CHAPTERS BEHAVIOUR OF A MODEL SQUARE FOOTING ON SOFT CLAY REINFORCED WITH SAND-COIR FIBRE COLUMN 5.1 General In countries were the availability and cost of synthetic reinforcing materials are the major constraining factors, the potential of natural materials such as coir fibre as a soil reinforcing element is worth examining. Unlike synthetic reinforcing materials, coir fibre is biodegradable; however, due to its high lignin content (about 40-46%), degradation takes place much more slowly than that in the case of other natural fibres in an earth context (Ayyar et aI., 2002). Biodegradability is an added advantage from the viewpoint of sustainable development and eco-friendliness. In this context also, the use of coir fibers for ground improvement assumes significance. Several studies on fibre reinforced soil have been reported in the literature. Works reported by Gray and Ohashi (1983), Freitag (1986), Maher and Gray (1990), Gopal Ranjan et al. (1996), Zornberg (2002) and Michalwoski and Cermak (2003) revealed that fibre reinforced soil is a composite material which can be advantageously utilized to improve the engineering behaviour of soil. The beneficial effect (significant gain in strength parameters and stiffness) of randomly oriented inclusions of coir fibres has been reported by Rao and Balan (2000). Banerjee et al. (2002) investigated the dimensional and mechanical properties of coir fibres as a function of fibre length. Babu and Vasudevan (2007) and Babu et al. (2008) also have reported about the beneficial effect of strengthening loose/weak soil through randomly oriented coir fibre inclusions. 94
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CHAPTERS
BEHAVIOUR OF A MODEL SQUARE FOOTING ON SOFTCLAY REINFORCED WITH SAND-COIR FIBRE COLUMN
5.1 General
In countries were the availability and cost of synthetic reinforcing materials
are the major constraining factors, the potential of natural materials such as coir fibre
as a soil reinforcing element is worth examining. Unlike synthetic reinforcing
materials, coir fibre is biodegradable; however, due to its high lignin content (about
40-46%), degradation takes place much more slowly than that in the case of other
natural fibres in an earth context (Ayyar et aI., 2002). Biodegradability is an added
advantage from the viewpoint of sustainable development and eco-friendliness. In this
context also, the use of coir fibers for ground improvement assumes significance.
Several studies on fibre reinforced soil have been reported in the literature.
Works reported by Gray and Ohashi (1983), Freitag (1986), Maher and Gray (1990),
Gopal Ranjan et al. (1996), Zornberg (2002) and Michalwoski and Cermak (2003)
revealed that fibre reinforced soil is a composite material which can be advantageously
utilized to improve the engineering behaviour of soil. The beneficial effect (significant
gain in strength parameters and stiffness) of randomly oriented inclusions of coir fibres
has been reported by Rao and Balan (2000). Banerjee et al. (2002) investigated the
dimensional and mechanical properties of coir fibres as a function of fibre length.
Babu and Vasudevan (2007) and Babu et al. (2008) also have reported about the
beneficial effect of strengthening loose/weak soil through randomly oriented coir fibre
inclusions.
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The stabilization of natural subsoil through inclusion of discrete, randomly
oriented fibers may be difficult, if not impossible, particularly when the vertical extent
of soil to be improved is large. Inclusion of fibers through provision of a columnar
reinforcement may be an effective alternative in such a situation. The work reported in
this Chapter examines whether soft clay soils (with water content nearer to liquid limit
water content) can be effectively stabilized/strengthened through installation of sand
coir fiber composite columns. Further, all the previous studies have examined the
effectiveness of coir fibers either through triaxial shear tests or one-dimensional
consolidation tests. Plate load testing which simulates static loading in a field situation
has not so far been used to investigate the response of coir fiber reinforced soft clays.
This Chapter presents the results of the research work in this direction.
Plate load testing of very soft clays (water content nearer to the liquid limit
water content) strengthened by columnar reinforcement of sand-coir fiber mixture,
with a program including single and multiple columns and with different values of
relative column area (defined as the ratio of the total cross-sectional area of the
columns within the plan area of the test plate to the cross-sectional area of the test·
plate) is presented in this Chapter. It may be pointed out that adoption of any ground
improvement technique would be most beneficial if it can result in a change in the type
of foundation from deep to shallow. Ground improvement studies are therefore, most
relevant in the case of very soft/weak soils. Hence, in the present study, a few
experimental results are presented with reference to very soft clays reinforced with
sand-coir fiber columns.
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5.2 Materials used
5.2.1 Soil
Two types of soils namely clay and uniformly graded coarse sand were used in
the present investigation. Processed China clay powder from English Indian Clays
Limited, Trivandrum, India, which is mineralogicaly kaolin clay was used to represent
the soft soil to be improved. The basic and index properties of the clay are presented in
the Table 5.1. The bulk unit weight and the water content values presented in the Table
correspond to the state at which the clay bed is prepared for testing. River sand
obtained from Trivandrum, India was used to prepare sand-coir fibre columns, the
properties of which are listed in Table 5.2.
5.2.2 Reinforcement
From the study on triaxial compression of clay reinforced with sand-coir fibre
core (Vinod et al. 2007), it was observed that the reinforcement effect is maximum at
fibre content of 1% and fibre aspect ratio has only a marginal influence on the extent
of soil improvement. Hence, all the experiments in the present study were conducted
with sand-coir fiber composite having fibre content of 1% and fibre aspect ratio of
83.3. The coir fibre selected for carrying out this investigation was natural coir fibre
obtained from a local coir-manufacturing unit near Trivandrum in India. The properties
of coir fibres used for" the study are presented in the Table 5.3.
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5.3 Laboratory model tests
5.3.1 General
The bearing capacity of soils and foundation settlements under different loads
were estimated by plate load tests in the laboratory. Since the actual loading in the
field are simulated in plate load tests, the conclusions are of great practical importance.
5.3.2 Test set up
The model plate load tests for the present investigation were conducted in the
laboratory in a steel test tank whose inside dimensions of the tank were fixed as
600mm length x 600mm width x 500mm depth. The tank was strengthened by a
number of channel shaped steel beams in both vertical and horizontal directions to
avoid lateral yielding during placement of soil bed and loading. Square rigid steel
plates of two sizes - lOOmmxlOOmm and 150mmx150mm and of 25mm thickness
were used as model footings. A thin layer of sand was cemented, using epoxy glue, to
the base of the model footings to make them rough.
5.3.3 Preparation of reinforced clay bed
Dry powdered clay.and water needed to fill the tank up to the required height
were mixed in required proportions. The clay was first pulverized and then mixed with
predetermined quantity of water. The moist soil was kept in air tight containers for a
period of one week, to allow for the uniform distribution of moisture in the clay. The
moist soil was placed in the tank up to 300m depth, in six equal layers and each layer
compacted uniformly with metal tamper so as to achieve the desired level. For each
layer, the calculated amount of clay needed to produce the desired bulk density was
weighed out and placed in the tank making use of a metal scoop. The soil was then
gently leveled and compacted to the proper depth using metal tamper, using the depth
markings on the sides of the tank as guide. The compactive effort to be given to each
97
of the six layers to obtain the desired bulk density was arrived at from initial trial
experiments. In order to verify the uniformity of the clay bed, undisturbed samples
were collected from different locations and the bulk unit weight and the moisture
content were determined. Values of the above parameters for samples collected from
different locations in the clay bed were found to be almost the same (as given in
Table 5.1).
A thin PVC pipe of desired relative column area was used to form the columnar
reinforcement of specified depth (lOOmm and 150 mm). The PVC pipe was embedded
in the clay bed to a depth of 10mm at the desired plan location [Fig 5.1 (a)]. Moist soil
to fill the remaining volume of the tank (excluding the volume of PVC pipe) up to the
specified height was placed in the tank around the pipe in two layers (for 100 mm
height; and three layers if 150 mm height) and uniformly compacted. The bulk unit
weight of clay placed in the upper 100mmll50 mm layer (i.e., in the reinforced zone)
was also checked by collecting samples as explained earlier. Calculated quantities of
sand and coir fiber to form the column with 1% fiber content and at the same bulk
density as that of clay were taken and divided into two/three equal parts. Each part of
sand was mixed with each part of coir fiber manually (hand mixing), taking maximum
possible care to get a uniform mixture. Each part of the mixture was placed inside the
PVC pipe and the pipe was lifted by one - half/ollt-third of the height of the column
(of height 100mm/150mm)~ The column material was then compacted using a metal
tamper. Since the compaction of any part of the mixture was done after lifting the
bottom of PVC pipe by one-half/one-third height as the case be, there was no void
formation between the column and the clay. The compactive effort that had to be given
to each layer was calibrated so that the sand-coir fiber column was placed at the same
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bulk density as that of clay. Upon filling the tank up to the required level, the surface
was leveled [Fig. 5.1 (b)]. Load tests were also conducted on clay bed with multiple
sand-coir fibre reinforced columns. A typical arrangement is shown in Fig. 5.1 (c)
5.3.4 Testing programme
After placing the test plate centrally over the prepared soil bed, another larger
plate was kept on top of the test plate for mounting of dial gauges. Two dial gauges
having least count of O.Olmm were placed on the diametrically opposite comers and
the readings were set to zero. This was followed by incremental loading with either
5kg, 2kg or lkg weight placed centrally above the test plate [Fig. 5.1 (d)]. Each load
increment was affected only when the rate of settlement under the previous load
increment was less than 0.1 mm/hr.
5.3.5 Test variables
Plate load tests were conducted on clay beds reinforced with single as well as
multiple sand-coir fiber columns with square plates of 100mm xlOOmm as well as
150mm x 150mm size. In all the tests wherein a single column was used, the same was
installed centrally beneath the location of the test plate [Fig. 5.2 (a)]. Plate load tests
conducted in this Series using 100mm square plate were with columns of diameter
32m, 50mm, 63mm and 75mm (the corresponding values of relative column area being
0.080, 0.196, 0.312 and 0.442 respectively). The tests carried out using 150mm size
plate had columns of 50mm, 75mm, 90mm and 110mm installed at the centre of clay
bed (corresponding values of relative column area beingO.087, 0.196, 0.283 and 0.422
respectively). Another Series of plate load tests were conducted with the installation
of four columns just inside the comers of the loaded area [Fig. 5.2 (b)]. Tests in this
Series with 150mm square plate were conducted with four identical columns, each of
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diameter 32mm, 40mm, 50mm and 63mrn (0.143, 0.223, 0.349 and 0.554 being the
corresponding values of relative column area). Only a typical plate load test with four
columns at the comers of the loaded area (diameter=32mrn; RCA=0.322) was
conducted using the 100mm size plate. Each of the above mentioned tests were done
for two column depths- 100mm and 150mm In addition to the above one test each was
performed, using 100 mm size plate and using a columnar reinforcement of 100mrn
depth with column configurations shown in Figs.5.2(c) and 5.2(d). The values of the
variables used in the parametric study are summarized in Table 5.4. The installation of
columns in the desired locations was ensured with the help of a configuration sketch
(plan), drawn to scale, showing the column positions in the clay bed. Plate load tests
were carried out on untreated clay bed as well.
5.4 Results and discussion
5.4.1 General
The degree of improvement obtained by using sand-coir fiber composite in the
form of columnar reinforcement in plate load tests on soft clay beds is discussed in the
subsequent sections. Influence of area as well as configuration of columns on the
pressure versus settlement behavior of soft clay is identified and isolated. The
mechanism of soil improvement and the possibility of expressing the degree of
improvement as a unique function of relative column area and normalized column
depth is also attempted.
5.4.2 Effect of central sand-coir fibre column on the pressure versus settlement
behaviour
Figs. 5.3 and 5.4 show the effect of columnar reinforcement of sand-coir fibre
mixture, installed centrally beneath the loaded area, on the pressure versus settlement
100
versus settlement curves presented is the average of two plate load test results under
identical conditions. It is evident from Figs. 5.3 through 5.6 that the response of the
reinforced clay bed is appreciably better than that of the untreated clay bed, the extent
of improvement increasing with increasing in relative column area. This is because, as
the reinforced soil bed is subjected to deformation, frictional interaction between sand
and coir fibres takes place resulting in the mobilization of tensile stresses in the fibre.
Mechanism of interlocking would also have developed in the sand coir fibre columns.
It is also seen from Figs. 5.3 through 5.6 that the initial modulus of the pressure versus
settlement curve of the reinforced soil bed, particularly with higher values of relative
column area, is much higher than that of clay bed without columnar reinforcement. In
order to get a quantitative picture about the extent of soil improvement, the
improvement due to the provision of sand-coir fiber column is represented using a non
dimensional strength improvement ratio (Dash et aI., 2003), as discussed in the
previous chapters. In the present study, the pressures corresponding to normalized
settlement [(settlement/width of plate) x100] of 10% have been compared for
reinforced and unreinforced conditions and the corresponding values of strength
improvement ratio are presented in Table 5.5. As could be seen from this table, the
results are quite encouraging in that strength improvement ratio of about 1.5 to 2.0
may, in many cases, eliminate the need of a deep foundation. It is also seen from Table
5.5 that for the same relative column area (0.196), strength improvement ratio obtained
using 150mm plate is smaller than that obtained using 100mm plate. The larger size of
101
the pressure bulb (significant zone) in the case of the150mm plate resulted in a lesser
strength improvement.
5.4.3 Effect of four corner sand-coir fibre columns on the pressure versus
settlement behavior
Performance of clay bed reinforced with four identical sand-coir fibre columns
installed just inside the comer locations of the loaded area [Fig. 5.2(b)] can be
observed from Figs.5.7 through 5.10. The strength improvement ratio values calculated
from these test results are presented in Table 5.5. A comparison of the values presented
in Table 5.5 indicates that the provision of columnar reinforcement just inside the
comers of the loaded area results in improved response when compared to centrally
located single column. For instance, the strength improvement ratio (B=150mm;
z/B=0.67) due to the provision of four columns at the comers of the plate is 2.17
corresponding to relative column area of 0.223, while the same is only 1.76 for a single
column of relative column area of 0.283. A better understanding on the response of
these two reinforced soil systems can be made from Figs. 5.11 through 5.14. It is clear
from these figures that, for any specific value of relative column area, four identical
sand-coir fibre columns installed just inside of the comers of the loaded area is a much
preferred choice of ground improvement. Further, strength improvement appears to be
a unique function of relative column area, for given values of normalized column
depth [depth of sand-coir fiber column (z) I width of the loaded area(B)], fiber content
and fiber aspect ratio.
5.4.4 Qualitative estimate of strength improvement
It may be recalled (Section 5.2.2) that fiber content of 1% and fiber aspect ratio
of 83.3 were used in all the experiments in the present study. Generalization of the test
102
(5.1)
results presented in Figs. 5.11 through 5.14 can, therefore, be made to make it
applicable to any value of normalized depth of columnar reinforcement (z/B) and
relative column area (RCA). The regression analysis carried out for this purpose
resulted in the following relationships with satisfactory values (0.933and 0.926
respectively) of correlation coefficient:
From the tests with single central column,
Strength improvement ratio = 0.953 + 0.122 (z/B) +2.835 (RCA)
Fig. 5.7 Pressure versus settlement response of untreated and reinforced clay bedwith four identical sand-coir fibre columns at the corners of the loaded area
(B= lOOmm; zIB=l.OO)
120
2824
---e---lUltreatedelay
-RCA=0.322
20Pressure(kPa)
12 168
..,,\ ..
\~,
\\,,,
\,,\\..,~,,,
\\
",,~
4
oo
5 ..
35
40
Fig. 5.8 Pressure versus settlement response of untreated and reinforced clay bedwith four identical sand-eoir fibre columns at the corners of the loaded area
Fig. 5.9 Pressure versus settlement response of untreated and reinforced clay bedwith four identical sand-coir fibre columns at the corners of the loaded area
(B = 150mm; z/B= 0.67)
122
---+-0.554
-a--O.349
282420Pressme (kPa)
12 1684
\..\\\ ..~,
\\,,
\\,..
\,
"In\I\I
.---------,\I
---&-- untreated clay ~
II
---RCA=O.143 \•II
---+- 0.223 \o
oo
5
10
Fig. 5.10 Pressure versus settlement response of untreated and reinforced claybed with four identical sand-coir fibre columns at the corners of the loaded
area(B= 150mm; z/= 1.00)
123
3
•
... single central collUlUl
• tom corner collullIl
1
o 0.1 0.2 0.3 0.4 0.5Rel~t:ive core area
Fig. 5.11 Comparison of strength improvement ratio for clay beds reinforcedwith single central column and four corner columns (B =100mm; zlB = 1.00)
124
3
•
... single central colullm
• four comer colullln
1
o 0.1 0.2 OJ 0.4 0,5
Relative core area
Fig. 5.12 Comparison of strength improvement ratio for clay beds reinforcedwith single central column & four corner columns
(B =IOOmm; zlB = 1.50)
125
3
•0
'+:1 2.5 •~I-<
~d>d>
S •d>
~2l-4p,
.5
~•
.§t/.) 1.5
& single central cohunn
• fom comer colmnn
1
0 0.1 0.2 0.3 0.4 0.5
Relative core area
Fig. 5.13 Comparison of strength improvement ratio for clay beds reinforcedwith single central column and four corner columns
(B =150mm; zIB = 0.67)
126
3 •
1.5
•
•
•
a\ singlecentral colmlln
• tom comer COhUlUl
1
o 0.1 0.2 0.3 0.4Relative core area
0.5 0.6
Fig.5.14 Comparison of strength improvement ratio for clay beds reinforcedwith single central column and four corner columns