Chapter 61

CHAPTER 6

Effect of Reinforcement and Plasticity Index on the CBR Value of Black cotton soil:

6.1: Introduction:

This Chapter presents the effects of plasticity index and also reinforcing effect of black

cotton soil on CBR values the effects of reinforcement and PI on CBR values are investigated in

both soaked and unsoaked conditions. The results shows that as the PI increase the CBR value

decreases and reinforcing black cotton soil with stabilisers will increase the CBR value.

Experimental programme:

A series of laboratory CBR tests are conducted on black cotton soil reinforced with randomly

distributed polypropylene fibers and coir fibers and also with other stabilisers like RBI-81,

GGBS, Optimum percentages of stabilisers are mixed with black cotton soil and CBR values are

obtained. The optimum percentage of stabilisers used is decided on the basis of higher UCS

value. The following table shows the optimum percentage of stabilizer.The results of soaked and

unsoaked CBR tests on reinforced and unreinforced soil specimens with different plasticity

indexes are presented in Table and figure

Effect of PPF on CBR

Polypropylene fibers are hydrophobic, non corrosive and resistant to alkalis, chemicals and

chlorides. The improvement of the engineering properties due to the inclusion of polypropylene

fibers was determined to be a function of a variety of parameters including fiber type, fiber

length, aspect ratios (length/diameter), fiber content, orientation, and soil properties. Attempts

were made by various researchers to determine the effect of each parameter on the different

engineering properties of the composite. The properties of polypropylene used in the present

study were number of C.B.R testes were carried out to know the effect of fiber on CBR value

clay mixes. By addition of 0.5% fiber only Maximum density is 1.848 g/cc for black cotton soil.

In case of black cotton soil treated with 1.0 % to 2.0 % of polypropylene fiber, it is found that

dry density decreases with increase in optimum moisture content. The CBR value is increased

from 3.73 % to 5.13 % when 2.0 % of polypropylene fiber is added to black cotton soil. Increase

in strength is due to the reinforcement phenomenon which is similar to coir fiber stabilization.

The CBR value increased by 1.37 times when compared to black cotton soil alone.

Table 1: CBR of Black Cotton Soil stabilised With PPF

Particulars CBR %, (Unsoaked) CBR %, (Soaked)

BC SOIL + 0.5% PPF 6.52 3.73

BC SOIL + 1.0% PPF 6.72 4.07

BC SOIL + 1.5% PPF 7.19 4.19

BC SOIL + 2.0% PPF 7.76 5.13

BC SOIL + 2.5% PPF 7.31 4.66

Fig 1 percentage PPF vs CBR (%)

Effect of Plasticity index on CBR:

Table produces the soaked and unsoaked CBR values of reinforced soil specimens with different

plasticity indexes. As it is observed with increase in PI, the CBR value decreases for both soaked

and unsoaked cases, because when the plasticity index of soil increases, the optimum moisture

content (OMC) increases and the maximum dry density (MDD) decreases. Therefore, the soil

strength decreases and its CBR value decline.

Particulars CBR %, (Unsoaked) CBR %, (Soaked) PI

BC SOIL + 0.5% Polypropylene Fibers 6.52 3.73 23.9





Table 2 CBR and Plasticity Index of BC soil stabilised with PPF

Fig 2 Plasticity index vs CBR (%) of black cotton soil stabilised with PPF

Effect of RBI-81 on CBR

The maximum dry density is 1.77 g/cc for black cotton soil treated with 8.0 % RBI Grade-81.

Increase in percentage of RBI Grade-81 from 2.0 % to 8.0 %, results in increase in dry density

with gradual increase in optimum moisture content. The CBR value increased from 3.73 % to

6.36 % when 8.0 % of RBI Grade-81 is added to black cotton soil. This increase in strength is

due to chemical reactions between soil and RBI Grade-81. Presence of polypropylene fiber in

RBI Grade-81 increases the strength by reinforcement phenomenon. The CBR value increased

by 1.23 times when compared to black cotton soil alone.

Table3: CBR of black cotton soil stabilised with Varying Percentages RBI-81

Fig 5: CBR Curves for Black Cotton Soil stabilized with Various Percentage of RBI-81

Effect of Plasticity Index on CBR

RBI-81 Grade Stabilizer CBR Values

0% 3

2 % 3.34

4 % 4.88

6 % 6.36

8 % 4.22






Table 4: CBR and Plasticity Index of BC soil stabilised with RBI-81

Fig 3 Plasticity index vs CBR (%) of black cotton soil stabilised with RBI-81

Effect of RBI plus PPF on CBR

RBI-81 Grade

Stabilizer

CBR Values

(Soaked)

CBR Values

(un Soaked)

Plasticity Index

0% 3 6 23.62% 3.34 6.5 20.74% 4.88 7.89 17.996% 6.36 9 12.618% 4.22 9 11.02

The MDD and OMC of Black Cotton Soil stabilised with PPF is 17.75 kN/m3 and 18.60%, for

this OMC and MDD the maximum value of CBR was found to be 6.24%.It is found that the

value of CBR increases 1.6 times with the addition of RBI plus PPF to the BC soil compare with

BC soil alone.

Table 5: CBR of black cotton soil stabilised with various percentages of RBI and PPF

RBI and PPF Stabilizer CBR in %

1% RBI+0.25% PPF 3.84

2% RBI+0.50% PPF 4.71

3% RBI+0.75% PPF 6.24

4% RBI+1% PPF 5.32

Fig 4: CBR for Black Cotton Soil stabilized with Various Percentage of RBI plus PPF







Table 6 CBR and Plasticity Index of BC soil stabilised with RBI-81 plus PPF

Fig 5 Plasticity index vs CBR (%) of black cotton soil stabilised with RBI-81plus PPF

Effect of coir fiber on CBR

RBI + PPF Stabilizer

CBR Values

(soaked)

CBR (Un soaked)

Plasticity Index

0 3 6 23.561%

RBI+0.25% PPF

3.846.54

20.52

2% RBI+0.50%

PPF4.71

6.9817.92

3% RBI+0.75%

PPF6.24

9.612.96

4% RBI+1% PPF 5.32 9 11.89

As per experimental programme CBR test is conducted. Table 7 presents the CBR values

of stabilized BC soil with different percentages of Coir fiber, and fig7 shows the Variation of

CBR with different percentages of Coir fiber

The maximum density is 1.77 g/cc for BC soil treated with 0.3 % coir fiber. In case of black

cotton soil treated with 0.2 % and 0.3 % of coir fiber, it is found that the dry density increases

with increase in optimum moisture content. Further increase in percentage of coir fiber from 0.4

% to 0.5 %, there is decrease in dry density. This is due to addition of coir fiber results increase

in tendency of moisture attraction. The CBR value is increased from 3.73 % to 6.98 %, when 0.5

% of coir fiber is added to black cotton soil. The increase in strength is due to the reinforcement

phenomenon of coir fiber. The CBR value increased by 1.87 times, when compared to black

cotton soil alone

Table 7: CBR of black cotton soil stabilised with varying percentages of Coir fiber

Sl no Particulars CBR %, (Soaked)

1 BC Soil Alone + 0.2% COIR FIBRE 4.03




Fig 6: CBR Curves for Black Cotton Soil with Various Percentages of Coir


Table 8 produces the soaked and unsoaked CBR values of reinforced soil specimens with

different plasticity indexes. As it is observed with increase in PI, the CBR value decreases for

both soaked and unsoaked cases, because when the plasticity index of soil increases, the

optimum moisture content (OMC) increases and the maximum dry density (MDD) decreases.

Therefore, the soil strength decreases and its CBR value decline.

Table 8 CBR and plasticity index of black cotton soil stabilised with coir fiber

Fig 7 Plasticity index vs CBR (%) of black cotton soil stabilised with Coir fiber

Sl no

Particulars

CBR %, (Soaked)

CBR %, (Un Soaked)

Plasticity Index

1BC Soil Alone + 0.2% COIR FIBER

4.03 8 22.86


4.34 8.5 21.26


4.96 9 18.37


6.98 12 13.07

Effect of GGBS on CBR:

Table 9: CBR of black cotton soil stabilised with varying percentages of GGBS

Fig 8: CBR Curves for Black Cotton Soil with Various Percentages of GGBS


BC Soil + GGBSCBR%

(Soaked)CBR%

(un Soaked)0% 3.73 3.985% 2.15 410% 3.86 4.615% 7.72 920% 8.58 9.325% 9.01 9.8






Table 10 CBR and plasticity index of black cotton soil stabilised with GGBS

Fig 9 Plasticity index vs CBR (%) of black cotton soil stabilised with GGBS

Effect of Cement on CBR

GGBSPI CBR% (Soaked) CBR% (un Soaked)

23.56 3.73 3.9813.185 2.15 415.05 3.86 4.6

17 7.72 914.42 8.58 9.312.09 9.01 9.8

As per experimental programme CBR test is conducted Table and Table presents the

CBR values of stabilized BC soil with different percentages of Cement, RBI Grade-81, PPF &

their combination respectively. Fig 4.14 and Fig 4.19 shows the Variation of CBR with different

percentages of Cement, RBI Grade-81, PPF & combination for BC soil respectively.

Table 11: Variation of CBR Values in Addition of Cement with Varying Percentages

Fig 10: CBR Curves for Black Cotton Soil with Various Percentage of Cement


Cement % CBR Values

2 4.56

4 7.92

6 11.64

8 8.32






Table 12: CBR and plasticity index of black cotton soil stabilised with Cement

Fig 11: Plasticity index vs CBR (%) of black cotton soil stabilised with Cement

Effect of cement plus PPF on CBR:

Cement % CBR (Soaked) CBR(Unsoaked) Plasticity

Index0 3 6 23.56

2 4.56 7 20.74

4 7.92 9 18.07

6 11.64 12 15.11

8 8.32 12 11.43

Table 13: CBR of black cotton soil stabilised with cement and PPF

Cement + PPF Stabilizer CBR Values

1% Cement+0.25% PPF 4.92

2% Cement+0.50% PPF 7.57

3% Cement+0.75% PPF 9.42

4% Cement+1% PPF 7.44

Fig 12: CBR Curves for Black Cotton Soil with Various Percentage of Cement+PPF

Effect of plasticity index on CBR: Table 14 produces the soaked and unsoaked CBR values of

reinforced soil specimens with different plasticity indexes. As it is observed with increase in PI,

the CBR value decreases for both soaked and unsoaked cases, because when the plasticity index

of soil increases, the optimum moisture content (OMC) increases and the maximum dry density

(MDD) decreases. Therefore, the soil strength decreases and its CBR value decline.

Table 14: CBR and PI of black cotton soil stabilised with cement plus PPF

Fig 13: Plasticity index vs CBR (%) of black cotton soil stabilised with Cement

Cement+PPF % CBR (Soaked) CBR

(Un Soaked)Plasticity

Index0 3 5 23.6

1% Cement+0.25%

PPF4.92

5.621.98

2% Cement+0.50%

PPF7.57

820.39

3% Cement+0.75%

PPF9.42

918.91

4% Cement+1%

PPF7.44

813.89

Effect of lime on CBR:

Table 15: CBR of black cotton soil stabilised with lime

ParticularsCBR %,

(Soaked)

BC Soil Alone + 2% LIME 4.96




Fig 14: CBR Curve for Black Cotton Soil with Various Percentages of Lime

Effect of plasticity index on CBR: Table 16 produces the soaked and unsoaked CBR values of

reinforced soil specimens with different plasticity indexes. As it is observed with increase in PI,

the CBR value decreases for both soaked and unsoaked cases, because when the plasticity index

of soil increases, the optimum moisture content (OMC) increases and the maximum dry density

(MDD) decreases. Therefore, the soil strength decreases and its CBR value decline.

Table16 CBR and Plasticity index of lime stabilised balck cotton soil:

Particulars CBR %, (Soaked) CBR %, (Un Soaked) PI

BC Soil Alone + 2% LIME 4.96 3.8 31.08


BC Soil Alone + 6% LIME 7.32 6 26.12


Fig15 : Plasticity index vs CBR (%) of black cotton soil stabilised with lime

CBR prediction model

General:

In India thickness of flexible pavement is designed on the basis of projected number of standard

axle loads during the design life, which is obtained using the present commercial vehicles per

day and its growth rate, along with the subgrade strength in terms of CBR.

From the results of the detailed laboratory investigations on the strength behaviour of stabilised

subgrade soil, it could be seen that the strength in terms of the CBR of the soil would increase

considerably. The strength mobilisation depends on many factors like the inherent strength

properties of the soil and also the type and percentage of stabilisers. Since there are various

parameters are contributing to the strength in terms of CBR, it is advantageous to develop a

model to predict the CBR of the reinforced black cotton soil. the geotechnical properties like

MDD, OMC and Atterberg limits are the major contributing parameters to the CBR value. Hence

considering CBR has an dependent variable and the other variables influencing it are

independent variable through the principles of multiple linear regression analysis a model can be

developed. The model thus developed can be effectively referred for further use.

Multiple Linear Regression Analysis (Johnson, 2001)

Multiple linear regression analysis is a statistical technique frequently used to develop prediction

equations to establish the relationship for a variable, which is known to respond to changes in

two or more other variables. The variable which is known to respond is (Y) commonly called the

dependent variable and the other variables influencing it are called the in dependent variable(x).

the function will be of the following form:

Y=a1x1+a2x2+………+amxm +a0

Here, x1, x2 ………… xm = independent varaibles,

a0 regression constant and

a1, a2,…………… am = regression coefficients of the independent variables.

The regression coefficients are determined from a given set (n) of observed values of Y and x1, x2 ………… xm by the method of least squares.

The analysis of variance approach is used to test the predictor equation. The total sum of squares

of deviations of the n observations from the mean is a measure of the degree to which the n

observations are spread around their average value. Smaller the standard error the better will be

the model. Co efficient of determination (R2) is another indicator of the strength of relationship.

It is the ratio of regression sum of squares to total of squares. R2 lies between 0 and 1. The closer

it is to 1, the better is the equation.

Development of Model:

The important property of the subgrade soil considered for the design of pavement is the CBR

value. The influencing parameters for the CBR values are MDD, OMC and Atterberg limits

therefore to approximately predict the CBR value for clayey type of soil through Multiple Linear

regression analysis using LINEST function (The LINEST function calculates the statistics for a

line by using the "least squares" method to calculate a straight line that best fits the data) by

inputting the laboratory test data obtained following Models were developed.

For polypropylene fibre stabilised soilCBR= -9.66MDD+0.275OMC+0.0747PI+15.48

Coir fibre stabilised black cotton soilCBR= -186.8MDD+36.12OMC+5.56PI-463.312

For RBI-81stabilised soilCBR=242.30MDD-6.16 OMC -0.85PI -270.92

RBI plus Polypropylene stabilised black cotton soilCBR=44.84MDD+1.17OMC+0.040PI-97.16

Flyash stabilised black cotton soilCBR=16.27OMC-0.50PI-268.90

Coirfiber plus flyashCBR= -4.52MDD-0.66OMC-0.814PI+43.26

Coirfiber plus limeCBR=8.66MDD+11.01OMC+1.19PI-249.48

Cement plus PPFCBR= 206.17MDD+6.8OMC-0.97PI-475.07

CementCBR= 4979MDD-95.17OMC+9.77PI-6905Cement plus RBI-81CBR= -117.84MDD+5.78OMC+0.546PI+80.83

Lime stabilised soilCBR= -78.21MDD-0.01OMC-0.63PI+156.22

Lime plus FlyashCBR=-26.96MDD+1.27OMC-0.49PI+80.834

Conclusions:

As it is observed that with increase in PI, the CBR value decreases for both soaked and

unsoaked cases, because when the plasticity index of soil increases, the optimum

moisture content (OMC) rises and the maximum dry density (MDD) decreases.

Therefore, the soil strength decreases and its CBR value declines.

The MDD and OMC of Black Cotton Soil are 1.670 g/cc and 20.08%.

The value of CBR is found to be 3.73% and the UCS parameter of the BC soil alone is

0.882 Kg/cm2.

The CBR value is increased from 3.73 % to 6.98 %, when 0.5 % of coir fiber is added to

black cotton soil. The increase in strength is due to the reinforcement phenomenon of

coir fiber. The CBR value increased by 1.87 times, when compared to black cotton soil

alone.

The CBR value is increased from 3.73 % to 5.13 % when 2.0 % of polypropylene fiber is

added to black cotton soil. Increase in strength is due to the reinforcement phenomenon

which is similar to coir fiber stabilization. The CBR value increased by 1.37 times when

compared to black cotton soil alone

Maximum density is 1.79 g/cc for black cotton soil treated with 8.0 % of cement. In case of

black cotton soil treated with 2.0 % to 8.0 % of cement, it is found that dry density

increases with increase in optimum moisture content. This is due to reduction in plasticity

and formation of matrix enclosing clay lumps.

The CBR value is increased from 3.73 % to 24.60 % when 8.0 % of cement is added to

black cotton soil. Due to increasing percentage of cement added, there is an increase in

strength and moisture movement in soil-cement mix and a decrease in volume change,

plasticity and also its tendency to swell and shrink. The CBR value is increased by 6.59

times, when compared to black cotton soil alone.

The maximum dry density is 1.77 g/cc for black cotton soil treated with 8.0 % RBI Grade-81.

Increase in percentage of RBI Grade-81 from 2.0 % to 8.0 %, results in increase in dry density

with gradual increase in optimum moisture content.

The CBR value increased from 3.73 % to 41.90 % when 8.0 % of RBI Grade-81 is added

to black cotton soil. This increase in strength is due to chemical reactions between soil and RBI

Grade-81. Presence of polypropylene fiber in RBI Grade-81 increases the strength by

reinforcement phenomenon. The CBR value increased by 11.23 times when compared to black

cotton soil alone.

The maximum density is 1.796 g/cc for black cotton soil treated with 20 % fly ash. In case of

black cotton soil treated with 10 % and 20 % of fly ash, the dry density increases with increase in

optimum moisture content. Addition of fly ash upto 30 % results in increased availability of free lime

content of fly ash, hence the dry density decreases with decrease in optimum moisture content.

The CBR value is increased from 3.73 % to 10.71 % when 10 % fly ash is added to black cotton soil.

The CBR value is increased by 2.87 times when compared to black cotton soil alone.

Chapter 61

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