Vibro compaction of granular soils 41
In a stage of fluidization, the shear strength of the soil is reduced but not
eliminated completely. Therefore, vibrations, although of course dampened,
can be transmitted through this zone where particle contacts are continuously broken and remade. As the acceleration transmitted from the vibrator
decreases with increasing distance from its source, several annular density
zones surrounding the vibrator can be defined as is shown in Figure 3.4b.
In water-bearing soils, fluidization occurs principally when the rate of
the pore water pressure increase that is induced by the vibrations exceeds
the rate of dissipation, until this pressure overcomes the normal pressure
acting between the particles. It may also occur in dry soils by the action of
water jetting or when the upward-directed vertical component of acceleration exceeds gravity. As modern machines easily produce accelerations in
excess of 10 g, fluidization is induced in the vicinity of the vibrator normally
as a combination of the two effects. Soil instability directly caused by the
action of acceleration in dry soils is referred to as fluidization, whereas in
saturated soils the vibrator-induced oscillations cause liquefaction depending on pore fluid pressure.
Soil properties that influence the vibro compaction process are
• Initial density.
• Grain size.
• Grain shape and grading.
• Specific particle gravity.
• Depth (influencing intergranular and normal stress level).
• Permeability.
Essential vibrator characteristics influencing compaction are frequency,
amplitude, and acceleration of the induced oscillations and out-of-balance
force. Finally the duration of the vibration also influences the degree of
compaction achieved.
Interpretations suggest that the fluidized zone, which is characterized
by minimum shear strength, is a measure of the soil’s transmissibility of
vibrations and thus is responsible for the radius of influence of the vibratory treatment. In the transitional or plastic zone, the dynamic forces are
not sufficient to fluidize the soil but are still strong enough to shear the soil
particles from each other at such a rate that they can find a closer packing. From the point of maximal achievable density (Figure 3.4b), attenuation of vibration occurs until it reaches certain threshold shear strength in
the ground where any further compaction is inhibited. Water saturation
reduces the effective stresses and therefore increases the radius of the compaction zone. It is for this reason that in dry soils the use of flushing water
and even flooding of the whole site extends the radius of compaction.
Practical experience gained from construction sites, where depth vibrators with different compaction frequencies were working side by side, has
shown that sand can generally be most effectively compacted by vibrating
In a stage of fluidization, the shear strength of the soil is reduced but not
eliminated completely. Therefore, vibrations, although of course dampened,
can be transmitted through this zone where particle contacts are continuously broken and remade. As the acceleration transmitted from the vibrator
decreases with increasing distance from its source, several annular density
zones surrounding the vibrator can be defined as is shown in Figure 3.4b.
In water-bearing soils, fluidization occurs principally when the rate of
the pore water pressure increase that is induced by the vibrations exceeds
the rate of dissipation, until this pressure overcomes the normal pressure
acting between the particles. It may also occur in dry soils by the action of
water jetting or when the upward-directed vertical component of acceleration exceeds gravity. As modern machines easily produce accelerations in
excess of 10 g, fluidization is induced in the vicinity of the vibrator normally
as a combination of the two effects. Soil instability directly caused by the
action of acceleration in dry soils is referred to as fluidization, whereas in
saturated soils the vibrator-induced oscillations cause liquefaction depending on pore fluid pressure.
Soil properties that influence the vibro compaction process are
• Initial density.
• Grain size.
• Grain shape and grading.
• Specific particle gravity.
• Depth (influencing intergranular and normal stress level).
• Permeability.
Essential vibrator characteristics influencing compaction are frequency,
amplitude, and acceleration of the induced oscillations and out-of-balance
force. Finally the duration of the vibration also influences the degree of
compaction achieved.
Interpretations suggest that the fluidized zone, which is characterized
by minimum shear strength, is a measure of the soil’s transmissibility of
vibrations and thus is responsible for the radius of influence of the vibratory treatment. In the transitional or plastic zone, the dynamic forces are
not sufficient to fluidize the soil but are still strong enough to shear the soil
particles from each other at such a rate that they can find a closer packing. From the point of maximal achievable density (Figure 3.4b), attenuation of vibration occurs until it reaches certain threshold shear strength in
the ground where any further compaction is inhibited. Water saturation
reduces the effective stresses and therefore increases the radius of the compaction zone. It is for this reason that in dry soils the use of flushing water
and even flooding of the whole site extends the radius of compaction.
Practical experience gained from construction sites, where depth vibrators with different compaction frequencies were working side by side, has
shown that sand can generally be most effectively compacted by vibrating