UNDERSTANDING THE PERFORMANCE OF DEEP MIXED COLUMN IMPROVED SOILS - A REVIEW

International Journal of Civil Engineering and Technology (IJCIET), ISSN 0976 – 6308 (Print),
ISSN 0976 – 6316(Online), Volume 6, Issue 3, March (2015), pp. 97-117 © IAEME
97
UNDERSTANDING THE PERFORMANCE OF DEEP
MIXED COLUMN IMPROVED SOILS - A REVIEW
Samuel Jonah Abbey, Samson Ngambi, Barisua Ebenezer Ngekpe
Faculty of Engineering and Computing, Department of Civil Engineering
Coventry University, United Kingdom
ABSTRACT
The essence of ground improvement is to enhance the engineering properties of weak soils to
provide stability and sufficient bearing capacity for construction and other engineering purposes.
Deep soil mixing is one method that has been widely employed for this purpose due to the
environmental nature and efficiency of this method. In deep soil mixing, the amount of binder to be
mixed with the weak soil depends on the initial moisture content of the surrounding soil. Several
researchers have reported on this method of soil improvement through laboratory experiments, in
situ testing and numerical modelling. Most literatures reviewed in this study have shown that
previous studies by different researchers on weak soil improvement using deep mixing techniques
have focused on the mechanical properties such as compressive strength, stiffness, modulus of
elasticity and consolidation behaviour of the improved ground. For example, Ali et al., (2012), Lin
and Wong, 1999, Fang et al., (2001), Porbaha et al., (2001), Yin, (2001), Porbaha, (2002), Farouk
and Shahien (2013), Tao, Jim and Jing, 2014) etc. Others focused on the effect of soil type, binder
type, binder content, water-cement ratio and area replacement ratio on the mechanical properties of
the improved ground, for example; Nur et al. (2013), Terashi et al., (1977), Kitazume and Terashi,
(2012), Farouk and Shahien, (2013) etc. This paper therefore serves as a quick guide on
understanding the performance of DMC improved ground based on what has been reported in
literatures.
1.0 INTRODUCTION
Location of new infrastructures on stable ground is becoming more difficult worldwide due
to over population in urban areas. Most areas have been built up and countries all over the world are
now being faced with the problem of non-availability of stable ground for construction purposes and
other engineering activities. As a result, most construction and building projects are being channelled
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ISSN 0976 – 6308 (Print)
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towards areas considered to be problematic due to the extent of underlying deposits of low strength.
Due to this reason, ground improvement has become imperative for infrastructure development
projects in the developed and developing countries.
For example in Japan, most constructions and building projects are located on soft alluvial
clay soils, artificially reclaimed lands consisting of dredged soft clay and highly organic soils,
(Kitazume and Terashi 2009). Ground conditions of this nature may pose serious problems of
excessive ground settlement due to low shear strength and high compressibility and this may impair
the performance and functions of the supported structure.
Figure 1.0: Installation process of a deep mixed soil column, (Massarsch and Topolnicki, 2005)
In DSM, the unstable soil is blended with cementitious and other additives to form a soil-
binder column to improve strength and reduce the compressibility of the weak soil. This method
mainly depends on increasing the stiffness of the native soil by adding a strengthening admixture
material such as cement, lime, gypsum and fly ash. Foundations constructed using DSM techniques
have been applied to support structures, embankments and excavations in countries like Japan,
Scandinavia and the United States, (Kitazume and Terashi 2013).The properties of the improved soil
column may reflect the characteristics of the native soil, mixing method, and the binder
characteristics (Bruce, 2001).
Deep soil mixing methods are categorized into "wet" deep mixing method (WDMM) and
"dry" deep mixing method (DDMM) depending on the moisture content of the native soil. In
WDMM, cementitious slurry is injected through large diameter to a specified improvement depth to
produce a soil-binder column. While in the DDMM, the common method is to rotate a mixing tool
into the native soil to break up the soil on the down stroke, and dry reagent such as cement or quick
lime or a combination of both, is pneumatically injected and blended with the soil by means of the
mixing tool on the up stroke to produce a soil-binder column.
The dry DMM is generally considered less expensive than the wet DMM and also, the
strength of the soil-binder column produced using dry DMM is considered to be less than the
strength achieved for the same soil type using wet DMM. Some typical applications of the DSM
includes; construction of embankment on soft soils, support of strip/pad and slab foundations,
excavation protection walls, slope stabilization, mitigation of liquefaction potential, cut-off walls and
barriers. Deep soil mixing method has also been employed in remediation of contaminated soils.

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Figure 2.0: Single and extruded overlapping deep mixed soil column (Massarsch and Topolnicki,
2005).
1.1 Application of Deep Soil Mixing Method
From literature, it can be said that deep soil mixing method of ground improvement is
environmentally friendly and its application in sealing and strengthening weak soils is growing
across the world. Significant improvement in the physical and mechanical properties of soft soils has
been achieved through the mixing of these soils with cement, lime, fly ash and other hydraulic
binders to produce a soil-binder column. The resulting soil material possesses higher strength, lower
compressibility and lower hydraulic conductivity, (Terashi, 2009). Typical applications of the deep
mixing method in soil improvement include the following;
• Embankment stability
• Embankment settlement reduction
• Foundations of structures
• Braced excavation
• Bridge abutment
• Mitigation of liquefaction potential
• Cut slope stability
• Impact to nearby structures

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Figure 3.0: Typical areas of application for deep mixing method (Terashi, 2009)
2.0 DEEP SOIL MIXING COLUMN IMPROVED GROUND
In the application of deep soil mixing method of improvement of problematic soils, the soil
columns could be installed in blocks, single columns, panels or stabilised grids depending on the
purpose of improvement as shown in Figure 4.0, (Kitazume and Terashi, 2012). The installation
pattern may differ from one project to the other. The diameter of these columns ranges from 0.5m to
0.75m at spacing of about 1m to 1.5m centre to centre. A 1.0m diameter column having cement
content of 200 to 300 kg/m3
and unconfined compressive strength of about 2 to 4MPa has been used
to support a five storey building in Japan, (Ali, Kamarudin and Nazri, 2012). The bearing capacity
and other improved parameters of the soil column may be affected by changes in diameter of the
column, cement content, replacement ratio or the testing methods.
Figure 4.0: Different types of DMC installation pattern, (Kitazume and Terashi, 2012).

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2.1 Strength of DMC Improved Ground
The unconfined compressive strength of deep mixed column improved ground may depend
on various factors such as the type of soil, binder content, curing method, mixing time and other
factors. Figure 5.0, (Kitazume and Terashi, 2012) shows the range of binder dosage and the
corresponding compressive strengths for different soil types.
Figure 5.0 Compressive strengths of improved soils for different range of binder dosage, (Kitazume
and Terashi, 2012).
The compressive strength of the treated soil may also be influenced by temperature and other
environmental changes. Temperature may influence the chemical reaction between the soil and the
binder. During soil binder reaction, the heat evolution of different binders may vary. In cement and
lime improvement methods, the required reaction temperature comes from these binders and does
not fully depend on the soft soil temperature. For curing period, EuroSoilStab, (2002) stated that the
strength development in cement improved soil is during the first month and for lime improved soils;
the strength development goes on for several months depending on the rate of pozzolanic reactions
between the weak soil and lime.
3.0 FACTORS INFLUENCING STRENGTH OF DMC IMPROVED GROUND
3.1 Effect of soil type
Soil type influences the unconfined compressive strength of a lime improved soil as shown in
Figure 6.0. The Figure shows the influence of soil type on the unconfined compressive strength of a
hydrated lime improved soils collected from different locations. The improved soils were mixed at
different moisture content and cured for a period of 91 days curing (Kitazume and Terashi, 2012).
Figure 6.0, (Kitazume and Terashi, 2012) show that the type of soil clearly influences the unconfined
compressive strength increase, irrespective of nature of the soil (marine or on-land soils). Terashi et
al., (1977) reported that the grain size distribution of soil also influences the unconfined compressive
strength of quicklime improved soil.

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Figure 6.0 Influence of soil type on strength at 91 days curing period, (Kitazume and Terashi, 2012).
Farouk and Shahien (2013) carried out an experimental investigation on ground improvement
using soil-cement columns. The first part of their study, considered two different natural silty soils
extracted from the Delta of the River Nile. They investigated the effect of soil type, cement dosage
and water-cement ratio on the strength of the cemented Nile delta soil. To investigate the effect of
soil type, two soil types “Shobra-alnamla” (Sh soil) and “Talbantqaisar” (Tal soil) were used. These
samples were mixed with the same cement dosage rate of 240kg/m3
at different w-c ratios. They
stated that cement dosage rate within the range of 120-240kg/m3
could be used effectively in
stabilizing silty soils. Winterkorn and Fang (1975) reported that cement dosage rate should range
between 5% - 16% of the weight of the untreated soil but, Farouk and Shahien (2013) adopted
cement dosage rate of 16.8% of the weight of the untreated soil. The second part of their study was
modelling of the soil-cement column system, where the untreated soil was modelled in a rigid box
made of steel and a steel loading frame with load gauges attached as shown in Figure 7.0, (Farouk
and Shahien 2013).
Figure 7.0: The Schematic Diagram of the Loading System (Farouk and Shahien 2013).

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The soil columns were installed and aligned vertically in positions by means of wooden
forms. A mattress of 10mm of the untreated soil was built at the top of the soil column (improved
ground) and overlaid a 480݉݉ × 100݉݉ × 20݉݉ rigid steel plate to simulate the behaviour of a
strip footing on the improved soil. The steel plate was loaded by means of a hydraulic jack until
reaching soil column failure.
The result of their analysis revealed that as the water-cement ratio increases, the compressive
strength of the soil column decreases for both soils. Figure 8.0: shows the variation of compressive
strength with water-cement ratio.
Figure 8.0: Variation of compressive strength of stabilized soil with water cement ratio (Farouk and
Shahien 2013).
They claimed that, for both soil types, the effect of w-c ratio on the modulus of elasticity of
the soil column is negligible within the range of w-c ratio investigated.
Their analysis on the effect of soil type on compressive strength shows that at equal w-c ratio,
“Sh” soil exhibited a higher compressive strength compared to “Tal” soil. Their study also revealed
that at higher w-c ratio, “Sh” soil behaves like a ductile material and a brittle material at lower w-c
ratio. Figure 8.0, shows that the “Tal” soil exhibits almost ductile behaviour at all used values of w-c
ratio, which they attributed to the higher fines content present in the “Tal” soil.

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Figure 9.0: Stress strain relationship of cemented “Tal” soil at different water cement ratios (Farouk
and Shahien 2013).
Farouk and Shahien (2013) concluded that compressive strength and stiffness of the
improved soil decreases with an increase in fine content of the unimproved soil. The clay content of
“Sh” soil was found to be greater than that of “Tal” soil by 5%, which implies that fines content has
more effect on the elastic behaviour of the improved soils than the clay content.
The humic acid of the unimproved soil can affect the unconfined compressive strength and
this has been investigated by Okada et al., (1983). An artificial soil was prepared by mixing varying
amounts of humic acid to Kaolin clay of liquid limit and initial water content of 50.6 % and 60.6%
respectively, and improved with 5% hydrated lime. It was observed that an increase in quantity of
humic acid resulted to a decreases in strength, irrespective of the type of humic acid. The amount of
humic acid appears to have critical influence on the strength of hydrated lime stabilized soil; this
might be due to the presence of humic acid in some clay and sludge at marine and on-land areas. The
unconfined compressive strength of cement improved soil depends highly on the type of cement, but
decreases considerably with increasing humic acid content irrespective of the binder type. The
strength reduces to about 1/3 when the content humic acid gets to about 5%. Potential hydrogen (pH)
of the original soil also influences the unconfined compressive strength of a lime improved soil, such
that a decrease in pH results to a corresponding decrease in strength.
3.2 INFLUENCE OF WATER CONTENT
According to Terashi et al., (1977), the water content of the surrounding soil affects the
unconfined compressive strength of improved soil. When curing period is increased, the water
content in the original soils provides maximum strength shifts toward the dry side but, the strength of
the improved soil decreases considerably at increasing initial water content greater than the liquid
limit. (Kitazume and Terashi, 2012) stated that in marine construction in Japan, this might not cause
serious problem because in most cases, the natural water content of normally consolidated Japanese
marine clay almost equals its liquid limit but however, care should be taken on clay soils with water
content higher than its liquid limit such as on-land reclamation areas with pump dredged clay. In
deep mixing method, the amount of binder is a percentage of the mass of dry soil in a given mass of
the weak soil depending on the adopted mix ratio. This mass of dry soil is directly proportional to the
mass of weak soil and inversely proportional to the initial moisture content. After installation of

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these columns, any changes in the physical properties of the surrounding soil such as change in
degree of saturation, due to seasonal fluctuation of soil moisture, may affect the performance of the
deep mixed column. This issue is what this research tends to also investigate.
3.3 Influence of amount of binder
The unconfined compressive strength of a weak soil increases with an increase in cement
content, Nur et al. (2013). Terashi et al., (1977) studied the influence of amount of binder on the
unconfined compressive strength of lime improved soil as shown in Figure 10.0, Terashi et al.,
(1977). Two types of marine soils (reclaimed soil and marine clay) with moisture contents 102.5%
and 120% and liquid and plastic limits of 92.4% and 46.9%, and 78.8% and of 49.1%) respectively,
were improved and cured for several days. For the reclaimed soil, the unconfined compressive
strength increases almost linearly with amount of quicklime, irrespective of the duration of curing.
While, for the marine clay, clear peak strength was reached and the amount of binder at this strength
becomes even larger with longer curing time as shown in Figure 10.0, Terashi et al., (1977).
Figure 10.0: Influence of amount of binder on strength in quicklime soil improvement, (Terashi et
al., 1997).
To investigate the effect of cement dose on the compressive strength of DMS column,
(Farouk and Shahien, 2013) conducted a test on ‘Sh’ soil mixed with different cement dosage rate of
160, 200, 240, 340 and 440Kg/m3
at a constant w-c ratio of 1.25. The effect of w-c ratio was
investigated by preparing different samples of the ‘Sh’ soil at different w-c ratios of 0.80, 1.00, 1.25
and 1.5 at a constant cement dosage rate of 240kg/m3
. Their result on the effect of cement dosage
rate on compressive strength of the soil column revealed that, at higher cement dosage rate, the
improved soil exhibits a brittle behaviour. While at lower dosage rate, the behaviour tends to be
ductile. They also revealed that the compressive strength and cement dosage of the improved soil are
exponentially correlated.

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3.4 Influence of Mixing Time
During mixing, it is expected that the mixing time should be an index, describing how the
soil and binder have been sufficiently mixed. Though, other factors such as type of mixer and
characteristics of the original soil to be improved in the laboratory may also influence the degree of
mixing. The unconfined compressive strength of both cement and lime-cement improved soil may be
influenced by this index. In a quicklime stabilization test conducted on clay with varying initial
water contents and liquid and plastic limits of 87.8% and of 49.7% respectively, mixing was done at
arbitrary mixing time and at 10 minutes mixing time, (Terashi et al., 1977).
The ratio of strength of stabilized soil prepared at arbitrary mixing period to those prepared at 10
minutes mixing time was recorded as the strength ratio. They observed that the strength ratio
decreases considerably when the mixing period is less than 10 minutes at lower binder factor, and
increases at mixing time greater than 10 minutes as shown in Figure 11.0, Terashi et al. (1977) . It
was proposed that a laboratory mixing time of about 10 minutes and the use of appropriate soil mixer
should be adopted.
Figure 11.0: Influence of mixing period on unconfined compressive strength, (Terashi et al., 1997)
3.5 Influence of Curing Period
Curing period is another factor that influences the unconfined compressive strength of lime
improved soft clay soils as shown in Figure 12.0, (Terashi et al., 1977). This figure shows the
influence of curing time (in days) on the unconfined compressive strength of different types of clay
improved by quicklime at a constant binder factor of 10% (Terashi et al., 1977). In Figure 12.0, the
curing time is plotted in a logarithmic scale along the abscissa while the unconfined compressive
strength is plotted on the ordinate. The figure shows that the increase in strength of the improved soil
is much dependent upon the type of soil even at constant binder dosage, but the strengths of the
improved soils increase almost linearly with logarithm of the curing time.

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Figure 12.0: Influence of curing period on unconfined compressive strength of quicklime improved
soil, (Terashi et al., 1997).
4.0 BEHAVIOUR OF DEEP MIXED COLUMN IMPROVED GROUND
Sukum et al., (2012) investigated the consolidation behaviour of a soil-cement column
improved ground using experimental and numerical (PLAXIS 2D) methods. In their laboratory
investigation for the preparation of the model ground, soft Bangkok clay was mixed with water up to
a water content of about 162%. The model ground was placed in a cylindrical stainless steel mould
with diameter and height of 300mm and 450mm respectively and the soil-cement column was
installed at the centre of the mould with a 30mm thick sand place at the base of the mould. A vertical
consolidation stress of about 20kPa was applied through an acrylic plate with 102mm diameter hole
at the middle and 8mm thick, placed on the model composite ground with no horizontal
displacement permitted.
The result of their laboratory investigation revealed that the moisture content of the model
ground reduces to about 60% and the final settlement of the composite ground is dependent on the
stress state. It has also been reported that at larger area replacement ratio (ratio of area of column to
area of surrounding soil) that settlement reduces with an accelerated rate of consolidation. Alwi,
(2007) reported that the extent of settlement reduction depends not only on area replacement ratio
but also on stress concentration ratio (ratio of soil-cement stiffness to that of the surrounding soil).
Ali et al., (2012) carried out a numerical analysis using PLAXIS 3D on settlement of mat foundation
on group of cement columns in peat. The analysis of their result revealed that the vertical settlement
of peat-cement column decreases as the length of the column increases. Sukum et al., (2012) also
observed that the stress on the column reduced due to failure of the soil-cement column and a
significant increase in stress on the model ground, leading to an increase in excess pore pressure and
the development of cracks on the soil-cement column at failure accelerated the dissipation of the
excess pore pressure. According to Soyez et al., (1983) lime columns reduces maximum

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embankment settlement up to 50% compared to unimproved soil and an increase in column spacing
leads to small reduction in settlement. Holm et al., (1983) carried out a full scale test on lime
columns improved soft clay under embankments using 0.5m diameter columns with 1.4m spacing.
The improved soft clay has undrained shear strength of 6 to 9kPa and a modulus of compression of
about 60 to 175kPa. Their investigation revealed that at an applied load of 50kPa, there was a
settlement attenuation of about 50% after 2.6 years.
Carlsten (1996) stated that the diameters of lime and lime cement columns of stabilized clay
ranges between 0.5–0.6 m. During lime stabilization, finely milled, burnt lime is mixed with the
unimproved soft clay by means of a lime column machine. While in lime-cement columns
production, standard Portland cement is added to the lime usually in a proportion of lime/cement of
50/50 percent per weight. The compression modulus and shear strength of lime and lime/cement
columns are considerably higher than that of the unimproved clay. He also stated that these columns
increase the bearing capacity of the surrounding soft clay depending on the spacing between them.
Yuewen (1996) reported that a deep mixed cement column of 0.5m diameter, 20% cement
content and 22% area ratio have been used to improve the bearing capacity of a surrounding soft soil
up to 520 to 650kPa and these columns were successfully used to support a 12 to 15 storey building
in China. Rogers and Glendinning (1997) attributed the increased bearing capacity of the improved
soft clay soils to the strength of the lime columns due to confining pressure provided by unimproved
surrounding soil. According to them, such lateral support increases considerably with depth and the
degree of vertical confinement of the clay close to the surface determines the method of designing
these lime columns.
In a static laboratory and static field tests on the deformation properties of lime-cement
column conducted by Massarsch (2005), it was revealed that the modulus of elasticity values
obtained from laboratory studies were about 2 to 3 times greater than those obtained from in-situ
tests. This shows that the actual long term performance of a lime-cement column still has to be
properly investigated because the conditions of the surrounding soil may change over time after
installation. According to Massarsch (2005) the modulus of elasticity at 50% of the failure load
depends on the unconfined compressive strength of the soil column and this can be expressed
mathematically as:
E50= 160 qu, col
Kivelo (1994) conducted a static field load test on a lime-cement column of 0.5m diameter
installed in a soft, plastic clay soil of undrained shear strength of about 18-20kPa and water content
of 43%. The mix ratio of lime-cement was 50%. The deformation in the column at varying depth as
load increases was measured and the reduction in modulus of elasticity of the column as a function
of the imposed load was also measured. It was observed that the reduction in elastic modulus of the
column was depended on strain such that an increase in axial strain resulted to a decrease in elastic
modulus. The axial strain that will develop in a lime-cement column over time will depend on the
interaction of the column with the surrounding soil. Therefore any changes in the properties of the
surrounding soil such as change in degree of saturation may affect the long term performance of the
column.
Yan, Jie and Gang, (2013) adopted a mechanical and hydraulic coupled 3D finite element
analysis approach to study the consolidation behaviour of soft soils fully-penetrated by deep mixed
columns. The column and the soft soil were modelled as elastic materials using one quarter of a unit
cell due to symmetry. They observed that the consolidation rate increases due to an increase in
column modulus, area replacement ratio and permeability of the column. They also stated that an

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increase in time and column modulus increases the stress concentration ratio and due to interfacial
lateral deformation between the column and the surrounding soil, the deformation of the column was
not one-dimensional.
It was also stated that settlement of the DMC foundation reduces with larger column modulus
and area replacement ratio but increase as soft soil thickness increases. In this study, the interfacial
lateral deformation between the column and the surrounding soil was not deeply investigated as the
possible changes in the nature of the surrounding soil were not considered.
Tao, Jim and Jing (2014) employed analytical approach to study the consolidation of a
composite ground improved with partially penetrated impervious columns. The accuracy of their
proposed approach was examined using finite element method and it was observed that the
consolidation rate of the composite ground depended highly on the ratio of column length to soil
thickness (penetration ratio), such that as this ratio increases, the consolidation rate increases.
Yin and Fang (2006) in their work on the consolidation behaviour of composite foundation
consisting of a cement-soil column and unimproved soft marine clay, observed that the excess pore
pressure dissipation rate in the soft-cement column was faster than that in improved soft clay due to
higher permeability of the soil-cement column and so the soil-cement column acted as a vertical
drain. In other words, the soil-cement accelerated the rate of consolidation due to its higher stiffness,
resulting to a greater coefficient of consolidation rather than a higher value of coefficient of
permeability. Therefore, deep mixed soil-cement column can speed up consolidation process of the
composite even at equal values of permeability coefficient between the soil-cement column and the
surrounding ground.
Stefan et al. (2012) studied the failure behaviour of soft clay soil improved with lime-cement
columns under lateral loading condition using single and overlapping column patterns. There
experimental work was carried out in a shear box as shown in Figure 2.16, (Stefan et al. (2012),
where the columns were installed in laboratory prepared clay. In their numerical studies, the soft clay
was modelled using the Drucker-Prager model, while the DMC(s) were simulated using the damage
plasticity model.
According to Stefan et al. (2012), the damage plasticity model was adopted to simulate the
failure mechanism of the improved soil due to irreversible plastic and damage deformation during
loading. They manufactured the laboratory clay by mixing kaolin soil with water up to 90% water
content and the clay slurry was poured into a 0.9m high circular steel box with a diameter of 0.5m as
shown in Figure 13.0. They tested the unimproved clay, 12 single DMC(s) and two rows of six
DMC(s) each and each column having 500mm length and the overlaps between columns in the two
rows were varied in three steps. A movable steel plate with a sealing plastic film was then placed on
top of the slurry and the steel box sealed at the top with a massive steel plate. The slurry was
consolidated with a 50 mm thick drainage sand layer placed at the bottom of the box to achieve a
95% degree of consolidation. Stefan et al. (2012) adopted the dry deep mixing technique using a
binder that consisted of 30% quicklime and 70% Portland limestone cement respectively.

International Journal of Civil Engineering and Technology (IJCIET), ISSN 0976
ISSN 0976 – 6316(Online), Volume 6, Issue 3, March (2015), pp.
Figure 13.0: Schematic of the Shear Box Test, (
The top half of the box was free to move laterally. Steel weights were used to apply lateral
traction force which was increased in stages.
columns failed due to bending, and there was progressive cracks initiated by the columns furthest
away from the centre, in the direction of the imposed load. Their study also revealed a significant
increase in shear resistance for the columns installed in rows due to higher bending resistance. They
also, stated that failure was due to a combination of shear and bending and there was a reduction in
tensile strength of the column in the overlap zone, which g
and the failure becomes a bending failure.
Stefan et al. (2012) observed that the
is most likely that shear resistance is being overestimate in the curre
shear failure is considered. It has been investigated that the main failure mechanisms were captured
in the damage plasticity model up to a very reasonable extent.
small deformations, rows of columns will provide a higher shear resistance than single columns. This
is a clear fact because columns still possesses higher flexural resistance after induced cracking in the
rows of columns. They concluded that the overlapping zones between the li
an influence on the shear resistance. According to
showed the complex failure mechanisms of a lime
their study, therefore creates gap for
Dennes and Glen (2005) carried out a case history of full
clay ground supporting a 6.0 m high reinforced test embankment. The deep mixed columns were
installed in the soft clay using the jet mixing t
pressure of 20 MPa and excess pore pressure build up during installation was monitored. They
compared with another test embankment of approximately the same height constructed previously on
surrounding unimproved soft clay foundation. They also monitored settlements as well as the lateral
movement during and after the embankment construction. The result of their analysis revealed that
International Journal of Civil Engineering and Technology (IJCIET), ISSN 0976
6316(Online), Volume 6, Issue 3, March (2015), pp. 97-117 © IAEME
110
Schematic of the Shear Box Test, (Stefan et al. (2012)
traction force which was increased in stages. The result of their study revealed that the single
se in shear resistance for the columns installed in rows due to higher bending resistance. They
tensile strength of the column in the overlap zone, which gave rise to a reduction in shear resistance
and the failure becomes a bending failure.
Stefan et al. (2012) observed that the model tests and the numerical analyses indicated that, it
is most likely that shear resistance is being overestimate in the current design practice where only
in the damage plasticity model up to a very reasonable extent. Stefan et al. (2012)
of columns will provide a higher shear resistance than single columns. This
rows of columns. They concluded that the overlapping zones between the lime
an influence on the shear resistance. According to Stefan et al. (2012) the finite element analyses
showed the complex failure mechanisms of a lime-cement column but this was not investigated in
their study, therefore creates gap for further investigation.
Dennes and Glen (2005) carried out a case history of full-scale deep mixing improved soft
installed in the soft clay using the jet mixing technique and the cement slurry was injected at a
roved soft clay foundation. They also monitored settlements as well as the lateral
© IAEME
Stefan et al. (2012)
The result of their study revealed that the single
se in shear resistance for the columns installed in rows due to higher bending resistance. They
ave rise to a reduction in shear resistance
model tests and the numerical analyses indicated that, it
nt design practice where only
Stefan et al. (2012) claims that under
of columns will provide a higher shear resistance than single columns. This
me-cement columns have
the finite element analyses
cement column but this was not investigated in
scale deep mixing improved soft
echnique and the cement slurry was injected at a
roved soft clay foundation. They also monitored settlements as well as the lateral

111
deep mixing columns effectively reduced the settlement by almost 70%, while lateral movement of
the foundation soil was reduced by about 80%. The local differential settlement between the deep
mixing columns and the unimproved surrounding soil amounted to about 8–20% of the average total
settlement of the improved ground, and this could induce downdrag skin friction on the deep mixed
columns.
4.1 Deep Mixed Column under Embankment Loading
Paulo, Joao and Antonio (2011), carried out a numerical analysis study on an embankment
built on a normally consolidated soft soil reinforced with deep mixing columns using a coupled soil-
water formulation. They adopted the modified Cam Clay model concerned with Biot’s consolidation
theory to simulate the soil behaviour using a two dimensional FE code developed at the university of
Coimbra. The DMC was modelled using linear elastic laws with parameters based on laboratory
results.
Kitazume (2009) reported that linear elastic laws do not allow yielding of top of the DMC to
be simulated due to low confining stress and high loads, and the failure due to bending caused by
horizontal movement of the column that may occur at the columns founded under the toe of the
embankments. But, Abusharar, Zheng, and Chen (2009), stated that linear elastic model could be
adopted because the reduction in strength properties using safety factors causes stress level to be
considerably lower than the yielding stress of the DMC material.
Paulo, Joao and Antonio (2011), revealed that the application of DMC to strengthen the
foundation of an embankment structure, reduces settlement and small differential settlement. They
also found that with the use of DMC, consolidation was reduced because higher stiffness of the
column is associated with greater coefficient of consolidation. An increase in young’s modulus of
the DMC reduces settlement and consolidation time irrespective of the higher vertical effective
stresses on the DMC. Their results also revealed that in both mechanical and hydraulic terms, the
behaviour of the soil-DMC system is wholly controlled by the properties of the column. Sari et al.
(2009) adopted a finite element modelling approach to study the consolidation behaviour of an
embankment supported by multi columns of cement-fly ash- gravel, soil-cement and lime columns as
shown in Figure 14.0, (Sari et al. 2009). They modelled the embankment fill and the surrounding soil
as linear elastic-perfectly plastic materials with the Mohr-Coulomb failure criterion and the multi-
columns as linear elastic materials. The result of their analysis showed that soft ground treated with
multi-columns can reasonably reduce total and differential settlement and restricts lateral movement
of the supported embankment, thereby increasing its stability.
Comparing their numerical results with field test result, they observed that the embankment
settlement was more prominent at the centre than at the toe of the embankment and that the long
term stability of the embankment is enhanced by the cement-fly ash-gravel-lime column due to its
higher compression modulus. This study has revealed that the long term stability of an embankment
can be enhanced by deep mixed multi-columns due to its high compression modulus. At the long
term, there is a possibility that the moisture content of the surrounding soil might change due to
seasonal fluctuation of soil moisture. But the study has not incorporated the possible effect of this
change of degree of saturation of the surrounding (change in moisture content) on the long term
performance of this column.

112
Figure 14.0: A multi-column supported embankment, (Sari et al. 2009).
Paulo, Joao and Antonio (2011) observed that when the permeability of the deep mix column
exceeds the permeability of the virgin soil, the deep mix soil behave like a vertical drain controlling
the entire water flow of the soil-deep soil column system.
According to Seksun et al. (2012), when the ratio of the permeability of the improved soil
and the surrounding soil equals unity, the soil-cement column will not act as a drain and because the
yield stress and the stiffness of the improved ground is enhanced by the soil-cement column, the
improved soil goes into over consolidation state under the influence of the applied vertical stress.
Also, water flow occurs entirely in the soil when the permeability of the soil exceeds that of the deep
mix column and the column drains horizontally to the soil.
They concluded that the consolidation time of the soil-deep mix column depends on
permeability and stiffness of the deep mix column. How this consolidation time will be affected by
time varying loading will also be considered in this research. This study has not considered the long
term performance of the deep mix column in mitigating the embankment settlement. It has been
based on parametric studies and therefore, creates gap for further research studies.
Song –Yu et al (2012) carried out a field investigation to study the performance of a T-
shaped DMC, supporting an embankment over a soft ground. They conducted several field tests on a
high way embankment built on a soft clayey soil supported by a T-shaped deep mixed columns
(TDMC) and the conventional DMC. In situ and laboratory studies such as vane shear and piezocone
penetration tests were included in the study. The test site was divided into three different sites, which
they referred to as site A, B and site C and each site having a width of 50m and length of 100m
respectively. The strengths of sites A and B were improved using a T- shaped deep mixed column
while site C was improved using the conventional deep mixed column, serving as the control section.
Figure 15.0, Song –Yu et al (2012) shows the columns arrangements with the corresponding spacing
(S), lengths (L) and diameters (D) at each site.

113
Figure 15.0: Geometry of Columns at Different Test Sites, (Song –Yu et al 2012).
They adopted a water-cement ratio of 0.55 and a cement dosage rate of 255kg/m3
for the
production of the soil columns, slightly above the cement dosage rate of 5% - 16% of the weight of
untreated soil recommended by Winterkorn and Fang (1975).
They reported that during column installations, the cement slurry was injected at a pressure of
30Mpa and a maintained rotational speed of 60rpm. The installed columns were then subjected to
static plate load test 28 days after installation with a sand mat of 100mm thickness placed between
the soil columns and loading plates before loads were applied. The T- shaped deep mixed columns
have larger diameter at shallow depths compared to their diameters at deeper depths. Loads were
applied with increment ranged from 20 to 30 kPa and maintained until two consecutive readings less
than 0.1mm in each hour was obtained.
Song –Yu et al (2012) stated that prior to the filling of the embankment, a monitoring section
was selected at the centre of each test site and instruments such as earth pressure cells, piezometers
and inclinometers were located at the monitoring section as show in Figure 16.0.

114
Figure 16.0: Location of Monitoring Instruments at Site A, (Song –Yu et al 2012).
The result of their investigation revealed that the T- shaped deep mixed columns installed at
sites A and B resulted in cement savings by weight up to about 15 and 7% and also, save in
construction time of about 28 and 19% compared with that of the conventional deep mixed column
at the control section (site C). It was revealed that the bearing capacities of the T-shaped deep mixed
columns placed at sites A and B were found to be greater than the bearing capacity of the
conventional deep mixed column. This implies that T-shaped deep mixed columns have the tendency
of yielding stiffer foundations than the conventional deep mixed columns in shallow supported
ground.
They also, stated that the conventional deep mixed column exhibited a higher stress
concentration ratio than the T-shaped deep mixed column supported ground but the efficacy of the T-
shaped deep mixed column supported ground was higher than that of the conventional deep mixed
column. It was also reported that the T-shaped deep mixed column supported ground performed
better in terms of settlement than the conventional deep mixed column because it gave lesser
settlement values at sites A and B. Song-Yu (2012) concluded that the use of T-shaped deep mixed
columns for ground improvement under embankment loading conditions provided a viable,
economic and technological sound solution but, they have not considered the effect of changes in
strength and modulus of elasticity on the long term performance of both the T-Shaped and the
conventional DMC that may occur due to change in loading orientation during and after construction
of the embankment.
5.0 CONCLUSION
The essence of ground improvement is to enhance the engineering properties of weak soils to
provide stability and sufficient bearing capacity for construction purposes. In deep soil mixing, the
amount of binder to be mixed with the weak soil depends on the initial moisture content of the
surrounding soil. Several researchers have reported on this method of soil improvement through
laboratory experiments, in situ testing and numerical modelling. Most literatures reviewed in this
study have shown that research on deep soil mixing had focused on the mechanical properties of the
improved soil. Such as the compressive strength, stiffness, modulus of elasticity and consolidation
behaviour of the improved ground. For example, Ali et al., (2012), Lin and Wong, 1999, Fang et al.,

115
(2001), Porbaha et al., (2001), Yin, (2001), Porbaha, (2002), Farouk and Shahien (2013), Tao, Jim
and Jing, 2014) etc.Others focused on the effect of soil type, binder type, binder content, water-
cement ratio and area replacement ratio on the mechanical properties of the improved ground, for
example; Nur et al. (2013), Terashi et al., (1977), Kitazume and Terashi, (2012), Farouk and
Shahien, (2013) etc. In all these studies, the interaction of the improved soil columns with the
surrounding weak soil after installation has received little or no attention. Especially, with the
possibility of a change in the degree of saturation of the surrounding soil over time due to
environmental changes.It is most likely that the strength, stiffness and the resistance to shear
deformation of the improved material might change over time, and these might impair the long term
performance of the improved soil columns in terms of interfacial behavioural mechanism.
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