Vibro compaction of granular soils 35
The centrifugal force F results from the rotational speed ω of the eccentric weight with the mass M and an eccentricity of e, and acts as a lateral
impact force on the surrounding soil causing its compaction:
F M e
= ⋅ ⋅ ω
2
(3.1)
The centrifugal force F ranges between 150 kN for smaller vibrators and
more than 700 kN for the heaviest depth vibrators. When the machine is in
normal working conditions, it is restrained by the ground and oscillation
amplitudes, and surface accelerations are much less despite the constant
centrifugal force F.
Table 3.1 provides an overview of vibrator and motor characteristics of
depth vibrators in use today. It is, however, only a selection of a surprisingly large variety of depth vibrators and reflects the ongoing research
and development in this field, driven by geotechnical objectives but
very often restrained by mechanical and material limits. Hydraulically
driven vibrator motors had until recently certain operational advantages
because any desired frequency changes are relatively easily effected.
However, operational faults may cause unwanted oil spills posing a
potential hazard to groundwater and soil unless biologically degradable
hydraulic oil is used.
ω
ω
ω
D + 2a
2a
2
1
3
4
M
e
F
e = Eccentricity of bob weight
M = Mass of bob weight
a = Vibration amplitude
D = Vibrator diameter
Δm
F = M⋅e⋅ω²
f = Δm⋅a⋅ω²
ω = 2⋅π⋅n, n[Hz]
Figure 3.2 Principle of vibro compaction and vibrator accelerations in a horizontal plane.
The centrifugal force F results from the rotational speed ω of the eccentric weight with the mass M and an eccentricity of e, and acts as a lateral
impact force on the surrounding soil causing its compaction:
F M e
= ⋅ ⋅ ω
2
(3.1)
The centrifugal force F ranges between 150 kN for smaller vibrators and
more than 700 kN for the heaviest depth vibrators. When the machine is in
normal working conditions, it is restrained by the ground and oscillation
amplitudes, and surface accelerations are much less despite the constant
centrifugal force F.
Table 3.1 provides an overview of vibrator and motor characteristics of
depth vibrators in use today. It is, however, only a selection of a surprisingly large variety of depth vibrators and reflects the ongoing research
and development in this field, driven by geotechnical objectives but
very often restrained by mechanical and material limits. Hydraulically
driven vibrator motors had until recently certain operational advantages
because any desired frequency changes are relatively easily effected.
However, operational faults may cause unwanted oil spills posing a
potential hazard to groundwater and soil unless biologically degradable
hydraulic oil is used.
ω
ω
ω
D + 2a
2a
2
1
3
4
M
e
F
e = Eccentricity of bob weight
M = Mass of bob weight
a = Vibration amplitude
D = Vibrator diameter
Δm
F = M⋅e⋅ω²
f = Δm⋅a⋅ω²
ω = 2⋅π⋅n, n[Hz]
Figure 3.2 Principle of vibro compaction and vibrator accelerations in a horizontal plane.