40 Ground improvement by deep vibratory methods
With increasing radial distance from the vibrator, the ground vibrations
are attenuated by the forces acting between the soil particles. Compaction
is therefore only possible when their frictional contact is broken by overcoming the residual frictional strength of the soil. Only then will the soil
particles rearrange themselves to find a state of lower potential energy,
that is, from loose to dense. For this purpose, a minimum dynamic force
is required with a dependent acceleration sufficiently high to break the soil
strength. Where, despite continuing transmission of attenuated weak vibrations, the resisting forces within the soil prevent further compaction, there
is a limit to how far from the vibrator effective vibrations reach.
It has been found that the stability of the structure of granular soils is
destroyed by dynamic stresses when a critical acceleration of more than
0.5 g is reached. With increasing accelerations, the shear strength of the sand
decreases until it reaches a minimum between 1.5 and 2 g. At this point,
the soil is fluidized, and a further increase of acceleration causes dilation.
Figure 3.4a shows the idealized relation between shear strength of the soil
and the induced acceleration.
3.0g
1.5g
0.5g
Pretreatment
density
Distance from vibrator
Soil density
4
3
2
1
Particle acceleration
Shear strength
Dilatory behavior
Compaction
Fluidization
Dilatory
behavior
1—Fluidized zone
2—Plastic zone
3—Compaction zone
4—Elastic zone
4
3
2
1
(a)
(b)
Figure 3.4 (a,b) Idealized response of granular soils to vibration. (After Rodger, A.A., Vibrocompaction of cohesionless soils, Internal Report, R.7/79, Cementation Research
Limited, Croydon, UK, 1979.)
With increasing radial distance from the vibrator, the ground vibrations
are attenuated by the forces acting between the soil particles. Compaction
is therefore only possible when their frictional contact is broken by overcoming the residual frictional strength of the soil. Only then will the soil
particles rearrange themselves to find a state of lower potential energy,
that is, from loose to dense. For this purpose, a minimum dynamic force
is required with a dependent acceleration sufficiently high to break the soil
strength. Where, despite continuing transmission of attenuated weak vibrations, the resisting forces within the soil prevent further compaction, there
is a limit to how far from the vibrator effective vibrations reach.
It has been found that the stability of the structure of granular soils is
destroyed by dynamic stresses when a critical acceleration of more than
0.5 g is reached. With increasing accelerations, the shear strength of the sand
decreases until it reaches a minimum between 1.5 and 2 g. At this point,
the soil is fluidized, and a further increase of acceleration causes dilation.
Figure 3.4a shows the idealized relation between shear strength of the soil
and the induced acceleration.
3.0g
1.5g
0.5g
Pretreatment
density
Distance from vibrator
Soil density
4
3
2
1
Particle acceleration
Shear strength
Dilatory behavior
Compaction
Fluidization
Dilatory
behavior
1—Fluidized zone
2—Plastic zone
3—Compaction zone
4—Elastic zone
4
3
2
1
(a)
(b)
Figure 3.4 (a,b) Idealized response of granular soils to vibration. (After Rodger, A.A., Vibrocompaction of cohesionless soils, Internal Report, R.7/79, Cementation Research
Limited, Croydon, UK, 1979.)