105
Chapter 4
Improvement of fine-grained
and cohesive soils by vibro
replacement stone columns
4.1 VIBRO REPLACEMENT STONE COLUMN
TECHNIQUE
We have seen from the description of the development of the deep vibratory processes that vibro stone columns extend the limits of application of
these ground improvement methods from predominantly noncohesive into
fine-grained cohesive soils. With regard to the grain size distribution of the
soils to be improved, vibro replacement stone columns are used when conditions prevail as represented by areas outside of zone B in Figure 3.13. In
the following, it will be shown that the load-carrying capability of soils so
improved depends primarily from the interaction of the stone columns with
the surrounding soil. The columns are supported by the soil which they
replace and which requires therefore a certain minimum strength.
With increasing fines content in the grain size distribution of sands,
the spacing between compaction probes needs to be decreased to obtain
sufficient compaction when using the vibro compaction technique, as discussed in Chapter 3. At the same time, the addition of coarse backfill material helps to achieve the desired density. However, when the fines content
exceeds about 10%, the vibrations emanating from the depth vibrator can
no longer separate the soil particles from each other to achieve a closer
density, owing to their cohesive forces. Improvement of such soils, which
are generally relatively soft and impervious, can be effected by introducing stone columns. Two methods are currently being distinguished by the
strength range of the soils to be improved, generally in terms of their undrained shear strength c u .
The vibro replacement wet method is employed only in water-bearing
soft soils within a strength range of around c u = 10–30 kN/m 2 . In these
soils the depth vibrator normally sinks by its own weight, and that of the
extension tubes, to the desired depth, helped only by the low pressure,
large volume bottom jets. Collapsing soil from the side walls of the hole is
transported in the water flow to the surface where an effective sludge and
water management has to take care of it. To avoid pollution, only water
that has been largely freed from soil particles is allowed to be disposed
DOI: 10.1201/9781315372341-4
Chapter 4
Improvement of fine-grained
and cohesive soils by vibro
replacement stone columns
4.1 VIBRO REPLACEMENT STONE COLUMN
TECHNIQUE
We have seen from the description of the development of the deep vibratory processes that vibro stone columns extend the limits of application of
these ground improvement methods from predominantly noncohesive into
fine-grained cohesive soils. With regard to the grain size distribution of the
soils to be improved, vibro replacement stone columns are used when conditions prevail as represented by areas outside of zone B in Figure 3.13. In
the following, it will be shown that the load-carrying capability of soils so
improved depends primarily from the interaction of the stone columns with
the surrounding soil. The columns are supported by the soil which they
replace and which requires therefore a certain minimum strength.
With increasing fines content in the grain size distribution of sands,
the spacing between compaction probes needs to be decreased to obtain
sufficient compaction when using the vibro compaction technique, as discussed in Chapter 3. At the same time, the addition of coarse backfill material helps to achieve the desired density. However, when the fines content
exceeds about 10%, the vibrations emanating from the depth vibrator can
no longer separate the soil particles from each other to achieve a closer
density, owing to their cohesive forces. Improvement of such soils, which
are generally relatively soft and impervious, can be effected by introducing stone columns. Two methods are currently being distinguished by the
strength range of the soils to be improved, generally in terms of their undrained shear strength c u .
The vibro replacement wet method is employed only in water-bearing
soft soils within a strength range of around c u = 10–30 kN/m 2 . In these
soils the depth vibrator normally sinks by its own weight, and that of the
extension tubes, to the desired depth, helped only by the low pressure,
large volume bottom jets. Collapsing soil from the side walls of the hole is
transported in the water flow to the surface where an effective sludge and
water management has to take care of it. To avoid pollution, only water
that has been largely freed from soil particles is allowed to be disposed
DOI: 10.1201/9781315372341-4