Volume 6 – Geotechnical Manual, Site Investigation and Engineering ...

Chapter 8 GROUND IMPROVEMENT
The vibrator penetrates the soil by means of water jets and once at full depth, it is gradually
withdrawn leaving behind a column of well compacted soil. Figure 8.7 illustrated the schematic
processs of vibro compaction.
To achieve a mass densification, the entire area is compacted by
column
points in a triangle or square pattern. This technique is well suited for the densification of
relatively clean (fines content up to about 10 to 15%)
granular soils such as sands and gravels. A
major benefit of this method is that no additional materials are necessary which makes it a very
economical technique. The extent and effectiveness of
the techniques in improving the compaction
of the soil can be determined easily by sounding tests such as cone penetration test or electric
piezocone.
Figure 8.7 The Schematic Process of Vibro Compaction
8.2.4.22 Vibro
Replacement (Stone Column)
Vibro replacement
is a technique used to improve sandy soils with high fines contents (>15%) and
cohesive soils such as silts and clays. In this method columns made
up of stones are installed in the
soft ground using the depth vibrator. The vibrator is used to first create a hole in
the ground
which is
then filled with stones as the vibrator is withdrawn. The stones are
then laterally displacedd into the
soil by
subsequent
re-penetration of the vibrator. In
this manner a column made up
of well
compacted stone fill with diameters typically ranging between 0.7
m and 1.1 m is installed in the
ground.
Two methods of installation namely the ‘wet’ and ‘dry’ methods are
used for installation of the stone
columns. In the wet method, water jets are
used to create the hole and to assist in penetration. In
the dry
method, the hole is created by the vibratory energy and induced pulll down force. Typical
installation process in the case of dry method is schematically shown in Figure 8.8. This technique of
soil improvement can be used for nearly all types of soils.
March 2009
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