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.

94

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.

95

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.

96

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

98

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

99

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)

From the tests with four comer columns,

Strength improvement ratio = 1.036 + 0.653 (z/B) +2.808 (RCA) (5.2)

Statistical analysis of the ratio of predicted to observed values of strength

improvement ratio yielded satisfactory values for ranking index and ranking distance

(0.266 and 0.217 for single column and 0.187 and 0.211 for four columns

respectively), suggesting that the correlation is acceptable for engineering applications.

The above relationships are purely qualitative in nature and if at all applicable,

are confined to the range of values of relative column area and normalized column

depth considered in the present study. With generation and examination of extensive

test data with plates/footings of different sizes corresponding to different values of

relative column area and normalized depth of reinforcement, equations for practical

usage can be arrived at.

Typical variation in the values of strength improvement ratio with increase in

normalized settlement of the model footing (B = 150mm) is presented in Fig. 5.15. It is

observed that strength improvement ratio (of clay beds reinforced with single as well

as multiple columns) decreases with increase in settlement. This is in contrast to the

published experimental results on reinforced soil beds (e.g., Sitharam et aI., 2007) and

103

the results obtained in the present study with coir geotextiles and nettings. But these

studies were on soil beds strengthened with horizontally laid reinforcements located at

some depth beneath the base of model footings. In such cases, some finite strain is

invariably needed for the reinforcement mechanism to mobilize/develop. In the case of

columnar reinforcement, since the reinforcement is placed in the vertical direction just

beneath the plate, its beneficial effect is felt immediately on commencement of

loading. However, with increase in settlement/loading, lateral displacement, bulging or

bending of the column takes place resulting in reduction in strength improvement. As

could be seen from Fig. 5.15, this reduction in strength improvement ratio is only

marginal in the case of clay beds strengthened by single central sand-coir fibre column,

while the same is much greater in the case of four corner columns. This is because, the

slight lateral movement/bulging of the corner columns might have resulted in a small

percentage of their cross-sectional area falling outside from directly beneath the plate,

leading to greater reduction in the extent of improvement. However, this does not

happen in the case of the central column since it is always directly beneath the test

plate.

The results of plate load tests (B=100mm) conducted on clay bed with the

reinforced column configurations shown in Figs. 5.2(c) and (d) are shown in Fig. 5.16.

All the columns used in these tests were of 75mm diameter and 100mm depth. For

comparison, the result obtained with the use of single central sand-coir fibre column of

the same diameter and depth and that for the untreated clay bed are also presented in

Fig. 5.16. It is seen that the pressure versus settlement behaviour corresponding to the

configuration referred in Fig. 5.2 (d) is less effective when compared to that for single

central column. The value of strength improvement ratio corresponding to normalized

104

settlement of 10% for the configuration presented in Fig. 5.2(d) is 1.95, while the same

due to the provision of single central column is 2.43. This is because, even though the

entire clay bed is reinforced with equally spaced sand-coir fibre columns in a uniform

pattern, the portion of the clay bed vertically beneath the plate is almost in an

unreinforced state. The cross-sectional area of columnar reinforcement, which comes

directly underneath the test plate, in this case, is only 0.18 percent of the area of the

test plate. These results probably suggest that the optimum benefit is obtained by

reinforcing the clay bed directly beneath the loaded area. It is also seen from Fig. 5.16

that the reinforcement configuration shown in Fig. 5.2(c) results in appreciable soil

improvement. The value of strength improvement ratio in this case, corresponding to

normalized settlement of 10% is 2.75. It may be recalled, however, that the

corresponding value of strength improvement ratio, when reinforced with single

central column of the same diameter was 2.43. The provision of eight additional

columns outside the loaded area [as per the pattern shown in Fig. 5.2(c)] thus results in

further improvement of only about 13%. The results obtained in this study suggest that

such an increased effectiveness can be economically achieved either through the use of

a single central column with slightly larger relative column area or by the use of four

corner columns with much lower value of relative column area, and hence, may be a

more preferred column configurations in field applications. It is also observed (Table

5.5) that sand-coir fibre columns installed at four corner of the model footing are very

effective in controlling settlements even at very low values of relative column area

whereas in the case of single central column, both settlement reduction and strength

improvement are appreciable only at higher values of relative column area. This can be

easily understood from the following example. Strength improvement ratio for four

105

corner colunm with relative area of 0.143 is 1.79 whereas a value of RCA of 0.283 is

needed to attain nearly same strength improvement ratio(1.76) in the case of a single

central column.

All the results presented have scale effects and can be considered to be only

qualitative in nature. Large scale field tests are needed to quantify the strength

improvement ratio for any given value of normalized settlement. But, still these

laboratory model test results provide insight into the basic reinforcing mechanisms that

establishes the pressure versus settlement behaviour of footings resting on soft clay

beds strengthened by sand-coir fiber columns. The present study clearly suggests that

the proposed method of ground improvement results in stabilization of soft clay beds

and is worth in-depth research and field studies.

5.5 Concluding remarks

Coir, which combines the properties of fiber strength and biodegradability,

remains an under-utilized material of great potential value for soil improvement in

those parts of the world where it is cheaply and abundantly available. This chapter

presents the results of a study made to examine whether soft clay soils can be

effectively stabilized Istrengthened through installation of sand-coir fiber composite

columns. Plate load tests which simulate static loading conditions in the field were

carried out for this purpose. Based on the results of tests carried out on soft clay bed

with different reinforcement configurations and column sizes, the following

conclusions are drawn:

(1) Pressure versus settlement behaviour of soft clay bed reinforced with

sand-coir fiber columns is appreciably better than that of untreated clay

106

bed. The results are quite encouraging in that a strength improvement

ratio of 1.5 to 2.0 would, in many situations, be sufficient enough to

eliminate the need of a deep foundation.

(2) Provision of columnar reinforcement outside the loaded area results in

soil improvement to some extent; however, it may not tum out to be cost­

effective, in comparison with the provision of the same within the loaded

area.

(3) For a chosen value of relative column area, provision of four identical

sand-coir fiber columns just inside the comer locations of the proposed

loaded area appears to be an optimum choice of column configuration for

soil improvement.

(4) Strength improvement ratio shows a decrease at higher settlements/

pressures.

(5) A framework for prediction of the degree of soil improvement in terms of

the relative column area, and normalized column depth can be developed

in the following form:

From the tests with single central column,

Strength improvement ratio = 0.953 + 0.122 (z/B) +2.835 (RCA)

From the tests with four comer columns,

Strength improvement ratio = 1.036 +0.653 (z/B) +2.808 (RCA)

It is hoped that the research work reported in this Chapter would provide a

basis for the potential use of coir fibers'in geotechnical engineering practice.

107

Table 5.1 Properties of the clay used in the study

Liquid limit (%) 66

Plastic limit (%) 33

Plasticity Index (%) 33

Specific Gravity 2.6

Sand content (%) 6.6

Silt content (%) 2004

Clay content(%) 73

Water content (%) 60.0

Bulk unit weight (kN/m3) 15.94

Table 5.2 Properties of the sand used in the study

Size 2mm-4.75 mm

Specific Gravity 2.65

Effective Size(mm) 2.7

Uniformity Coefficient 1.37

Coefficient of Curvature 0.90

Bulk unit Weight(kN/m3) 15.94

Table 5.3 Properties of the coir fibre used in the study

Diameter (mm) OJ

Length(mm) 24

Aspect Ratio 83.3

Tensile Strength(N/mm2) 80

108

Table 5.4 Summary of variables used in the parametric studyType of reinforcement: Sand-coir fibre column

Width Depth ofColumnar reinforcement

Diameter ofConfiguration

of plate sand-fiberz/B Number of

sand - coirRelative

10 column sand-coir columnmm(B) in rnrn(z) fibre column

fibrearea(RCA)

column(rnm)32 0.080

50 0.196100 1.00 1 63 0.312

75 0.442100 32 0.080

50 0.196150 1.50 1 63 0.312

Fig.5.2(a)75 0.44250 0.08775 0.196

100 0.67 1 90 0.283110 0.422

150 50 0.08775 0.196

150 1.00 1 90 0.283110 0.422

100 1.00 4 32 0.322100 150 1.50 4 32 0.322

32 0.14340 0.223

100 0.67 4 50 0.349Fig.5.2(b)

63 0.554150 32 0.143

40 0.223150 1.00 4 50 0.349

63 0.554

Fig. 5.2 ( c) 100 100 1.00 9 75 0.442

Fig. 5.2 (d) 100 100 1.00 16 75 0.180

109

Table 5.5 Strength improvement ratio for the different Test SeriesReinforcement type: Sand -coir fibre column

Width of Depth of Diameter RelativeStrength improvement ratio

test plate, B column,z of column z/B column (siB = 10%)(mm) (mm) (mm) area(RCA)

100 Untreated clay - - 1.00

32 0.080 1.18

50 0.196 1.71100 63 1.00 0.312 2.00

10075 042 2.43

32 0.080 1.33

50 0.196 1.88150 63 1.50 0.312 2.04

75 0.442 2.51

150 Untreated clay - -. 1.00

50 0.087 1.301--..

75 0.196 1.25100 90 0.67 0.283 1.76

150110 0.422 2.08

50 0.087 1.44

75 0.196 1.74150 90 1.00 0.283 2.13

110 0.422 2.21

100 100 32 1.00 0.322 2.30

100 150 32 1.50 0.322 2.55

32 0.143 1.79

40 0.223 2.17100 50 0.67 0.349 2.54

15063 0.554 2.83

32 0.143 1.87

40 0.223 2.33150 50 1.00 0.349 2.67

63 0.554 2.93

100 100 75 1.00 0.442 2.75

100 100 75 1.00 0.442 1.95

110

Fig. 5.I(a) Testing tank showing a PVC pipe iDsertaI iato "e clay bed

111

Fig. 5.1(b) TestiJIg 1aDk showing clay bed reinforml widl ceab'al sand-coir fibrecoIlUIUI

112

Fig. S.l(c) Testing tank showing clay bed reinforced widt me sand - coir fibercolumns

113

Fig. S.l(d) Test setap for plate load test

114

· '~.----------

All DiMensions In MM

Fig. 5.2 Configuration of sand- coir fibre column(s) in plate load test

115

Pressure(kPa)0 4 8 12 16 20

024 28

~~

\•,•,

\:; •,•\ ,- ,•

:~,

0' \,- ,~ 10 b•~

,\

5 \ ---o---ulltremed clay,\- •

~,\

-II-RCA==O.080t/l,

15 l

~,,,

'fJ,, -'-0.196

~ ••~,

g •,\ -0-0.312::1 20 •

;Z; q•\ -'-0.442,,

\•,')- •,_J •\,,,,

(:)

30

Fig. 5.3 Pressure versus settlement response of untreated and reinforced clay bedwith a central sand - coir fibre column ( B=100mm; z!B=1.00)

116

282420Pressure(kPa)12 1084o

o,,,,,

I

5,,,

I ---e---llJlh'eated clayI,,\I

".-.. \

~,

--.-RCA=0.080~e.,., 10 I-a I

I,~

, --a-0.196,,Q1 \

\

'E ,I

~0.31215 \q) ... ,!I.l ,

I'0 ,,q) , ---+-- 0.442!I.l I

i,II,

§ 20 ...,(\l,

Z ..,I,,

\ .,25

,,,,,,,,0

10

Fig. 5.4 Pressure versus settlement response of untreated and reinforced clay bedwith central sand-coir fibre column (B =100mm; zIB =1.50)

117

Fig. 5.5 Pressure versus settlement response of untreated and reinforced clay bedwith central sand-coir fibre column (B= 150 mm; zlB= 0.67)

118

2824

_RCA=0.087

-'-0.196

---e--- untreated clay

-8-0.283

-tr-0.422

Pressure (kPa)12 16 2084

,,..,,,,,,,,,~,,,..,,,,,,,,,,,

G?\.,,,,,,

•G?,,,,,

•,,,,,•,,,,o

oo

5

25 -

Fig. 5.6 Pressure versus settlement response of untreated and reinforced clay bedwith a central sand-coir fibre column (B= 150 mm;z/B=1.00)

Il9

--O--lUltreated clay

-+- RCA=0.322

84oPressme (kPa)12 16 20 24

t----fl~~~~-.-......---+=:~-r----r---r----r-~-=,~..--:28.......'Q,,..,,,..,

\\,

\,t;,,,,,,,,,,,

\,,\\,

~\,,,,,,,,,,,,

o

o

5

25

10

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

(B =100 mm;zIB= 1.50)

121

282420Pressure(kPa)

12 1684

___ 0.123

o

-+-0.349

-0-0.554

o r"L.----J (--,-ju-~~:Q--'--'Q-..•,

\•,\ ,\ ,..q,,,,..

\\,

\\,,..

q,,,,I,,

I,;),----------, \--..: ..-- untreated clay \,,-.tr-RCA=0.143 "

b

5

25

10

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

(B =150mm; zIB = 1.00)

127

6

5

o

B= 1501mn; z!B=l.OO -e-RCA =0,087-0.196-'-0.283---1-0.422---b--- 0.143---8--0.223

\

---;K--- 0.349----- 0.554

__ Single central colullmFour comer columns

o 5 lOIS 20 25Normalised settlement (%)

Fig. 5.15 Variation of strength improvement ratio with normalised settlement

128

o 4 8Pressure (kPa)

12 16 20 24 28

-+-Fig5.2(d): RCA = 0.180

-+-Fig.5.2(a): RCA = 0.442

--..- Fig.5.2(c); RCA = 0.442

--~-- untreated

o __-~,,:-t~--"'~'--==:r-~-r--,r------r--.--r---...--..----.........."0

\\\\\\\\\\\\\\\\\\

b\\\\\\\,\

\\\\\\\\\\\,\\

b\\\,,,,

\,\,,

\\,,6

5

I'Q'~ 10-a~a1~ 15t/'J

"Cl4)t/'J

~~ 20S:z

25

JO

Fig. 5.16 Comparison of pressure versus settlement response of reinforced claybed with different column configurations

129