chapter 8

DISCUSSIONS AND CONCLUSIONS

Discussions

Through the experimental analysis following discussions are done:

Black Cotton soil alone

From Table 4.1, the physical properties test on Black Cotton Soil has shown that Liquid

Limit is 45.70 and Plastic Limit of Soil is 22.14. Hence the soil used for the investigation

can be treated as medium clayey soil.

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%, the UCS parameter of the BC soil alone is 0.882

Kg/cm2.

From Table 4.16 it is observed that, the K-value is 3.914 Kg/cm3 and E-value is 138.768

Kg/cm2 in case of BC Soil alone.

Coir Fibers

From Table 4.2, it is observed that addition of coir fiber from 0.2 % to 0.5 % (at an

increment of 0.1%) results in the reduction of liquid limit from 45.70 to 39.96 and also the

increase in percentage of plastic limit from 22.14 to 26.89. Liquid limit decreases with

increase in percentage of coir fiber indicates minimum water content is required for soil to

flow under application of lower shearing force. In case of plastic limit, moisture necessary

for soil to remain in plastic state increases with increase in percentage 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.

Unconfined compressive strength is 11.808 kg/cm2 for 28 days cured specimen, when BC

soil is treated with 0.5 % coir fiber. It is observed that addition of 0.5 % coir fiber shows

significant increase in the unconfined compressive strength with 28 days curing period.

Addition of coir fiber denotes an increase in reinforcement action with increase in curing

period.

From Table 4.17, K-value is increased from 3.914 kg/cm3 to 3.968 kg/cm3 and E-value is

increased from 138.768 kg/cm2 to 175.584 kg/cm2 when 0.5 % of coir fiber is added to

black cotton soil.

Polypropylene Fibers

From Table 4.3, it is observed that addition of polypropylene fibers from 0.5% to 2% (at an

increment of 0.5%) results in the increase of liquid Limit from 43.90 to 50.00 and also the

increase in percentage of plastic limit from 20.00 to 21.97. Increase in liquid limit with

percentage of polypropylene fiber indicates water content required for soil to flow under

application of lower shearing force increases with increase in percentage of polypropylene

fiber.

Maximum density is 1.848 g/cc for black cotton soil treated with 0.5 % of polypropylene

fiber. 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.

Unconfined compressive strength is 2.323 kg/cm2 for zero day cured specimen, when black

cotton soil is treated with 2.0 % polypropylene fiber. It is observed that addition of 2.0 %

polypropylene fiber shows gradual increase in unconfined compressive strength with zero

day curing periods. This is due to the increase in reinforcement action by addition of

polypropylene fiber.


increased from 138.768 kg/cm2 to 368.16 kg/cm2 when 2.0 % of polypropylene fiber is

added to black cotton soil.

Cement

From Table 4.5, it is observed that addition of cement from 2.0 % to 8.0 % (at an increment

of 2.0 %) results in the reduction of liquid limit from 44.90 to 39.29 and also the increase in

percentage of plastic limit from 24.16 to 27.86. With higher percentage of cement, calcium

ions (Ca++) migrate to the surface of the clay particles and displace water and other ions.

Soil becomes friable and granular, making it easier to work and compact. At this stage

plasticity index decreases dramatically, as does its tendency to swell and shrink.

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.

Unconfined compressive strength is 19.34 kg/cm2 for 28 days cured specimen, when black

cotton soil is treated with 8.0 % cement. It is observed that addition of 8.0 % cement shows

gradual increase in unconfined compressive strength with 28 days curing periods. Steep

increase in unconfined confined compressive strength is observed which is due to increase

in cement added increases strength with increase in curing period.


increased from 138.768 kg/cm2 to 210.984 kg/cm2 when 8.0% of cement is added to black

cotton soil.

RBI Grade – 81

From Table 4.8, it is observed that the addition of RBI Grade-81 from 2.0 % to 8.0 % (at an

increment of 2.0%) results in the reduction of liquid limit from 44.60 to 38.90 and plastic

limit from 23.90 to 26.66. Liquid limit decreases with the increase in percentage of RBI

Grade-81. This is due to presence of cementitious particle in RBI-Grade-81 resulting in

reduction of moisture movement. Also plasticity index decreases rapidly with increase in

addition of RBI Grade-81 to black cotton soil.

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.

When black cotton soil treated with 8.0 % of RBI Grade-81, unconfined compressive

strength is 17.22 kg/cm2 for 28 days cured specimen. Increase in unconfined compressive

strength is due to increase in percentage of RBI Grade-81 from 2.0 % to 8.0 % and increase

in curing period. Presence of chemicals in RBI Grade-81 and polypropylene fibers

increases the bonding between soil and RBI Grade-81 and hence the strength increases.

From table 4.21, K-value is 3.21 kg/cm3 and E-value is 141.60 kg/cm2 for 8.0 % of RBI

Grade-81 added to black cotton soil.

Fly ash

From Table 4.10, it is observed that the addition of fly ash from 10.0 % to 30.0 % (at an

increment of 10.0%) results in the reduction of liquid limit from 43.70 to 33.90 and plastic

limit from 21.12 to 24.98. The addition of fly ash results in the decrease of liquid limit due

to the effect of reduction in the diffused double layer thickness as well as decrease in effect

of dilution. Decrease in plastic limit is due to increase in addition of fly ash, the amount of

soil to be flocculated decreases and the finer particles of fly ash get incorporated in the

voids of flocculated soil.

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.

From Table 4.18, K-value is 3.28 kg/cm3 and E-value is 145.14 kg/cm2 when 20 % of fly ash is

added to black cotton soil.

Ground granulated blast furnace slag (GGBS)

From Table 4.13, the addition of GGBS from 5.0 % to 25.0 % (at an increment of 5.0%)

results in the reduction of liquid limit from 42.35 to 35.30 and plastic limit from 29.16 to

23.21. Increase in addition of GGBS results in decrease of liquid limit due to the effect of

dilution. Plastic limit also decreases due to the finer particles of GGBS get filled into the

voids of soil and resulting in lower water content to remain in plastic state.

The maximum density is 1.809 g/cc for black cotton soil treated with 25 % GGBS. In case of black

cotton soil treated with 5.0 % to 25.0 % of GGBS, the dry density increases with gradual decrease

in optimum moisture content. This is due to increase in amount of GGBS requires lesser optimum

moisture content to obtain higher dry density. Higher amount of GGBS decreases the repulsive

pressure of soil particles, reducing the resistance to compactive effort resulting in higher densities.

The CBR value is increased from 3.73 % to 9.01% when 25 % of GGBS is added to black cotton

soil. Increase in CBR of soil is due to cementatious action by pozzolanic compounds present in

GGBS. The CBR value is increased by 2.41 times when compared to black cotton soil alone.

Unconfined compressive strength is 9.877 kg/cm2 for 28 days cured specimen, when black cotton

soil is treated with 10 % GGBS. The 28 day curing period shows higher strength which means that

the unconfined compressive strength increases with higher curing periods. The variation in strength

is due to variation in cohesion of the soil with increasing addition of GGBS.

From Table 4.13, K-value is increased from 3.914 kg/cm3 to 6.72 kg/cm3 and E-value is increased

from 138.768 kg/cm2 to 297.36 kg/cm2 when 10 % of GGBS is added to black cotton soil.

Combination of Fly ash and GGBS

From Table 4.15, the addition of fly ash and GGBS from 0.0 % to 12.0 % (at an increment of

4.0%) results in the reduction of liquid limit from 45.70 to 41.01 and plastic limit from

22.14 to 21.56. Due to the presence of pozzolanic compounds, increase in addition of fly

ash and GGBS results in decrease of liquid limit and plastic limit which reduces the effect

of diffused double layer.

The maximum density is 1.83 g/cc for black cotton soil treated with 4 % fly ash and 4% GGBS.

Optimum moisture content is increasing gradually with increase in addition of fly ash and GGBS.

Due to increased availability of free lime, increases the repulsive force of soil particles. Hence there

is an increase in compactive effort and density decreases.

The CBR value is increased from 3.73 % to 9.44 % when 12 % fly ash and 8 % GGBS is added to

black cotton soil. This increase in CBR is due to increase in activity of calcium ions with clay

particles and displace water and other ions. The CBR value is increased by 2.53 times, when

compared to black cotton soil alone.

Unconfined compressive strength is 7.292 kg/cm2 for 28 days cured specimen, when black cotton

soil is treated with 4 % fly ash and 8 % GGBS. Increase in unconfined compressive strength is

observed when fly ash and GGBS are added to black cotton soil. Strength of soil is improved

considerably with curing period due to pozzolanic reactions present in fly ash and GGBS

compounds.

From Table 4.23, K-value is 3.52 kg/cm3 and E-value is 155.76 kg/cm2 when 4 % fly ash and 8 %

GGBS are added to black cotton soil.

Conclusions

1. Fibers such as Coir and Polypropylene when used in BC soil improve strength and

prevent brittle failure.

2. Addition of stabilizers such as cement, RBI Grade-81, Fly ash and GGBS to the BC soil

reduces the liquid limit and plasticity index of soil.

3. When cement, RBI Grade-81, Fly ash and GGBS are used in the BC soil improves the

unconfined compressive strength of the specimen’s increases as the content of these

stabilizers increases. Adding of these stabilizers to the BC soil reduces the liquid limit

and increases plastic limit of soil.

4. Even though the cost of RBI Grade-81 stabilizer is more when compared to other

conventional stabilizers such as Cement, Fly ash and GGBS it gives more strength to the

pavement thereby decreasing pavement thickness.

5. Thickness required for pavement with stabilized soil subgrade is much lesser than un-

stabilized soil subgrade.

6. RBI Grade-81 is preferred for the case viz higher CBR to improve the soil characteristics

and for higher life span of the pavement.

Scope for further studies

In this paper stabilization is done on the basis of CBR, UCS values are evaluated, for

further studies other parameters such as other strength defining tests such as direct shear

tests and triaxial test can be opted.

Unconfined compressive strength test have been done on curing period for 0,3,7,14 and

28 days only. For further studies, curing period can be increased by 56 days and so on for

a period of 180 days.

Studies can be done only on evaluation of K-value by conducting Plate load test by

increasing thickness of the stabilized layers and various others layers of pavement can be

incorporated.

This paper limited to some amount of stabilizing agent, amount of stabilizer treated with

BC Soil can be increased and higher value of K and E can be achieved, which may be

beneficial in reduction in pavement thickness and cost of construction in clayey areas.

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chapter 8

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