator and at opposite sides generally just above the
coupling.
Motor drive is either electric or hydraulic, powered by a generator or
power pack which is generally mounted as a counterweight on the rear of
the crane carrying the depth vibrator. Common power ratings of vibrator
motors are between 50 and 180 kW, with the largest machines developing
up to 360 kW. The rotational speed of the eccentric weight, which can also
be split into two or more parts for structural reasons, is with electrically
driven machines determined by the frequency of the current and the polarity
DOI: 10.1201/9781315372341-3
Chapter 3
Vibro compaction of granular soils
3.1 THE DEPTH VIBRATOR
The design principles of modern depth vibrators have changed little since
the time they were invented and then further developed to suit the needs of
practical use on-site. The vibrator is essentially a cylindrical steel tube with
external diameters ranging between 300 and almost 500 mm, containing
internally as its main feature an eccentric weight at the bottom, mounted
on a vertical shaft that is linked to a motor in the body of the machine
above. The length of the vibrator is typically between 3 and 4.5 m and its
weight ranges from 1500 to about 4500 kg. Figure 3.1 displays these features in a cross-sectional presentation.
When set in motion, the eccentric weight rotates around its vertical axis
and causes horizontal vibrations that are needed for the vibro compaction
method. The dynamic horizontal forces are thus applied directly to the surrounding soil through the tubular casing of the vibrator, with the machine
output remaining constant regardless of the depth of penetration; this is the
key factor distinguishing vibro compaction from other methods employing
vibratory hammers with their vertical vibrations.
A flexible, vibration-dampening device or coupling connects the vibrator with follower tubes of the same or slightly smaller diameter, providing
extension for deep penetration into the ground. These tubes contain water
and power lines for the motor, occasionally also air pipes for jets located
at the nose of the vibrator and at opposite sides generally just above the
coupling.
Motor drive is either electric or hydraulic, powered by a generator or
power pack which is generally mounted as a counterweight on the rear of
the crane carrying the depth vibrator. Common power ratings of vibrator
motors are between 50 and 180 kW, with the largest machines developing
up to 360 kW. The rotational speed of the eccentric weight, which can also
be split into two or more parts for structural reasons, is with electrically
driven machines determined by the frequency of the current and the polarity
DOI: 10.1201/9781315372341-3