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

Chapter 8 GROUND IMPROVEMENT
Figure 8.8 Schematic Showing the Installation of Stone Columns (Dry
Method)
The Vibro Replacement technique provides an economical and flexible solution, which can readily be
adaptedd to varying ground conditions. Vibro
Replacement techniquee can improved the soil conditions
in various ways, among which are:
• Compaction of the subsoil and increase in density
• Improvement in the
stiffness of
the subsoil to decrease excessive settlement
• Improvement in the
shear strength of the subsoil to decrease the risk of failure
• Increase in the mass of the subsoil to mitigate ground vibrations
• Ability to carry very
high loads since columns are highly ductile
• Rapid consolidationn of the subsoil
Stone column improvement shall not be treated as structural solution. Dense stone columns
installed
and the surrounding soil is considered as a composite matrix. Shear strength consider after
treatment is not limited to stone column but subjected to overall strength increase. Overall
composite strength shall be considered in stability design. The common design approach adopted in
stone column is using Priebe’s
method which developed by Heinz J. Priebe 1995 from Keller. In
Priebe’s
method, improvement
factors are
calculated to be column spacing, diameter, constraint
modulus and etc. The common
diameter of stone column adoptedd in Malaysia ranges from
900mm
to 1200mm diameter. Depth of
treatment is subjected to loading, soil stratum,
need for settlement
/stability.
Testing
of the soil improvement, after installation of the stone columns in coarse-grained soils is
usually performed with either static or dynamic penetrometer tests (CPT or DPT). However for stone
columns constructed in fine-grained soils it is common practice to carry out load
tests directly on the
columns.
8.2.5
Deep Soil Mixing
(Lime Column)
Deep soil mixing (DSM) technology is a development of
the lime-cement column method, which was
introduced almost 30 years ago. It is a form of soil improvement involving the introduction and
mechanical mixing of in-situ soft and weak soils with a cementitious
compound such as lime, cement
or a combination of both in different proportions. The mixing of the cementitious compound is
facilitated with a rotary paddle as shown in Figure 8.9. The mixture
is often referred to as the
8-8
March 2009