42 Ground improvement by deep vibratory methods
frequencies that are close to what we might call their natural frequency. For
this reason, specialist contractors have developed vibrators capable of compacting granular soils using frequencies as low as 25–30 Hz. Occasionally,
the accompanying reduction of centrifugal force with frequency was found
to be advantageous in optimizing the compaction effect.
Theoretical study of these observations—both in surface and deep
compaction—that treat vibro compaction as a plastic–dynamic problem,
confirm some fundamental findings that have been acquired in practice
under operational conditions: for example, at constant impact force the
effective range of the vibrations increases with decreasing vibrator frequency, whereas the degree of compaction increases with an increasing
impact force. The studies generally aim to develop some kind of on-line
control of the vibro compaction by continuously evaluating information
obtained from the vibrator movements during compaction. Fellin (2000)
suggested making simultaneous measurements of horizontal acceleration
in two orthogonal directions at the vibrator tip and coupling. These would,
together with the phase angle Φ of the rotating mass, completely describe
the vibrator motion when working in the ground. This information could
indeed eventually provide the operator with valuable information to better control and direct the compaction work on-site. So far, this additional
vibrator instrumentation has only been realized in exceptional cases to support special investigations and scientific research programs.
The study of surface compaction using vibratory rollers also shows an
optimal compaction of the soil at its resonance frequency which will generally be between 13 and 27 Hz. These findings compare favorably with
the experience obtained from vibro compaction projects indicating that
sand and gravel are compacted best by using agitating frequencies of below
30 Hz which are close to their natural frequency.
Model tests carried out in saturated sand also indicate that resonance
of the vibrator–soil system could lead to optimal compaction of the surrounding soil. However, in practice, the control of resonance at any point
and time during compaction requires sophisticated on-line measurement of
vibrator data such as the phase angle Φ between the position of the rotating mass and the vibrator movement and its control during compaction by
changing the vibrator frequency f. Figure 3.5 gives the principle of the slip
angle measurement, which would form the basis of this control mechanism, with Φ = π/2 at resonance. The complexity of such an undertaking in practice becomes evident when remembering the interdependency of
vibrator performance characteristics and soil properties with time during
compaction.
Today, numerical simulations of the vibro compaction process can also
help to better understand this method. It is a well-established fact that
granular soils can be better compacted by repeated shearing rather than by
compression, and that the degree of compaction achieved depends in the
first place on the shear strain amplitude. If the induced strain is too low, the
frequencies that are close to what we might call their natural frequency. For
this reason, specialist contractors have developed vibrators capable of compacting granular soils using frequencies as low as 25–30 Hz. Occasionally,
the accompanying reduction of centrifugal force with frequency was found
to be advantageous in optimizing the compaction effect.
Theoretical study of these observations—both in surface and deep
compaction—that treat vibro compaction as a plastic–dynamic problem,
confirm some fundamental findings that have been acquired in practice
under operational conditions: for example, at constant impact force the
effective range of the vibrations increases with decreasing vibrator frequency, whereas the degree of compaction increases with an increasing
impact force. The studies generally aim to develop some kind of on-line
control of the vibro compaction by continuously evaluating information
obtained from the vibrator movements during compaction. Fellin (2000)
suggested making simultaneous measurements of horizontal acceleration
in two orthogonal directions at the vibrator tip and coupling. These would,
together with the phase angle Φ of the rotating mass, completely describe
the vibrator motion when working in the ground. This information could
indeed eventually provide the operator with valuable information to better control and direct the compaction work on-site. So far, this additional
vibrator instrumentation has only been realized in exceptional cases to support special investigations and scientific research programs.
The study of surface compaction using vibratory rollers also shows an
optimal compaction of the soil at its resonance frequency which will generally be between 13 and 27 Hz. These findings compare favorably with
the experience obtained from vibro compaction projects indicating that
sand and gravel are compacted best by using agitating frequencies of below
30 Hz which are close to their natural frequency.
Model tests carried out in saturated sand also indicate that resonance
of the vibrator–soil system could lead to optimal compaction of the surrounding soil. However, in practice, the control of resonance at any point
and time during compaction requires sophisticated on-line measurement of
vibrator data such as the phase angle Φ between the position of the rotating mass and the vibrator movement and its control during compaction by
changing the vibrator frequency f. Figure 3.5 gives the principle of the slip
angle measurement, which would form the basis of this control mechanism, with Φ = π/2 at resonance. The complexity of such an undertaking in practice becomes evident when remembering the interdependency of
vibrator performance characteristics and soil properties with time during
compaction.
Today, numerical simulations of the vibro compaction process can also
help to better understand this method. It is a well-established fact that
granular soils can be better compacted by repeated shearing rather than by
compression, and that the degree of compaction achieved depends in the
first place on the shear strain amplitude. If the induced strain is too low, the