CHAPTER – 1INTRODUCTION

General

Reinforced soil is a composite construction material consisting of soil fill strengthened by

inclusion of tensile elements. The reinforcing elements may be in different forms e.g.

metal sheets, mats, fiber reinforced plastics, geosynthetics, etc.

Fiber reinforcement is an age-old technique used in the past for making un-burnt clay

bricks (reinforced with straw). Also significant research is being conducted since 1960s

on soil reinforcement with natural and man made fibers. Fibers specifically engineered

for soil reinforcement are available now (e.g. fiber grids by synthetic industries) and have

been successfully used in many construction projects in various countries. It has been

widely reported that the properties (especially the shear strength) of the soil can be

enhanced by fiber reinforcement, resulting in more stable soil structure with high load

bearing capacities.

This project aims at using Geosynthetics for reinforcing soil and study the variation in

the reinforcement effect at different depths in case of planar sheet element form and at

different aspect ratios in case of randomly distributed fiber form.

1.2 Pavement

A Pavement is a hard crust constructed over the natural soil for the purpose of providing

a stable and even surface for the vehicles. Basic requirement of a good pavement is to

provide a stable non-yielding surface for the movement of heavy vehicles. A Pavement

layer that distributes the wheel loads through the largest area per unit thickness of the

layer is the most efficient.

Role of subgrade in pavement design-The thickness of the pavement depends upon the

properties of the subgrade. A thicker pavement is required over a weaker soil.

1

1.3 CBR Value

The CBR values are used to determine the total thickness of the flexible pavement and

thickness of various layers. The required thickness of construction above a material

decreases as the CBR value increases. The design curves give the required thickness of

construction above a material of a certain CBR value. These design curves are based on

the data collected on a number of pavements which performed satisfactorily.

1.4 Significance of reinforced soils

To improve the bearing capacity of poor soil before construction of the pavement, i.e. to

make use of unsuitable land for pavement construction. The CBR value of soil can be

increased by reinforcement which results in reduced thickness of the pavement which

means an economic design.

1.5 Geosynthetics

Many civil engineering problems in coastal areas and hilly regions need stabilization of

soft soil. To tackle such problems Geosynthetics emerged as a good solution.

Geosynthetics are made of synthetic materials such as polythene, polyester, and nylon.

Geosynthetics have high tensile strength. These can be used to increase the load carrying

capacity of the soil.

1.6 Statement of the problem

1.6.1 Objective

The primary objective of this study is to evaluate the reinforcement mechanism in

unsoaked and soaked conditions in terms of CBR value. The secondary objective is to

compare the reinforcement effect of a given Geosynthetics in planar element form and in

randomly distributed fiber form. It further includes the optimum fiber content and the

aspect ratio that is yielding maximum CBR for the materials used in this dissertation.

2

1.6.2 Necessity

CBR value is the basis for the design of pavements. Higher CBR value gives economical

pavement layers. It is therefore necessary to improve the CBR value. In this direction

efforts are made in this project to improve the CBR value through reinforcement function

of Geosynthetics. In view of the above, there is great necessity for this project work.

1.6.3 Scope

Scope of this project work is limited to conducting tests on one type of Sand collected

from River Krishna in Krishna district and one type of Geosynthetics only. Sand is tested

in both soaked and unsoaked conditions. The CBR tests are conducted in this dissertation.

Reinforcement in planar sheet form is limited to one layer only. The moulding moisture

content was maintained at OMC of the base material Sand. It is considered that, the

inclusion of fiber do not change the moulding density significantly.

1.7 Organization of the Thesis

This thesis has been organized into 4 Chapters.

1 Literature review related to unreinforced and reinforced soils is given in Chapter

2.

2 Chapter 3 deals with methodology adopted.

3 Chapter 4 deals with results and discussion.

4 Chapter 5 deals with the observations and conclusion.

3

CHAPTER-2LITERATURE REVIEW

General

Many civil engineering problems in coastal areas and hilly regions need stabilization of

soft soil. Some of these structures may be temporary for establishing roads and rail

communication links and in some accessibility by itself possesses a major problem. To

tackle such problems in civil engineering, geosynthetics emerged as a good solution.

To improve the mechanical properties of soils, a variety of materials are used for

reinforcement e.g. metallic elements, geosynthetics and others.

Mechanism of shearing resistance

Relative sliding between particles is a major factor contributing to deformation in a soil

mass. Hence, the resistance that the soil can offer to deformation will have to come

mainly from the shear resistance between the particles at their contact points. At these

points of contact, forces of attraction exist between the surface atoms of the particles. If

the normal force acting on the particles increases, the number of contact points may

increase leading to an increase in the number of bonds, thus causing the total shear

resistance to increase. The reverse may happen when the normal force decreases. It can

be said that this case is analogous to the shear resistance of a solid block of a material

like steel, which is frictional in nature. However, a part of the shear resistance may, in

some cases, be present even when the normal force is reduced to zero. This part of the

shear strength is ascribed to true cohesion between the particles and is perhaps due to

sustained contact between particles for a long duration of time.

To express the frictional resistance in case of soil masses, one can consider the analogy

of the force transmitted by two solid bodies in contact with each other. The Figure 0-1

Mechanism of shearing resistance shows the normal force N perpendicular to the surface

of contact and the tangential force, τ parallel to the surface. When the block begins to

4

slide, the shearing force will have reached its maximum value. τmax is equal to μ N where

μ is called the coefficient of friction.

Figure 0-1 Mechanism of shearing resistance

In soils like sands and other granular materials, the resistance to sliding on any plane

within the soil mass depends on the normal force and the angle of internal friction.

However, in case of these soils, there is also the rolling friction in addition to the sliding

friction. Even more important in sands is the phenomenon of interlocking, which

contributes appreciably to the frictional resistance, especially if the relative density of the

sand deposits is high.

Figure 0-2 Interlocked soil particles(a) Before Shear (b) After Shear

The above Figure 0-2 Interlocked soil particles illustrates the concept of interlocking in

soil particles. It can be seen that soil particles interlock into each other and the planes

through their contact points are not horizontal, but undulating (Figure 0-2 (a)). In order

5

that shear failure should occur, the interlocking between particles must break up through

rearrangement of soil particles

Figure 0-2 (b) in dense sand, the interlocking is considerable and consequently, the value

of Ф is also greater than it is for loose or medium sands.

Modified Failure Envelope

Mohr’s Failure Envelope is generally obtained by drawing a common tangent to Mohr

circle at failure. This method of obtaining the failure envelope is not convenient as it

becomes difficult to draw the required tangent touching all the circles. A modified failure

envelope as shown below is more convenient. A modified failure envelope is a plot

between p and q values at failure where

231 σσ +

=p ------------------------------------------ Equation 0-1

231 σσ −

=q -------------------------------------Equation 0-2

The coordinates of top point of the Mohr circle corresponding to the maximum shear

stress are (σ1+σ3)/2 and (σ1-σ3)/2 and are therefore equal to p and q.

6

Figure 0-3 Conventional and Modified Failure Envelopes

In Figure 0-3(a), the points 1, 2 and 3 give the maximum shear stresses reached in the

three tests at the time of failure. These points are transferred to p-q plot in Figure 0-3(b),

and a line is drawn through these points. The line makes an angle α′ with the p-axis and

has an intercept a′ on q-axis. This line is known as the modified failure envelope.

A relationship between the shear strength parameters c′ and φ ′ and the parameters of

the modified envelope α′ and a′ can be obtained by comparing the Equations.

'' tanαφ =Sin -----------------------------------Equation 0-3

''' cos ac =φ -------------------------------------Equation 0-4

The main advantage of the modified failure envelope is that the stress conditions at

failure are represented by one point instead of a Mohr circle. As the averaging of

scattered pointsis easier than drawing a common tangent to a number of circles, it is more

convenient than the Mohr-Coulomb failure envelope.

7

Shear Characteristics of Cohesion less Soil

The shear strength of cohesion less soils, such as sands and non-plastic silts, is mainly

due friction between particles. In dense sands, interlocking between particles also

contributes significantly to the strength.

The stress-strain curve for dense sands exhibits a relatively high initial tangent modulus.

The stress reaches a maximum value at its peak at a comparatively low strain and then

decreases rapidly with an increasing strain and eventually becomes more or less constant.

The stress-strain curve for loose sands exhibits a relatively low initial tangent modulus.

At large strains, the stress becomes more or less constant. The dense sand shows initially

a volume decrease in a drained test, but as the strain increases, the volume starts

increasing. The loose sand shows a volume decrease throughout.

In the case of loose sand, the specimen bulges and ultimately fails by sliding

simultaneously on numerous planes. The failure is known as the plastic failure. In the

case of dense sand, the specimen shows a clear failure plane and the failure is known as

the brittle failure.

The failure envelope for dense sand can be drawn either for the peak stresses or for the

ultimate stresses. The value of the angle of shearing resistance (Ф) for the failure

envelope for peak stresses is considerably greater than that for the ultimate stresses. In

the case of loose sands, as the peak stress and the ultimate stress are identical, there is

only one failure envelope. The angle of shearing resistance in very loose state is

approximately equal to the angle of repose. The angle of repose is the angle at which a

heap of dry sand stands without any support. It has been established that air-dry sand

gives approximately the same value of (Ф) as the saturated sand. As it is easier to

perform tests on dry sand, tests can be performed on dry sand instead of saturated sand.

Review of Previous Studies

Goel (1978) summarized the work done by various research workers to find the effect of

particle size, shape, orientation, gradation, soundness, compactness of particles, type of

the test, ratio of box size to the maximum particles size and normal stress on the sand and

gravelly material. He concluded the angle of internal friction depends upon ratio of

8

specimen size to maximum particle size, shape of particle, void ratio, gradation,

confining pressure etc. The findings arrived at by different research workers regarding

the influence of different factors on shear strength parameters are not the same.

Fagnoul (1969) conducted triaxial and direct shear tests on phylitic angular material. The

sizes of sample were 20x20x7.8cm; 5x5x2cm in direct shear test and of 5cm and 50cm

diameters in triaxial test. He concluded that the specimen size to maximum particle size

ratio up to 5 and testing speed from 0.14 to 0.79 mm/minute have no effect on shear

characters of the sample. Shear strength of the specimen is affected by finer fraction, if

fines are more than 50% in well graded material. The shearing resistance does not vary

greatly with mean size of the material except for very fine particles, when the grain size,

distribution and porosity remain constant. The shear stress deformation curve is

influenced by the density considerably. The value of the angle of shearing resistance

found out by triaxial tests is lower as compared to the direct shear values.

Sowers and Sowers (1961) summed up the phenomenon of shearing resistance as

follows: When a normal load acts on a soil mass, the soil particles respond in two ways

simultaneously. Some of the particles deform more or less elastically. Some particles

undergo local crushing due to development of high stresses at their point of contact. Due

to these two responses, namely elastic distortion and crushing, translation and rotation of

the grains take place altering the pattern of voids in the soil mass. When a shear force is

made to act on the soil mass, two additional responses of the soil grains become evident.

Firstly the particles tend to roll over one another. There is a resistance to the rolling,

depending on the angle of contact and the normal stress acting at the point. When the

resistance is overcome the particles tend to roll.

The practice of building houses and roads on fibre-reinforced earth and constructing

earth walls with different types of reinforcing intrusions is an old concept. Rope fibres

and bamboo were used to strengthen rural road bases and the soil below low-cost low-

rise buildings. In recent times, this concept has been extended to other materials, like

fabrics and membranes, often termed geotextiles and geomembranes for reinforcing the

9

soil for improvement of its properties (Narain and Ratnam, 1985; Datye and Nagaraju,

1985).

The concept of reinforced earth is based on the principle of Vidal (1969) by which the

introduction of reinforcing elements in a soil mass increases the shear resistance of the

medium. In the early studies the use of reinforced earth was confined to retaining walls.

The reinforcements used consist of intrusions ranging from discrete fibres to strips, grids,

woven and non-woven fabrics of metals, paper and polymeric materials.

The first significant study on foundations has been done by Binquet and Lee (1975) who

concluded that the bearing capacity of sand increases to three times or more with

moderate amount of aluminum foil strips reinforcement. Yang (1972) and Andrews et al

(1978) also made similar studies using a single layer of mesh reinforcement.

A significant study on bearing capacity of sand reinforced with coir rope was done by

Ramanatha Ayyar, Joseph, and Beena, in the year 1988 and conclusions were drawn.

From the results of bearing capacity tests on model square footing on a deep

homogeneous sand bed with coir rope reinforcements and bamboo strip anchorages, the

following conclusions are drawn.

1. The ultimate bearing capacity can be improved by using coir rope

reinforcement’s up to about 2.5 times.

2. The most significant factor in increasing the ultimate bearing capacity is the

vertical spacing of reinforcement layers. A vertical spacing of 0.25B or less may

be adopted for increasing BCR by more than 100%.

3. The ultimate bearing capacity is not as sensitive to horizontal spacing of

reinforcing ropes as to the vertical spacing of reinforcing layers. However it is

found that beyond X/B=1, the reinforcements are ineffective. Considering the

economy and performance/B=0.5 is recommended as the optimum horizontal

spacing for reinforcing ropes in a layer.

4. The area under reinforcing has very limited influence on bearing capacity. This

area of reinforcing should be as small as possible for economical and practical

10

considerations but should cover the area of stress influence to be effective. An

area of (3B)2 is suggested for optimum results.

5. It can be seen that when the anchorages are kept closer, within the range 3B to

5B, the CBR increases.

6. Only marginal increase is obtained by adopting “mesh” pattern.

7. For a specific load, the settlement is less in reinforced sand than in unreinforced

sand.

8. The increase in volume of reinforcement certainly increases the bearing capacity

but considerably increases the settlement at failure.

An investigation was undertaken to study the behavior of reinforced earth in improving

the bearing capacity and settlement resistance under square and strip footing by

Sreekantaiah, (1988). Locally available river sand was used along with aluminum foil

strips as reinforcing material. The tests were conducted at a density of 1500kg/m3.The

parameters selected were, depth of the top layer of reinforcement below the footing and

the number of layers of reinforcement. It has been concluded that by a suitable

arrangement of the reinforcing strips, the bearing capacity and settlement resistance of

sand could be improved up to 3 times that of the unreinforced sand.

Experimental results reported by various investigators (MCgown et al 1978; Verma and

Char 1978; Gray and Ohashi 1983; Al refeai 1986; Gray and Maher 1989; Gray and

Maher 1990; Al refeai 1991) have shown that fibre reinforcement causes significant

improvement in strength and stiffness to sand. More importantly, fibre reinforced soil

exhibits greater extensibility and small loss of lost strength (i.e. greater ductility in the

composite material) as compared to sand alone or sand reinforced with high modulus

inclusions (Gray and Ohasi 1983; Gray and Al refeai 1986).

The increase in strength and stiffness is reported to be a function of sand characteristics,

particle size, shape, gradation, fibre characteristics, weight fraction, aspect ratio, skin

friction and modulus of elasticity (Gray and Maher 1989 and Maher and Gray).

11

They reported that the strength of sand increases with increase in fibre content, aspect

ratio, and soil surface friction. A better gradation i.e. increase in coefficient of

uniformity, lower sphericity and smaller average grain size (D50) of sand result in higher

fibre contribution to strength (Maher and G ray 1990, Hoare 1979) .Analyzing the results

of a series of laboratory compression and CBR tests on a sandy gravel reinforced and

randomly distributed synthetic fibres<2% by weight observed that the presence of fibres

increased the apparent angle of internal friction and ductility of the soil particularly at

low confining stress.

Verma and Char have reported similar observations in 1978 through triaxial tests on mild

steel reinforced in medium/fine sand. They observed an increase in angle of internal

friction from 36 to 45 degrees, with increase in fibre content from zero to 7% by volume.

Andersland and Khattak 1979 tested kaolinite clay reinforced with paper pulp (cellulose)

fibres in triaxial testing under confining stress of 245kpa to 441kpa.On the basis of both

stiffness and un-drained strength of clay. The effective angle of internal friction of

reinforced soil was reported to range from 20 degrees for unreinforced clay to 80.4

degrees for samples of fibre only.

Setty and Rao(1980) and Setty and Murthy(1990) carried out triaxial tests, CBR tests and

tensile strength tests on silty sand and black cotton soil reinforced with randomly

distributed polypropylene fibres. The results indicated that both the soils showed

significant increase in cohesion intercept and a slight decrease in angle of internal

friction (i.e. overall effect is to increase shear strength) with an increase in fibre content

up to 3% by weight.

Hausman (1976) hypothesized that the strength characteristics of reinforced sand

depends on the increased apparent friction angle. Brown (1977) observed that the

ultimate strength increased with decreasing the distance between the fabric discs and that

the discs which were placed at the two ends of the samples did not influence the ultimate

strength. Lee (1976) carried out a limited unconfined compression tests on compacted

12

clay samples reinforced with these narrow strips of malar type. The results showed an

increase in the ductility of sample but no increase in compression strength. The stress

strain relationships for various materials have been well documented by Holtz et al

(1982). The stress strain behaviour of reinforced soil depends on the type and form of

reinforcing elements and interactions between soil and metal strips.

Banerjee et al (2002) investigated the dimensional and mechanical properties of coir

fibres as a function of fibre length. A sufficient quantity of retted coir fibres was

collected from one particular pit in Kerala and subjected to certain tests to find out fibre

length distribution, determination of thickness, distribution of coir fibres in a husk,

tensile properties and flexural rigidity. From their studies it was concluded that the

length, thickness and linear density of fibres obtained from this particular type of husk

range from 50mm to 250mm, 130μ to 325 μ and 19tex to 60tex respectively. The longer

fibres are in general thicker than shorter ones. The fibre thickness is highly variable

along its length. The breaking load and work of rupture increases perceptibly for fibres

longer than 149mm. However the fibres shorter than 150mm exhibit very similar values.

The flexural rigidity of coir fibre that is longer than 149mm exhibit sharp increase with

the increase in fibre length.

Venkatappa Rao and Balan (2000) after conducting Drained triaxial test on specimens of

sand reinforced with coir fibres (25mm to 50mm) up to 1% reported a significant gain in

strength parameters and stiffness.

Varghese et al (1989) investigated the possibility of increasing the bearing capacity of

cohesion less soils by reinforcing with coconut fibres through model studies. It has been

observed that the bearing capacity of foundation soil will be the maximum when the

reinforcement is kept at a depth of 0.41 times the width of the foundation.

Guha (1995) reveal that coir fibre differs from jute fibres in an aspect other than

durability, jute fibres exhibit moderately high modulus as well as high tenacity and very

low elongation at break whereas coir fibres behave exactly in the opposite manner,

13

namely moderately low modulus, low tenacity and very high elongation break. This

difference persists irrespective of the length of coir fibre.

14

CHAPTER-3METHODOLOGY

The methodology adopted in this project work is as given below :

3.1. Characterization of materials used

3.2. Scheme of experiments

3.3 Procedure of experiments

3.1 Characterization of Materials

The test materials used in this work are:

Sand: The investigation was carried out on locally available Krishna river sand, from

Krishna District, Andhra Pradesh, which is rich in finer particles as comparing with

medium and coarser particles. It had a specific gravity of 2.67, coefficient of uniformity

(Cu) of 2.45 and coefficient of curvature (Cc) of 1.06.

The effective size of the particle (D10) is 0.57mm. The dry density of sand is 1.664

gm/cm3. The soil properties are shown below.

Table 3.1 Properties of Krishna River Sand

Specific gravity 2.67

Dry density (gm/cc) 1.664

OMC 14.00%

Uniform coefficient(Cu) 2.45

Coefficient of curvature(Cc) 1.06

D10(mm) 0.57

Classification of the soil: SW

15

Fibres: Polypropylene fibre is mixed with sand in various proportions. They were cut

into lengths of 10 mm, 20 mm and 30 mm to maintain the aspect ratios 5, 10 and 15

respectively. The width of fibre observed was 2 mm.

Picture 3.1 Polypropylene fibre in Planar sheet element form

Picture 3.2 Polypropylene fibre of Aspect Ratios 15, 10 and 5

16

Table 3.1 Properties of fibre used in this investigation

Product Name

SKAPS Nonwoven

Civil Geotextiles

GT-142 4.5 oz

Weight (g/sqm) 153

Width (mm) 2.00

Thickness (mm) 0.45

Grab Tensile strength (kN) 0.533

Grab Elongation (%) 50

Puncture Resistance (kN) 0.311

3.2 Scheme of Experiments

Table 3.3 CBR Values of unreinforced soil

Condition of the soil Un-soaked SoakedCBR Value

Table 3.4 CBR Values of reinforced soil (Fiber in planar element form)

D/B Ratio CBR ValueUn-soaked Soaked

0.250.501.001.50

Table 3.5 CBR Values of reinforced soil (Fiber in randomly distributed form)

17

Aspect Ratio % Substitution CBR ValueUn-soaked Soaked

5 0.251.001.502.00

10 0.251.001.502.00

151.001.502.00

3.3 Test Procedure of CBR

3.3.1 Equipment

• CBR mould, inside diameter=150mm, total height=175mm

• Spacer disc

• Rammers for heavy compaction(4.89kg, drop 450mm, 5 layers)

• Slotted masses

• Cutting collar, steel, which can fit flysh with the mould both inside and outside

• Penetration piston, 50mm diameter, 100mm long.

• Loading Device, capacity 50kN, equipped with a movable head at a uniform rate

of 1.25mm per minute.

• Two dial gauges, accuracy 0.01mm.

• IS Sieves, 4.75mm and 20mm size.

18

Picture 3.3 Accessories required for CBR Mould preparation

Picture 3.4 Rammer used in the compaction of soil sample (Heavy Compaction)

19

Picture 3.5 Compacting Apparatus used in the preparation of CBR Mould

3.3.2 Procedure

1. Sieve the sample through 20mm IS Sieve. Take the sample passing 20mm IS

Sieve for the test.

2. Take about 4.5 to 5.5kg of the sample; mix it thoroughly with water content equal

to optimum moisture content.

3. Insert the spacer disc over the base, with the central hole of disc at the lower face

after fixing the base plate and collar to the mould.

4. Take the soil sample in the mould. Compact it properly.

5. After compacting the soil, remove the collar and flush the soil surface to the level.

Now fix the collar to the mould and place a filter paper over the soil surface.

20

6. Place annular masses to produce a surcharge equal to the mass of base material

and wearing course of the pavement expected.

7. Immerse the mould assembly in a tank full of water and keep the mould in tank

undisturbed for 24 hours (for soaked condition).

8. Allow the mould to drain off for 15 minutes (soaked mould).

9. Place the mould containing the specimen, with the base plate in position.

10. Seat the penetration plunger at the center of the specimen to establish full contact

between the plunger and the specimen.

11. Apply the load on the plunger Keep the rate of penetration as 1.25mm/min.

12. Record the load corresponding to penetrations of 0.0, 0.5, 1.0, 1.5, 2.0, 2.5, 3.0,

3.5, 4.0, 4.5, 5.0, 5.5, 6.0, 8.0, 10.0 and 12.0mm.

Picture 3.6 CBR Test Apparatus

21

3.3.3 Calculations

Load (or pressure) sustained by the specimenat 2.5 or 5.0mm penetration

CBR% = *100

Load (or pressure) sustained by standard aggregateat corresponding penetration level

3.3.4 Standard values

The standard load values are obtained from the average of a large number of tests on

crushed stones.

Table 3.3 Standard load values for CBR test

Penetration(mm) Load(kg) Pressure(kg/cm2)

2.5

5.0

1370

2055

70

105

3.3.5 Reporting CBR Value

Normally CBR value at 2.5mm penetration which is higher than that at 5.0mm is reported

as the CBR value of the material. If the CBR value obtained from the test at 5.0mm

penetration is higher than that at 2.5mm, then the test is to be repeated for checking, if the

test again gives similar results, the higher value is reported as the CBR value.

The average CBR value of three test specimens is reported to the first decimal, as the

CBR value of the material. If the variation in CBR value between he three specimens is

more than the prescribed limits, tests should be repeated on additional three samples and

the average CBR value of six specimens is accepted.

22

3.4 Test Programme

The CBR tests were conducted on all specimens.

Picture 3.7 Testing the CBR Mould

23

Picture 3.8 Tested CBR Mould(Reinforcement in Randomly Distributed fibre form)

Picture 3.9 Soil sample removed from the mould after testing (Reinforcement in Randomly

Distributed fibre form)

24

Picture 3.10 Tested CBR Mould (Reinforcement in Planar Sheet Element form)

Picture 3.11 Soil sample removed from the mould after testing (Reinforcement in Planar Sheet

Element form)

25

CHAPTER-4RESULTS AND DISCUSSION

This chapter presents results of the experimental work carried out and discusses the same in detail.

Table 4.1 CBR Values of unreinforced soil

Condition of the soil Un-soaked SoakedCBR Value 28.00 23.70

Table 4.2 CBR Values of reinforced soil (Fiber in planar element form)

D/B Ratio CBR ValueUn-soaked Soaked

0.25 46.70 39.900.50 41.60 39.101.00 38.90 35.501.50 34.00 32.10

Table 4.3 CBR Values of reinforced soil (Fiber in randomly distributed form)

Aspect Ratio % Substitution CBR ValueUn-soaked Soaked

5 0.25 32.4 30.61.00 35.0 32.11.50 35.5 31.12.00 30.2 28.8

10 0.25 44.0 40.91.00 43.8 40.41.50 47.4 44.52.00 41.7 39.3

15 0.25 31.9 36.51.00 37.5 43.81.50 41.6 40.12.00 29.5 27.9

26

Analysis of results

Table 4.4 Reinforcement in planar sheet element form

D/B Ratio CBR ValueUnsoaked %

improvementSoaked % improvement

0 28 100 23.7 1000.25 46.7 166.79 39.9 168.350.50 41.6 148.57 39.1 164.981.00 38.9 138.93 35.5 149.791.50 34 121.43 32.1 135.44

Table 4.5 Reinforcement in randomly distributed fiber form

Aspect Ratio

% Substitution CBR ValueUnsoaked % improvement Soaked % improvement

5 0.25 32.4 115.71 30.6 129.111.00 35.0 125 32.1 135.441.50 35.5 126.79 31.1 131.222.00 30.2 107.86 28.8 121.52

10 0.25 44.0 157.14 40.9 172.571.00 43.8 156.43 40.4 170.461.50 47.4 169.29 44.5 187.762.00 41.7 148.93 39.3 165.82

15 0.25 31.9 113.93 36.5 154.011.00 37.5 133.93 43.8 156.541.50 41.6 148.57 40.1 169.22.00 29.5 110 27.9 126.16

4.1 Unreinforced soil

The CBR values for unreinforced sand were tabulated in the table 4.1.

4.2 Reinforced soil (Fiber in planar element form)

The CBR values for reinforced sand with planar sheet element fibre at D/B ratio of 0.25,

0.50, 1.00 and 1.50 were tabulated in the table 4.2. In general it is observed that inclusion

of fibre increased the CBR value of the soil both in soaked and unsoaked conditions,

which confirms the ability of fibre to strengthen the sands. The CBR plots obtained were

shown in Graph 5.3 to Graph 5.10.

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4.3 Reinforced soil (Fiber in randomly distributed form)

The CBR values for reinforced sand with randomly distributed fibre of aspect ratios 15,

10,5 and % substitution of 0.25, 1.00, 1.50 and 2.00 for each aspect ratio were tabulated

in the table 4.3. In general it is observed that inclusion of fibre increased the CBR value

of the soil both in soaked and unsoaked conditions, which confirms the ability of fibre in

randomly distributed form also to strengthen the sands.

28

CHAPTER-5OBSERVATIONS AND CONCLUTIONS

Based on the results obtained in this work, the following important observations are

made.

5.1 Observations relating to Planar Sheet Element form

As shown in the figure given below, placement of Geo-textile at any level up to 1.50 B,

where B is lateral dimension of the plunger of CBR apparatus, showed an improvement

in both un-soaked and soaked CBR values.

However, the maximum improvement of 167% was achieved when the single layer of

Geotextile was placed at 0.25 B. This clearly explains the fact that, the reinforcement

effect was maximum when the geotextile was placed at 0.25 B because it intercepts the

plastic zones more effectively.

Graph 5.1

DEPTH OF PLACEMENT OF PLANAR SHEET ELEMENT (Vs) % IMPROVEMENT IN CBR VALUE

0

50

100

150

200

0 0.25 0.5 1 1.5

DEPTH OF PLACEMENT AS (D/B) RATIO

% IM

PR

OV

EM

EN

T IN

CB

R

VA

LU

E

UN-SOAKED

SOAKED

29

5.2 Observations relating Percentage of Fiber

As shown in the figure given below, there is improvement in un-soaked CBR value when

randomly distributed fiber was mixed with sand.

However, the maximum improvement occurred for all aspect ratios when the fiber was at

1.50% by weight of soil solids. The maximum of maximum improvement of 169%

occurred when the aspect ratio was maintained as 10.

% OF FIBER (VS) % IMPROVEMENT IN UN-SOAKED CBR VALUE

0

50

100

150

200

0.25 1 1.5 2

% FIBER

% I

MP

RO

VE

ME

NT

IN

UN

-S

OA

KE

D C

BR

VA

LU

E

ASPECT RATIO=5

ASPECT RATIO=10

ASECT RATIO=15

Graph 5.2

30

Similar mechanism was observed pertaining to soaked CBR value as shown in the figure

given below :

% FIBER (VS) % IMPROVEMENT IN SOAKED CBR VALUES

0

50

100

150

200

0.25 1 1.5 2

% FIBER

% I

MP

RO

VE

ME

NT

IN

CB

R

VA

LU

E

ASPECT RATIO=5

ASPECT RATIO=10

ASPECT RATIO=15

Graph 5.3

The decrease in improvement when the fiber content was increased from 1.50 to 2.00

may be due to the fact that, at 2.0% the fiber quantity is higher enough to effect more

fiber-fiber interaction than fiber-sand interaction.

5.3 Observations relating to Aspect Ratio

As shown in the figure given below, maximum improvement for any percentage of fiber

occurred when the aspect ratio was at 10. This signifies the fact that, at aspect ratio, the

anchorage effect was optimum.

31

ASPECT RATIO (VS) % IMPROVEMENT IN UN-SOAKED CBR VALUE

0

50

100

150

200

5 10 15

ASPECT RATIO

% I

MP

RO

VE

ME

NT

IN

UN

-S

OA

KE

D C

BR

VA

LU

E0.25% FIBER

0.50% FIBER

1.50% FIBER

2% FIBER

Graph 5.4

5.4 Observations pertaining to effectiveness of reinforcement in Planar form to Fiber form:

The investigations carried out in this project work considered one woven geotextile only.

It is used in planar sheet element form as it is and in the fiber form after deriving fiber out

of the geotextile. Hence the material properties are same except the form. The results

showed maximum improvement in un-soaked CBR value to the extent of 168% in planar

element form and 169% in Randomly distributed form. However, in terms of weight of

reinforcement used in planar form is much lower than that in Randomly distributed form.

Hence, the investigations made in this project indicate effectiveness in sheet element

form.

5.5 Observations pertaining to soaked and un-soaked CBR values of reinforced sand

The investigations carried out in this project work clearly indicated effectiveness of

reinforcement function even under soaked conditions. Further, the difference between

soaked and un-soaked conditions was marginal signifying the fact that the materials used

in this work are non-plastic and hence plasticity of soils which influence the bearing

resistance in soaked conditions is not existing in the present reinforced sands.

32

5.6 Conclusions

Based on the investigations carried out in this project work the following conclusions are

made:

1 The CBR value can be improved by reinforcing the material.

2 The improvement in CBR value was observed when reinforcement

material was used in planar sheet element form as well as in randomly

distributed form.

3 The maximum improvement in un-soaked CBR value was 168% in planar

sheet element form when placed at a depth equal to 0.25 times size of the

plunger and 169% in randomly distributed form with an aspect ratio of 10

and at a fiber content of 1.50%.

4 The investigations showed that, reinforcement in sheet element form is

more economical and effective.

5 The investigations showed that, reinforcement effect is present even under

soaked conditions.

6 The investigations showed that when the materials are non-plastic, there

will not be serious drop in CBR value under soaked conditions.

33

Graph 5.5 Load vs Penetration for Unreinforced soil in Unsoaked condition

0

100

200

300

400

500

600

700

0 1 2 3 4 5 6 7 8

PENETRATION (mm)

LO

AD

(kg

)

LOAD (Kg)

34

Graph 5.6 Load vs Penetration for Unreinforced soil in Soaked condition

0

100

200

300

400

500

600

700

0 1 2 3 4 5 6 7 8

PENETRATION (mm)

LO

AD

(kg

)

LOAD (Kg)

35

REFERENCES

1. Al-Refeai, T. and Al-Suhaibani, A., 1998, Dynamic and Static Characterization of

polypropylene fibre-reinforced dune sand, Geosynthetics International, vol., 5, 443-

456.

2. Banerjee, P.K., R. Chattopadhyay, and A. Guha (2002) Investigations into

homogeneity of coir fibres, Indian journal of fibre and textile research, vol. 27.

3. Gray D. H and Al-Refeai (1986), Behaviour of fabric versus fibre reinforced sand,

Journal of Geotechnical Engineering, vol 112.

4. Radoslaw L.Michalowski and Aigen Zhao, Failure of fibre-reinforced granular soils,

Journal of Geotechnical Engineering, 1996, vol.122, 226-232.

5. Shashi K Gulhati and Manoj Datta, (2005), Geotechnical Engineering, Tata McGraw-

Hill publishing company Ltd. (565-568).

6. Scott, “Effect of material properties on compactability and bearing capacity”,

Transportation Engineering Journal, No.4, Vol 120, pp. 570-587.

7. Yong.R.N and B.P.Winterkorn, “Soil Properties and Behaviour”, Elsevier Scientific

Publishing Ci., Amster Dam, 1975.

8. Arora.K.R, “Soil Mechanics and Foundation Engineering”, 1980, pp.357-375, 773-778.

9. Khanna.S.K. and C.E.G.Justo, “Highway Engineering”, Nem Chand & Bros, Roorkee,

1984.

10. IRC: 37, “Guidelines for the Design of Flexible Pavements”, IRC, 1970

36