Study of the efficiency of stone columns in soft clay considering the effect of clay minerals in soil

International Journal of Civil Engineering and Technology (IJCIET), ISSN 0976 – 6308 (Print),
ISSN 0976 – 6316(Online), Volume 5, Issue 9, September (2014), pp. 241-251 © IAEME
INTERNATIONAL JOURNAL OF CIVIL ENGINEERING
AND TECHNOLOGY (IJCIET)
ISSN 0976 – 6308 (Print)
ISSN 0976 – 6316(Online)
Volume 5, Issue 9, September (2014), pp. 241-251
© IAEME: www.iaeme.com/Ijciet.asp
Journal Impact Factor (2014): 7.9290 (Calculated by GISI)
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IJCIET
©IAEME
STUDY OF THE EFFICIENCY OF STONE COLUMNS IN SOFT CLAY:
CONSIDERING THE EFFECT OF CLAY MINERALS IN SOIL
Esraa A. Mandhour1, Saad N. Al-Saadi2, Saad F. Ibrahim3
1Assistant Professor, Department of Physics, College of Science, University of Thi-Qar, Iraq
2Professor, Department of Geology, College of Science, University of Baghdad, Iraq
3Professor, Department of Highway and Transportation Engineering, College of Engineering,
University of Mustansiriyah, Iraq
241
ABSTRACT
This paper presents the behaviors of stone columns in soft clay. A laboratory model tests
were conducted on three different soils brought from different sites in Iraq to guarantee the
difference between these soils in amount of clay minerals. The effect of increasing of the number of
stone columns with difference of amount of clay minerals of soil were studied in this research, the
columns were constructed on a triangular pattern at 2D spacing inside steel container of (500mm ×
300mm × 350mm).
From laboratory results, the bearing improvement ratios (BIR) and settlement reduction ratios
(SRR) were determined and on bases of these results the stone columns give more efficiency in soils
which contain high amount of montmorillonite with few amount of kaolin than soils which have low
amount of montmorillonite with high amount of kaolin.
Keywords: Stone Columns, Soft Cay, Clay Minerals.
1. INTRODUCTION
The growing urban expansion and continuous population growth led to the creation of many
projects in locations with poor and incapable soils such as filled ground sites and areas containing
recently deposited alluvium.
Over the years, many techniques have been developed to improve the properties of weak soil
deposits such as stone columns, sand drains, blasting compaction, vibro-compaction, etc. [1]-[2]. The
stone columns technique is considered more efficient and suited among a number of techniques for
improving soft clays, silts and loose silty sand [3]-[4].

In Iraq, there are many sites have soft cohesive soils especially in the middle and south
242
parts [5].
In this paper, the effect of amount and type of clay minerals on act and efficiency of stone
columns were studied with take into consideration the effect of the number of stone columns.
2. PROPERTIES OF USED MATERIALS
Three soft soil were collected from three sites from different cities in Iraq: Duekhla quarry in
Al – Anbar city (Soil1), Baghdad University at Al – Jadriya area in Baghdad city capital of Iraq
(Soil2) and from Al – Nasiriya city at area lies near birth hospital (Soil3) as shown in Fig.1.
Fig.1: Illustrates locations of the study areas on Iraq map
The physical and chemical properties of the used soils for model are given in Table 1. The
properties of the sand which used as a bottom layer (30mm in thickness) in the base of the container
and the gravel which used as a backfilling materials in all stone columns are given in Table 2.
The particle sizes of gravel for the columns are chosen according to guide lines of Nayak
which suggests that they should be in the range of (1/6 – 1/7) diameter of column [6]. The physical
and chemical properties for these materials were determined according to the American
specifications for testing and material in addition to British standard [7]-[8]-[9].

Table 1: Physical and chemical properties of the used soils
243
Chemical properties
Physical properties
Soils Unified soil classification system (USCS)
Grain size analysis (%) Properties index (%) Soil symbol
pH 7.5
CH
Clay 82 LL 60
Soil1 Silt 18 PL 29 OM 2.9
Sand 0 PI 31 SO3 0.4
pH 7.7
CL
Clay 69 LL 42
pH 7.7
CL
Clay 59 LL 37
Note: LL: Liquid limit, PL: Plastic limit, PI: Plasticity index, CH: Inorganic clay of high plasticity,
CL: Inorganic clay of low plasticity, pH: Power of hydrogen, OM: Organic matter, SO3: Sulphates.
Table 2: Some physical properties of the sand and gravel used in test model
properties
Note: Gs: Specific gravity, D10: Diameter for 10% finer by weight of the sample, D30: Diameter for
30% finer by weight of the sample, D60: Diameter for 60% finer by weight of the sample, Cu:
Coefficient of uniformity, Cv: Coefficient of curvature, SP: poorly - graded sand, GP: poorly - graded
gravel.
3. CLAY MINERALS IN THE USED SOIL
Clay minerals are very small crystalline substances (less than 2μm in diameter) which
composed in general of alumina, silica and water with lesser quantities of iron, magnesium, sodium
and potassium [10]-[11]-[12].
The basic structure of clay minerals consist of two crystal sheets: the tetrahedral (silica)
sheets and the octahedral (alumina) sheets as shown in Fig.2.
These sheets are stacked together with different bonding and different metallic ions in the
crystal lattice to form the different clay minerals [10]-[12].
Materials
Sand Gravel
Gs 2.6 2.6
uniformity
Diameters of particles (mm)
D10 0.28 5.8
D30 0.40 7.0
D60 0.59 7.1
Cu 2.1 1.3
Cv 0.96 1.05
Unified soil classification system (USCS) SP GP

International Journal of Civil Engineering and Technology (IJCIET), ISSN 0976
ISSN 0976 – 6316(Online), Volume 5, Issue 9, September (2014), pp.
(a)
(c)
Fig. 2 Diagrammatic
241-251 © IAEME
(b)
(d)
sketch showing. (a) Single silica tetrahedron,
(b) The sheet structure of silica tetrahedron arranged in a hexagonal network
(c) Single octahedral unit. (d) The sheet structure of the octahedral units [10].
There are three main groups of clay minerals:
3.1 Kaolinite
Kaolinite is a common phylosilicate minerals composed of one tetrahedral sheet and one
octahedral sheet. In layer of kaolinite the tetrahedral and octahedral sheets are held together tightly
by oxygen atoms and these layers are held to
kaolinite is no expansion when the clay is wetted. Therefore, the water do not enter between the
structure layers of kaolinite.
3.2 Montmorillonite
together by hydrogen bonding. In turn, the structure of
Montmorillonite is composed of two sheets of te
linked by shared oxygen atoms. The structure of montmorillonite is expansion and swelling when
wetted. Therefore, the water enters the interlayer space and force the layers apart and this is due to
the little attraction between oxygen atoms in the bottom tetrahedral sheet with those in the top of
another of tetrahedral sheet.
3.3 Illite
Illite has a structure similar to montmorillonite therefore it is composed of two tetrahedral
sheets with central octahedral sheet but the interlayer are bonded together with potassium atoms. The
degree of swelling is considerable less for montmorillonite and more than kaolinite.
Table 3 illustrates the percentage of the dominant clay minerals for used soils
tested by X – Ray diffraction. Soil1 contains high percentage of kaolinite with little montmorillonite
and illite while soil2 and 3 have lower percentage of kaolinite and with higher amounts of
montmorillonite and illite. Clay samples were test
244
tetrahedral and one sheet of octahedral sheet
raction tested according to [13]-[14].
– 6308 (Print),

network,
gether trahedral which were

Table 3: The percentage of clay minerals of the used soils
Percentage of valid clay minerals
(a) (b)
Fig. 3: Experimental setup. (a) Testing apparatus. (b) Overview of testing apparatus
245
Soils
Kaolinite Montmorillonite Illite
Soil1 90 - -
Soil2 37 42.5 20.5
Soil3 23.2 45.7 31.3
4. PROGRAM OF TESTS
4.1 Testing Equipment
Steel container of size 500mm x 300mm x 350mm was used to carry out the model tests.
Figure 3 illustrates the testing apparatus and the steel loading frame which was designed and used in
this study.
4.2 Preparation of the Test Bed of Soils
The tests were carried out on specimens of dry soils which mixed with enough water at
moisture content equal to 0.35 to get the desired shear strength 8.0kPa, 7.8kPa and 7.5kPa for soil1,
soil2 and soil3 respectively.
The soils were packed in layers inside the steel container to get the final thickness about
220mm. a layer of poorly graded sand was placed at the bottom of the container below layers of soil
with final thickness about a 30mm and unit weight 16kN/m3 as shown in Fig.4.
220mm
30mm
250mm
Soil
Sand
Fig. 4: Beds of soil inside a container

300mm
Soil

Soil
75mm

(a) (b)
300mm
Soil
75mm
(c) (d)

Fig. 5: Overview of arrangement of stone columns. (a) Untreated soil.
(b) Treated soil with single stone column. (c) Treated soil with two stone columns.
(d) Treated soil with four stone columns
246
4.3 Columns Installation
The tests were carried out on single columns and groups of two and four columns. The
columns were installed with diameter (D) about 25mm and length (L) of 100mm placed at spacing
2D center to center with depth factor (L/D) equal to 4.
Steel bearing plate of size (292mm x 75mm) as shown in Fig.3 was placed over the columns
and these columns supported plates were loaded during the load test. The arrangement of stone
columns is illustrated in Fig.5.
300mm
Soil
500mm
212.5mm
212.5mm
Footing
area
292mm
500mm
212.5mm
75mm
212.5mm
292mm
500mm
212.5mm
212.5mm
300mm
292mm
500mm
212.5mm
75mm
212.5mm
Soil
292mm
5. ROLE OF CLAY MINERALS IN IMPROVED SOIL WITH STONE COLUMN
Stone columns have been used as drain wells to speed up the consolidation of compressible
soils which subjected to new load and the compressive stresses cause a lowering of pore space and
expel excess water in the case of saturated soils.
The stone columns which confined by surrounding soil and this allows the column to develop
high bearing pressure relative to the surrounding soil. In turn, the stone columns and the surrounding
soil behave as an integrated system with low compressibility and improved sustainability.
At the beginning, the new compressive stress is carried by the water and as the excess water
drains the compressive stresses are transferred to grains of soil and cause an increase in the inter
granular pressure. Increasing loads on foundation soils will create shearing stresses and this means
increasing shear strength with reduction in soil pore water [15].
In case of saturated or oversaturated soils subjected to applied stress, the water may migrate.
The properties of clays are effected with water content and in case of loaded clays, water is squeezed
out at a rate which depends upon the permeability [16].
Permeability of clays is very small because the size of the pores between the granular too
small. The montmorillonite has the lowest permeability compared with the other clay minerals while
the permeability of kaolinite is decreasing more gradual than montmorillonite with increasing
load [10].

S/B%
Fig. 6: Variation of (q/cu) with (S/B%) for different untreated soils
247
6. RESULTS AND DISCUSSION
The model tests were carried out to investigate the behavior of single and groups of stone
columns under incremental vertical stress in comparison with model tests of untreated soil with stone
columns.
Four terms were used to analysis of results for each model test: the first term is bearing ratio
(q/cu) where q/cu represents the ratio of applied stress (q) to the un-drained shear strength of
soil(cu), whereas the second term is deformation ratio (S/B%) where S/B% represents the percentage
of settlement (S) to footing width (B). While third term is bearing improvement ratio
(BIR) = (qtreated/ quntreated), where qtreated (qt) represents the ratio of applied stress (q) to the un-drained
shear strength (cut) of treated soil while quntreated (qunt) represents the ratio of applied stress (q) to the
un-drained shear strength (cuunt) of untreated soil. The last term is settlement reduction ratio
(SRR) = (Streated/Suntreated), where Streated (St) represents settlement of treated soil while Suntreated (sunt)
represents settlement of untreated soil.
First of all, the model tests were performed on untreated soft soils samples of. Figure 6
represents the bearing ratio plotted against deformation ratio. In general, it can be seen that all soils
give the same pattern that means increase the deformation ratio with increasing of bearing ratio.

q/cu
In spite of the fact that all types of soils when treated with stone columns they give a similar
behavior of increase of bearing ratio at all deformation ratios. There is difference in response of these
soils to this treatment. Figure 7 shows that the bearing ratio in soil2 is more pronounced than soil1
and soil3.
From results of X-Ray diffraction, we can be seen that the construction of stone columns in
soil characterized by the presence or increase in the percentage of the montmorillonite mineral will
increase the q/cu for soil with loading depend on the mineralogical structure for these minerals as
montmorillonite has a very strong attraction for water and soils containing montmorillonite are very
susceptible to swelling as they change (increase) water content [17].
Therefore the presence of water in the interlayer molecular spaces of montmorillonite
squeeze out toward the stone columns with applied stress and the moisture movement cause increase
in shearing stress with reduction of the soil water.
This process takes place in kaolin but in low degree depend on it is crystal structure.
Therefore, the bearing ratio increases in soil2 and soil3 more than soil1.

In Fig.8, The values of the bearing improvement ratios (qt/qunt) indicate that the soil2 and
soil3 give large improvement in comparison with soil1 and the maximum values of bearing
improvement ratios are obtained at deformation ratio (S/B%) about (1-2%) then gradual decrease is
observed.
This is probably due to the difference in shear strength and mineralogy between the soils. The
high increase in the bearing capacity in soil2 and soil3 in the early stages of load application may be
explained by the fact that these soils (2 and 3) have high percentage of montmorillonite, while the
low increase in bearing capacity in soil1 may be explained by the absence of montmorillonite in this
soil and the presence of high percentage of kaolinite in it.
248
q/cu q/cu q/cu
S/B%
(a)
S/B%
(b)
S/B%
(c)
Fig. 7: Variation of (q/cu) with (S/ B %) for different soil, (a) Soil1,
(b) Soil2, (c) Soil3


Figure 9 shows that the maximum value of settlement reduction ratios (SRR) for soil3 are
lower than those detected from soil2 and soil3 respectively in early stage of loading, where with the
progress of stages of loading there is an increase in the settlement reduction ratio for all soils
till it value becomes semi leveled.
The increase in the number of stone columns act to decrease the settlement of soil and for
soils with any number of stone columns there are more reduction in the settlements in the soils that
contain high percentage of montmorillonite in the early stages of loading rather than the soils which
contain high percentage of kaolinite.
Therefore, the model tests for stone columns on three different types of soils revealed that the
stone columns give most efficiency in soils contain high amount of montmorillonite and few amount
of kaolin than those have low amount of montmorillonite and high amount of kaolin.
249
qt/qunt qt/qunt
S/B%
(a)
S/B%
(b)
S/B%
(c)
qt/qunt
Fig. 8: Variation of (qt/qunt) with (S/B %) for different soil, (a) Soil1, (b)
Soil2, (c) Soil3


q/cu
(a)
q/cu
(b)

q/cu
(c)
Fig. 9: Variation of (St/Sunt%) with (S/B %) for different soil, (a) Soil1, (b)
Soil2, (c) Soil3
250
7. CONCLUSION
St/Sunt% St/Sunt% St /Sunt%
For all types of soils and with increasing of stone columns, there are increase in bearing ratio
and bearing improvement ratio in addition to decrease in the settlement reduction ratio. The
construction of stone columns in soils that contain high amount of montmorillonite leads to increase
the bearing ratio and bearing improvement ratio relative to soils that contain high amount of kaolinite.
While the settlement reduction ratios decrease in soils contain high amount of montmorillonite
relative to soils that contain high amount of kaolinite.
8. ACKNOWLEDGEMENTS
The authors wish to thank and gratitude the Department of Civil Engineering in the
University of Baghdad especially laboratory of soil mechanic for technical and scientific support in
order to complete requirements of this research.

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