Vibro compaction of granular soils 81
with a content of fines below 10%. The soils represented by area A are very
well-compactable, but an increasing amount of coarse gravel and cobbles
also increases permeability leading to a total loss of water at about k =
10 −2 m/s. This may obstruct penetration of the depth vibrator to such an
extent that the desired depth cannot be reached in some or all parts of the
site. In cases like this, where the process may become uneconomic or even
impossible, to execute a precontract trial is recommended.
Such trials may also be advisable for soils falling into area C, where vibro
compaction is still possible but only with considerably extended compaction time. Speed and effectiveness of the vibro compaction process depend
largely on the permeability, k, of the sand. At a permeability below 10 −3 m/s,
penetration of the vibrator will increasingly be slowed down. While for
soils lying in areas A and B the necessary backfill material for compensation of the surface settlement resulting from the compaction process itself
can be taken from the surface, imported coarser (i.e., more suitable) backfill material is necessary when the soils to be compacted fall into area C.
Soils falling completely or partially in area D cannot be compacted by the
deep vibratory process. Vibro replacement stone columns (see Chapter 4) or
other foundation measures may become necessary in such a case.
The boundaries described above have all been established empirically
over many years of application, and it must be kept in mind that specialist
contractors often rely on considerable knowledge and experience when it
comes to borderline applications. In this context, the choice of the appropriate vibrator with specific characteristics can be decisive for the execution
of vibro compaction. A vibrator with particularly good penetration characteristics may be advisable for soils in area A, while vibrators compacting
at lower frequencies (below 30 Hz) will perform better in conditions of
area C—hence the development of depth vibrators with variable frequency
that combine both characteristics within one machine.
In addition to the particle size distribution of the soil, static CPTs may
also be used to establish compaction suitability of soils. Figure 3.34 shows
an empirical relationship proposed by Massarsch (1994) between cone penetration resistance and friction ratio, and defines the zone of compactable
soils to fall within friction ratios below 1% at a point resistance of at least
3 MPa. It also indicates a zone for soils that are only marginally compactable
with friction ratios between 1% and 1.5% at q c values between 1 and 3 MPa.
A suitability number (SN) based on grain size distribution has been proposed by Brown (1977), according to Equation 3.30:
SN
D
D
D
=
+
+
1 7
3
1
1
50
2
20
2
10
2
0 5
.
.
(3.30)
D 50 , D 20 , and D 10 are grain size diameters in millimeters at 50%, 20%, and
10% passing in a grain size distribution curve. It is suggested that a low SN
is better suited for vibro compaction than a high number of 40–50, above
with a content of fines below 10%. The soils represented by area A are very
well-compactable, but an increasing amount of coarse gravel and cobbles
also increases permeability leading to a total loss of water at about k =
10 −2 m/s. This may obstruct penetration of the depth vibrator to such an
extent that the desired depth cannot be reached in some or all parts of the
site. In cases like this, where the process may become uneconomic or even
impossible, to execute a precontract trial is recommended.
Such trials may also be advisable for soils falling into area C, where vibro
compaction is still possible but only with considerably extended compaction time. Speed and effectiveness of the vibro compaction process depend
largely on the permeability, k, of the sand. At a permeability below 10 −3 m/s,
penetration of the vibrator will increasingly be slowed down. While for
soils lying in areas A and B the necessary backfill material for compensation of the surface settlement resulting from the compaction process itself
can be taken from the surface, imported coarser (i.e., more suitable) backfill material is necessary when the soils to be compacted fall into area C.
Soils falling completely or partially in area D cannot be compacted by the
deep vibratory process. Vibro replacement stone columns (see Chapter 4) or
other foundation measures may become necessary in such a case.
The boundaries described above have all been established empirically
over many years of application, and it must be kept in mind that specialist
contractors often rely on considerable knowledge and experience when it
comes to borderline applications. In this context, the choice of the appropriate vibrator with specific characteristics can be decisive for the execution
of vibro compaction. A vibrator with particularly good penetration characteristics may be advisable for soils in area A, while vibrators compacting
at lower frequencies (below 30 Hz) will perform better in conditions of
area C—hence the development of depth vibrators with variable frequency
that combine both characteristics within one machine.
In addition to the particle size distribution of the soil, static CPTs may
also be used to establish compaction suitability of soils. Figure 3.34 shows
an empirical relationship proposed by Massarsch (1994) between cone penetration resistance and friction ratio, and defines the zone of compactable
soils to fall within friction ratios below 1% at a point resistance of at least
3 MPa. It also indicates a zone for soils that are only marginally compactable
with friction ratios between 1% and 1.5% at q c values between 1 and 3 MPa.
A suitability number (SN) based on grain size distribution has been proposed by Brown (1977), according to Equation 3.30:
SN
D
D
D
=
+
+
1 7
3
1
1
50
2
20
2
10
2
0 5
.
.
(3.30)
D 50 , D 20 , and D 10 are grain size diameters in millimeters at 50%, 20%, and
10% passing in a grain size distribution curve. It is suggested that a low SN
is better suited for vibro compaction than a high number of 40–50, above