fiber reinforced soil-report

1. INTRODUCTION

The geotechnical engineers design foundations and other structures on the soil after

investigation of the type of soil, its characteristics and its extent. If the soil is good at

shallow depth below the ground surface, shallow foundation such as footings and

rafts, are generally most economical. However if the soil just below the ground

surface is not good but a strong stratum exist at a great depth, deep foundations, such

as piles, wells and caissons are required. Deep foundations are quite expensive and

are cost effective only in the where the structure to be supported is quite heavy and

huge. Sometimes the soil conditions are very poor even at greater depth and it is not

practical to construct even deep foundation. In such cases various methods of soil

stabilization and reinforcement technique is adopted. The objective is to improve the

characteristics at site and make soil capable of carrying load and to increase the shear

strength decrease the compressibility of the soil.

In the investigation done by S A Naeini and S M Sadjadi,(2008) ,the waste polymer

materials has been chosen as the reinforcement material and it was randomly included

in to the clayey soils with different plasticity indexes at five different percentages of

fiber content (0%, 1%,2%, 3%, 4%) by weight of raw soil.CBR tests are conducted by

Behzad Kalantari, Bujang B.K. Huat and Arun Prasad, (2010) and their experimental

findings are analysed with the point of view of use of waste plastic fibers in soil

reinforcement. Effects of Random Fiber Inclusion on Consolidation, Hydraulic

Conductivity, Swelling, Shrinkage Limit and Desiccation Cracking of Clays

(Mahmood R. Abdi, Ali Parsapajouh, and Mohammad A. Arjomand,(2008) ) point to

the strength and settlement characteristics of the reinforced soil and compared with

unreinforced condition.

Moreover an environmental concern is also included by utilization of waste

plastic materials and they can be made useful for improving the soil characteristics

and to solve problems related to the disposal of waste plastic material.

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2. LITERATURE REVIEWS

2.1 CBR TEST

Fiber reinforced soil is subjected to CBR test by Behzad Kalantari, Bujang B.K. Huat

and Arun Prasad, (2010) and the results are published.

2.1.1 Test materials

Peat soil used in the study were collected as disturbed and undisturbed samples

according to AASHTO T86-70 and ASTM D42069 (Bowles, 1978; Department of the

Army, 1980) from Kampung, Jawa, western part of Malaysia. Binding agent used for

this study was ordinary Portland cement and its properties are presented in Table

1.Polypropylene fibers, shown in Fig. 1 were used as chemically non-active additive.

Table 1: Properties of polypropylene fibers (Sika.com, 2007)

Property Specification

Color Natural

Specific gravity 0.91

Fiber Length 12 mm

Fiber Diameter 18 micron

Tensile strength 300-440 MPa

Elastic modulus 6000-9000 (N mm2)

Water absorption none

Softening point 160° C

Fig.1 polypropylene fibers, (Kalantari, 2009)

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2.1.2 Experimental program

In order to examine the effect of cement admixtures and polypropylene fibers on the

CBR values of peat soil, index properties tests on the peat soil have been conducted.

The tests include: water content, liquid limit, plastic limit, organic content, specific

gravity and fiber content. Shear strength parameters of the undisturbed peat soil has

been found out by triaxial test and shear strength is found out by unconfined

compressive strength. Rowe cell consolidation test has been carried out to evaluate

the compressibility behavior of undisturbed peat soil. The CBR test has been carried

out on the stabilized peat soil (mixture of peat cement and polypropylene fibers) to

investigate the increase in strength of the samples. Peat soil samples used for the CBR

tests were at their natural moisture contents and therefore no water was added or

removed from the samples during the mixing process of peat, cement and

polypropylene fibers.

2.1.3 California Bearing Ratio (CBR)

CBR tests have been conducted on the undisturbed peat soil as well as stabilized peat

soil with cement and polypropylene fibers. For the stabilized peat soil with cement

(mixture of peat soil and cement) the soil samples used were samples at their natural

moisture contents of about 200%. Specified dosage of cement and polypropylene

fibers were mixed well with the peat soil for uniformity and homogeneity, before

molding the samples according to the specified standard. Stabilized peat soil samples

with cement and polypropylene fibers were placed in the CBR mold for air curing for

90 days. CBR tests were performed on samples under both, un-soaked and soaked

conditions.

2.1.4 Curing procedure

In order to cure the stabilized peat soil samples with cement and polypropylene fibers,

air curing technique has been used. In this technique, the stabilized peat soil samples

for CBR tests were kept in normal room temperature of 30±2°C and relative humidity

of 80±5% without any addition of water from outside. This technique is used to

strengthen the stabilized peat soil samples by gradual moisture content reduction,

instead of the usual water curing technique or moist curing method which has been a

common practice in the past for stabilized peat soil mixed with cement . The principle

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of using this air curing method for strengthening stabilized peat is that, peat soil has

very high natural water content and when mixed with cement has sufficient water for

curing or hydration process to take place and does not need more water (submerging

the samples in water) during the curing process. The technique used for curing

samples will cause the stabilized peat soil samples to gradually lose moisture content

during the curing period and become dry and thereby gain strength.

2.1.5 Cement dosages

For CBR (un-soaked and soaked) tests, each sample consists of peat soil at its natural

water content added with 15, 25, 30, 40 and 50% cement by weight of wet soil, with

and without polypropylene fibers as an additive. The amount of polypropylene fibers

used for the stabilized CBR soil samples was based on the result obtained from CBR

tests to be carried out to determine the optimum percentage by weight of the wet peat

soil samples.

2.1.6 Percentage of polypropylene fibers

The usual dosage recommended for cement mixes varies from 0.6-0.9 kg m3. In

this study, in order to find the optimum percentage of fiber content for the stabilized

peat soil that would provide the maximum strength, peat soil samples at their natural

water content were mixed with different percentages of cement and polypropylene

fibers and were cured in air for a period of 90 days and then CBR test was performed

on them. The samples examined for this purpose were prepared by adding 5, 15 and

25% cement and 0.1, 0.15, 0.2 and 0.5% polypropylene fibers. The sample which

showed the maximum value of CBR after 90 days of curing was chosen as the

optimum percentage of polypropylene fibers for further evaluation of strength of the

stabilized peat soil.

2.1.7 CBR test procedure for soaked condition

According to AASHTO T193-63 and ASTM D1883-73, the soaking period of CBR

samples for normal soil is 96 h or 4 days (Bowles, 1978). For this study, in-order to

investigate the CBR values of the soaked stabilized peat soil, a set of CBR samples

prepared with different dosages of cement and polypropylene fibers (15, 25, 40 and

50% cement with 0.15% of polypropylene fibers) to soil at its natural water content

were cured in air for 90 days and then soaked in water for a period of 5 weeks. During

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these five weeks of soaking period, the soil samples were weighed periodically for

possible weight increase due to increased saturation. When the samples attained a

constant weight and no further increase in weight was observed, it was assumed that

the samples became completely saturated. The samples were weighed every day for

the first 2 weeks, every 2 days during the next 1 week and every 5days for the last 2

weeks.

Results

2.1.8 Optimum percentage of polypropylene fibers

The results of increase in CBR values for different cement and polypropylene fibers

content are shown in Fig. 2. It appears that the samples with 0.15% polypropylene

fibers gives the maximum percentage increase in of CBR value (ratio of obtained

CBR value/highest CBR value) after curing for 90 days.

Fig.2 Increase in CBR values-Different cement and polypropylene fibers content

(Ismail, 2002)

Based on the results obtained, it is possible to that 0.15% of polypropylene fibers as

chemically non-active additive would provide the maximum CBR values for the peat

soil stabilized with cement. Also, based on the result of this test, 0.15% of

polypropylene fibers have been chosen as an optimum amount for the stabilization of

peat soil samples.

2.1.9 CBR soaking test

According to the results shown in Fig. 3, stabilized peat soil sample with 15% cement

reached 100% saturation and therefore constant weight at the end of four days of

soaking period. On the other hand, the sample with the maximum amount of cement

(50%) reached constant weight (100% saturation) at the end of six days of soaking.

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Based on the results of this test, all stabilized peat soil samples were submerged in

water for at least 6 days before performing the CBR tests under soaked condition.

Fig.3 weight increase during soaking Soaking time

(S. A. Naeini et al., 2008)

2.1.10 Effect of stabilization on CBR value

The results of CBR tests for stabilized peat soil samples with cement and

polypropylene fibers after air curing for 90 days are shown on Fig.4 . The CBR value

of undisturbed peat soil is 0.785%. With the addition of 50% cement, it increased to

34% for unsoaked condition and 30% for the soaked condition. With the addition of

0.15% polypropylene fibers with 50% cement, this increased to 38% and 35% for

unsoaked and soaked conditions. The results indicate that as cement amount in the

mixture is increased, the CBR values also increase and addition of polypropylene

fibers causes a further increase of the CBR values. Polypropylene fibers as additive

contributes more strength to the stabilized peat soil samples.

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Fig.4 CBR (%) values of undisturbed peat and different percentage of OPC and

polypropylene fibers for the stabilized peat soil cured for 90 days


The air curing technique of peat soil stabilized with cement and polypropylene fibers

increased the general rating of the in situ peat soil from very poor (CBR from 0-3%)

to fair and good (CBR from 7 to above 20%) (Bowles, 1978). Also, visual inspection

of soaked CBR samples depict that the polypropylene fibers not only increase the

CBR values but also contribute towards the uniformity and intactness to the stabilized

peat soil samples, as compared with the soaked samples with cement only.

2.2 SHEAR STRENGTH TEST

S. A. Naeini and S. M. Sadjadi, (2008) published the journal "effect of waste polymer

materials on the shear strength of unsaturated clays" and being a receiver of their

journal, their tests and results are analyzed.

2.3.1 Tested materials

Three clayey soils with different plasticity indexes used in the present experimental

testes were obtained from the three parts of Iran named as (soil A, soil B, soil C) They

are defined as high plasticity soils (CH) according to the Unified Soil Classification

System.

The grain-size distribution and engineering properties of the collected soils are

presented in table 2. The polypropylene fibers are shown in fig.5, fig 6.The rubber

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fibers used in this study were obtained from polymer west materials. The scrap tire

rubber fibers were supplied by local recapping Track Tyres producer in Qazvin city of

Iran.

Fig.5 Waste plastic strips Fig.6 Waste Tyre RubberChips


These fibers reproduced by shaving off the old tires into 150 mm and smaller strips

and then ground into scrap rubber. The product specifications of the polymer fibers

are given in Table 3.

Table 2: Engineering properties of collected soils


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Table 3: Physical and engineering properties of fibers


2.3.2 Testing program

This experimental work has been performed to investigate the influence of Plasticity

Index and percentage of waste polymer materials on the shear strength of waste

polymer materials on the shear strength of unsaturated clayey soils. For this purpose,

clayey soils with different plasticity Indexes were used and mixed with different

percentage of waste materials to investigate the shear strength parameters of

unreinforced and reinforced samples in terms of direct shear test.

In order to determine the shear strength parameters (C and φ) of unreinforced

and reinforced samples, a series of shear box tests at vertical normal stresses of 100-

300 KPa and strain rate of 0.2% mm/min were carried out in accordance with

ASTMD 3080.shear stresses were recorded as a function of horizontal displacement

up to total displacement of 17 mm to observe the post failure behavior as well.

Verification tests were also performed in order to examine the repeatability of the

experiments.

2.2.3 Results and Discussion

The shear stress-horizontal displacement curves obtained from the tests for reinforced

and unreinforced soils with the fiber content of 2% at normal stresses of 200 are

shown in Fig.7. It is seen that initial stiffness at the same normal stress for reinforced

and unreinforced soils remains practically the same. Therefore fiber reinforcements

have no discernible effect on the initial stiffness of the soils. It can be also seen that

the peak shear stresses are significantly affected by fiber content especially at high

normal stresses.

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Type polymerLength (mm) 7-12 mmCross-section rectangularThickness (mm) 0.25 mmWidth (mm) 0.35 mmDensity (μg/m3) 1.15

Fig 7 shear stress-horizontal strain for unreinforced and reinforced Soil B

with 2% fiber content. (S. A. Naeini et al., 2008)

The values of shear strength (τ) cohesion (c) and internal friction angle (φ) for both

unreinforced and reinforced soils obtained from tests showed that the addition

amounts of fiber have the significant influence on the development of cohesion and

internal friction angle and similar trends are found in three suit type with different

Plasticity Indexes.

It is indicated from Fig.8 that the variation of cohesion with percentage of fiber

content is a non-linear variation. The cohesion of fiber specimens increases while

increasing fiber content up to 2% and then decreases slightly with addition amounts of

fibers.

Fig.8 Effect of fiber content on cohesion of soils A, B and C.


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The increase in cohesion of soil-fiber matrix may be due to the increase in the

confining pressure because of the development of tension in the fiber, and the

moisture in the fiber helps to form absorbed water layer to the clay particles, which

enables the reinforced soil to act as single coherent matrix of soil fiber mass.

The decrease in cohesion of soil-fiber matrix with addition amount of fibers

(more than 2% fiber content) may be due to separation of clay particles due to the

addition of fibers. The maximum cohesion is observed at 2% fiber content as 110 kPa

for soil-A which is 1.12 times more than that of unreinforced samples, and 168 kPa

for soil-B which is a.05 times more than that of unreinforced samples and 194 kPa for

soil-C which is 1.04 times more than that of unreinforced samples.

These results showed that fiber reinforcement have more effect on soils with

low Plasticity Indexes. The variation of internal friction angle with fiber content,

illustrated in Fig.9 As seen, the variation of internal friction angle with tire rubber

fibers contents in showed a non-liner variation.

In general the internal friction angle value of each reinforced samples increased, and

these values in soil-A ranged from 27.3° to 37.4°, in soil-B ranged from 20.35° to

25.64°, and in soil-C ranged from 17.5° to 25.3°.

Fig.9 Effect of fiber content on friction angle of soils A, B and C.


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The effects of scrap tire rubber fibers on shear strength values of clayey soils

are given in Figure 10.for soil A, B and C respectively. The contents of fiber played

an important role in the shear strength. Figure 9 indicate that the shear strength values

of clayey soil-fiber mixtures have a tendency to increase first, after a peak value, the

shear strength values of these mixtures decrease. It was found that the shear strength

values of unreinforced samples increased due to the raise of 2% tire rubber fiber

content from 142 to 177 kPa, from 189 to 210 kPa, and from 210.7 to 229 kPa for the

clayey soils A,B and C, respectively.

The maximum shear strength value of soil-A (soil with lower Plasticity Index) being

177 kPa is 1.24 times more than that of unreinforced samples. These findings

indicated that the optimum tire rubber fiber content based on shear strength values is

2%.

Fig.10 Effect of fiber content on shear strength of soils A, B and C.


Materials used for consolidation, swelling, shrinkage, desiccation and

hydraulic conductivity tests

Mahmood R. Abdi, Ali Parsapajouh, and Mohammad A. Arjomand, (2008)

experimentally investigated the effect of waste polymer fibers in the soil stabilization

of soil by conducting consolidation test, swelling test, shrinkage limit, desiccation

cracks and hydraulic conductivity test.

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Soil Type: A soil comprised of a mixture of kaolinite and montmorillonite was used

in this research. Preliminary investigations conducted by the authors showed a

mixture of 75% kaolinite and 25% montmorillonite to be suitable. Not only it was

workable, it also showed pronounced consolidation settlement, swelling, hydraulic

conductivity, shrinkage limit and desiccation cracking characteristics. In order to be

brief, instead of referring to the above composition, the word "soil" is used here after.

All soil particles passed No. 200 sieve and hydrometer test data indicated 98%

passing 0.071mm, 82.6% passing 0.036mm, 76.6% passing 0.021mm, 50.1% passing

0.009mm and 15.3% passing 0.001mm. Atterberg limits (ASTM D: 4318-87) and

specific gravity (ASTM D: 854-87) tests were also carried out on representative

samples. The soil had a liquid limit of 110(%), plastic limit of 29(%), plasticity index

of 81(%), shrinkage limit of 21(%) and specific gravity of 2.68.

Fiber Type: Most of the researches carried out on fiber reinforcement of soils have

made use of polypropylene fibers. This is the most commonly used synthetic material

mainly because of its low cost and the ease with which it mixes with soils [19, 21, 23,

24]. Miller and Rifai [25] also reported that polypropylene has a relatively high

melting point ( ≈ 160°C), low thermal and electrical conductivity, high ignition point

(≈ 590°C), with a specific gravity of 0.91. It is also hydrophobic and chemically inert

material which does not absorb or react with soil moisture or leachate. Therefore, to

be consistent with earlier researches carried out, bearing in mind the foregoing

characteristics, polypropylene fibers having 5, 10 and 15mm lengths and contents of

1, 2, 4 and 8% by dry weight of soil were adopted in this research. Preliminary

investigations showed that longer and higher fiber contents could not be effectively

mixed with the soil and therefore were not investigated.

2.3 CONSOLIDATION TEST

2.3.1 Test procedure

In order to assess the effect of random fiber inclusion on consolidation settlement,

swelling and hydraulic conductivity, oedometer tests were Conducted according to

ASTM D2435-96. Earlier research conducted by Nataraj and McManis [21], Abdi and

Ebrahimi [23] and Miller and Rifai [25] had shown that fiber addition has little or no

effect on compaction characteristics. For that reason, in the current investigation all

samples were prepared using the same dry density and molding moisture content

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equal to 70% of the liquid limit. Initially several kilograms of kaolinite and

montmorillonite were weighed and thoroughly mixed in dry form by appropriate

proportions of 75 and 25 percent respectively. The soil was kept in a container and all

samples were subsequently made using the same mixture. For each particular mixture

initially enough soil and appropriate amount of fiber were weighed and thoroughly

dry mixed. Then, water was gradually added and mixing continued until a uniform

mixture was obtained. Samples were then molded directly into the confining ring and

tested according to ASTM standard procedure. Pressure increments of 50, 100 and

200kPa were used and verification of the results was assessed by randomly selecting

and testing duplicate samples of some mixtures. A maximum difference of 5% was

observed in results of duplicate samples tested which were considered acceptable.

2.3.2 Consolidation Settlements Results

Effects of random fiber inclusion on consolidation settlement of soil samples were

evaluated as function of fiber length, content and consolidation pressure. These

relationships are shown in Figures 1, 2 and 3 for fiber lengths of 5, 10 and 15mm

respectively. Prior to the fiber inclusion, consolidation settlement of unreinforced soil

sample was determined. This settlement is also shown in the above figures to be used

as a reference behavior for comparison with those from different fibrous samples. It

can be observed from Figures 1, 2 and 3 that at a Constant pressure, increasing the

fiber contents from 1 to 8% resulted in reducing consolidation settlement of the

samples. This is a common trend with all fiber lengths examined. Maximum and

minimum consolidation settlements of 7.5 and 2.6 mm were respectively measured

for the unreinforced sample and the sample reinforced by 8% fibers having 5mm

length (e.g., “Fig. 11”). This shows a reduction in consolidation settlement of

approximately 25%. Although increasing the fiber length from 5 to 10mm resulted in

slightly higher consolidation settlements, but in general this soil characteristic did not

appear to be very sensitive to the fiber lengths. It can be speculated that random fiber

inclusion resulted in increasing stiffness of the samples and subsequently reduced the

consolidation settlements.

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Fig.11 Variations of consolidation settlement with

Fiber content (Fiber length=10mm).

(Mahmood R. Abdi et al., 2008)

To support this speculation, laboratory triaxial compression tests conducted by

Consoli et al. [26] on fiber reinforced soils also showed a greater than 20%. In

contrast, unreinforced samples demonstrated an almost perfectly plastic behavior at

large strain. Their field plate load test results also showed a noticeable stiffer response

with increasing settlement. This potential applications of fiber reinforced soils in

shallow foundations, embankments over soft soils, and other earthworks that may

suffer excessive deformations. From the above figures it can also be seen that at

constant fiber contents, for all fiber lengths investigated, higher pressures resulted in

greater consolidation settlements. This is mainly attributed to the higher excess pore

water pressures initially generated and subsequently dissipated. Higher pressures also

grant greater potential for soil particles to slip and rearrange relative to each other,

resulting in greater deformations or settlements.

2.4 SWELLING TEST

Oedometer was used for swelling saturated on molding; they showed no affinity for

further water absorption after flooding the oedometer water bath. Therefore, they did

not exhibit much free swelling in order to be able to assess the effects of fiber

inclusions on this characteristic. Therefore, volume changes during the unloading

stage of the consolidation tests were measured and used as an indication of the

possible effects of fiber inclusion on swellings. The swellings presented were

measured after unloading the maximum consolidation pressure of 200kPa.

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2.4.1 Test result

The relationship between swelling and fiber content and length are presented in

Fig.12. It can be seen that by increasing the fiber content, the amount of swellings

decreased. The unreinforced sample produced the highest swelling of about 3.4mm.

This was reduced to approximately 1.5mm for the sample reinforced with 8% fibers

having 5mm length which is a substantial reduction in swelling. For constant fiber

contents, an increase in the fiber length from 5 to 10mm resulted in a slight increase

in swelling.

Fig.12 Variations of swelling with fiber content and length.


As a whole, however, the increase in the fiber length did not have a significant effect

on swelling reduction. This was particularly true when the fiber contents remained

constant. It can therefore be concluded that with the increase in fiber contents and

lengths, the soil/fiber surface interactions were increased. This resulted in a matrix

that binds soil particles and effectively resists tensile stresses produced due t swelling.

Resistance to swelling is mainly attributed to cohesion at the soil/fiber interfaces.

Puppala and Musenda [22] have reported that fiber reinforcement reduces the

swelling pressures in expansive soils. Reduced swelling pressures result in less

volumetric changes, which is exactly what has been observed in this investigation.

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2.5 SHRINKAGE LIMITS

Shrinkage limits of fiber reinforced and unreinforced samples were investigated using

the test procedure outlined in ASTM D4943-02. Because of standard sample size

limitations and the difficulty in soil-fiber mixing to obtain uniform distribution of

fibers within the soil, shrinkage limits of specimen reinforced with 8% fibers and

varying lengths could not be determined.

2.5.1 Test result

Variations of the shrinkage limits as function of fiber content and length are shown in

Fig. 13. It can be seen that increasing fiber contents and lengths resulted in increasing

the shrinkage limit of the samples. The resulted increase in the shrinkage limits

became more pronounced by increasing fiber length from 5 to 10mm as compared to

when it changed from 10 to 15mm. The shrinkage limit determined for the

unreinforced sample was approximately 21%. This was increased to 33% for the

sample reinforced with 4% fibers having 15mm length. This significant increase

means that samples reinforced with random inclusion of fibers experienced less

volumetric changes due to desiccation. Increase in the shrinkage limits means that

longer fibers having greater surface contacts with the soil have shown greater

resistance to volume change on desiccation. It can be said that random fiber inclusion

improved the soil tensile strength very effectively, thus resisting shrinkage on

desiccation.

Fig.13 Variations of shrinkage limit with Fiber content and length.


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2.6 DESICCATION CRACKS

Oedometer rings were used to investigate the effects of random fiber inclusion on

desiccation cracking of the soil. After molding, confining rings containing the

specimen were placed in open air in the laboratory at a temperature of about 30°C.

Samples were regularly weighed and when no changes in three consecutive

measurements were observed, they were considered completely dried. Then, samples

were used for observational examination of the extent of cracking.

2.6.1 Test result

Observational examination of samples after desiccation showed that by increasing the

fiber contents and lengths, the extent and depth of cracks were significantly reduced.

As an example, in Fig.14 surface cracking features of the unreinforced sample and the

sample reinforced with 8% fibers of 10mm length are shown for comparison.

Fig.14 Desiccation cracking:

(a) Unreinforced sample (b) Reinforced sample


It can be seen that extensive, deep and wide cracks were formed in the

unreinforced sample. The reinforced sample, however, has mainly experienced

separation from the metal ring with no visible sign of cracks forming within the

sample. This clearly shows the effectiveness of random fiber inclusion in resisting and

reducing desiccation cracking which is of paramount importance in surface cracking

of clay covers used in landfills. Therefore, it can be concluded that random fiber

inclusion seems to be a practical and effective method of increasing tensile strength of

the clayey soils to resist volumetric changes.

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2.7 HYDRAULIC CONDUCTIVITY

The relationship between hydraulic conductivity and fiber content is presented in

Fig.15. The hydraulic conductivity of the fibrous soil is dependent on the fiber

content, generally increasing with fiber content increase. The slight decrease of

hydraulic conductivity noted around 0.2% fiber content is within the limits of

experimental error, and should not be used to infer that minor fiber additions improve

the hydraulic conductivity. The increase in hydraulic conductivity was most

significant for fiber contents exceeding 1%.

Fig.15 : Hydraulic conductivity for various fiber contents.

(Carol J. Miller et al., 2004)

3 LITERATURE REVIEW ON MODEL ANALYSIS

Dushyant Kumar Bhardwaj and J.N.Mandal conducted a model analysis on the fiber

reinforced soil when subjected to centrifuge modeling and their response was noted.

3.1 PREPARATION OF THE MODEL

Centrifuge tests were performed on fly ash without and with fiber reinforcement at

slope angle, θ = 78.6°. Front and back sides of the container were covered with glass

plates. silicon grease was applied in the inner sides of the glass plates to minimize

the effect of friction. Figure 20 shows the dimensions of the slope model used in the

test for θ = 78.6°. Width of the model taken was 7.5 cm. Remaining portion was

covered using geofoam pieces. To minimize the friction in between the soil and

geofoam, plastic sheets were used, after applying silicon grease. All samples were

made at optimum moisture content. Because the height and the base width of slope

models were fixed due to restriction of container dimensions, therefore other

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dimensions of the slope models were taken in such a way that the inclination of slope

will remain 78.6°. All three potentiometers were adjusted in such a manner that their

locations were 2.5 cm, 4.0 cm and 5.5 cm respectively from the back face of sample.

No surcharge was used in this case; the sample was allowed to fail under self weight,

by increasing the RPM.

Fig.16 Dimensions of the slope model used in centrifuge test, for θ = 78.6°.

(Dushyant Kumar Bhardwaj et al., 2008)

3.2 TEST PROCEDURE

To observe the effect of fiber reinforcement in fly ash slope models all the centrifuge

tests were performed at 80 % compaction effort and all the necessary properties of fly

ash were calculated at 80 % compaction. Polypropylene fibers were mixed in the soil

1 % by dry weight of soil and water was taken according to the optimum moisture

content. After mixing the fiber in the soil at optimum moisture content, samples were

taken in three different and equal parts. Each part was compacted such that its width

should remain 2.5 cm to make the total width as 7.5 cm.

3.3 CENTRIFUGE MODELING

Small centrifuge present in IIT Bombay was used for the experiments. It is a

balanced beam type centrifuge. Potentiometers were used in the experiments to

measure the vertical displacements of the slope models.

Reading obtained from these potentiometers were not the actual displacements of the

slope models. To find out the actual displacements of the slope models, first these

potentiometers were calibrated.

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Unreinforced Soil

Fig. 17 (a) Before Failure (b) After Failure

Fig.17 Slope model for unreinforced soil, before failure at θ = 78.6°.

(Dushyant Kumar Bhardwaj et al., 2008)Figure 17 (a) and (b) show the unreinforced fly ash slope model before and after

failure (at θ = 78.6°) respectively. Data obtained from the centrifuge test, shows that

unreinforced slope fails at an angular velocity of 440 rpm and after 851 seconds from

the beginning of the test. Scale factor of unreinforced slope at 440 rpm was 50.

Reinforced Soil

(a) Before Failure (b) After Failure

Fig.18 Slope model for polypropylene fiber reinforced soil,

Before failure at θ=78.6°.


Figure 18 (a) and (b) show the polypropylene fiber reinforced fly ash slope model

before and after failure (at θ =78.6°) respectively.

Data obtained from the centrifuge test, shows that polypropylene fiber reinforced

slope achieves the angular velocity equal to that of unreinforced soil i.e. 440 rpm after

825 seconds from the beginning of the test. And finally polypropylene fiber

reinforced slope failed at 722 rpm after 1833 seconds from the beginning of the test.

Scale factor of polypropylene fiber reinforced slope at 722 rpm was 134.

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(a) Unreinforced (b) Reinforced

Figure 19 Variation of potentiometer reading with time.


With the help of potentiometer reading v/s time graph, reading of first potentiometer

at 852 seconds was 1.3 mm. From the calibration curve of first potentiometer, actual

displacement of model was 2.90 mm. For reinforced soil, with the help of

potentiometer reading v/s time graph, reading of first potentiometer at same scale

factor as that of unreinforced soil was 0.55 mm. From the calibration curve of first

potentiometer, actual displacement of model was 1.9 mm. After multiplying this

model displacement with the scale factor, prototype displacement was 95 mm. Results

of centrifuge tests and maximum vertical displacements for unreinforced and

reinforced soil are given in Table 8 and Table 9 respectively.

Table 8. Centrifuge test results at θ = 78.6°.


*g = Earth’s gravity; Re = Effective radius; ω = Angular velocity; N = Scale factor

Table 9 Maximum vertical displacements obtained from centrifuge tests.


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3.4 FACTOR OF SAFETY

Factor of safety of the slope models were found out by using student version of

software GEOSLOPE. This software uses the limit equilibrium theory to compute the

factor of safety of earth and rock slopes. Simplified Bishops method was used in

analysis the factor of safety. For the comparison of factor of safety between

unreinforced and reinforced slopes, factor of safety of all slope models were found out

at the same scale factor as that of unreinforced slopes. Values of minimum factor of

safety obtained from Bishop’s Method are given in Table 10.

Table 10 Factor of safety (FOS) obtained from Bishop’s Method.


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4. CONCLUSIONS

From a critical receiver of literature on the use of randomly distributed waste plastic

fibers for the stabilization of soil which are having very poor strength characteristics,

the following conclusions are drawn:

1. The soils are reinforced with randomly distributed polypropylene fibers

and the CBR values obtained for this type of soil is around 38% high than

the unreinforced soil. For the CBR test we have used cement as a binder,

even though the percentage of cement is very high fiber content is

responsible for the increase in CBR value.

2. The value of cohesion also increases due to the inclusion of fiber. The

variation of cohesion with percentage of fiber content is observed to be

non-liner . The value obtained for cohesion (c) indicates that soil obtained

is of very stiff nature.

3. In general angle of internal friction increased with fiber content. The

variation of with percentages of fiber contents leads to a conclusion that

the behavior of the fiber included soil can be non-liner variation because

the reinforcement materials exhibited a distribution with horizontal and

vertical directions to the shear surface.

4. The shear strength of fiber reinforced soil is improved due to the addition

of the waste polymer fibers and it is a non linear function. Up to a critical

fiber content shear strength increased considerably and later small

reduction is observed. However shear values are greater than unreinforced

soil.

5. The soil stabilization with waste fibers improves the strength behavior of

unsaturated clayey soils and can potentially reduce ground improvement

costs by adopting this method of stabilization.

6. The addition of randomly distributed polypropylene fibers resulted in

substantially reducing the consolidation settlement of the clay soil. Length

of fibers had an insignificant effect on this soil characteristic, where as

fiber contents proved more influential and effective.

7. With increase in fiber content the swelling after unloading is reduced to

almost half of the unreinforced situation. At constant fiber content the

length of fiber does not have much effect on swelling.

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8. The shrinkage limit is showing a rising graph with both the increase in

fiber content and fiber length. It indicates that the soil is susceptible to less

volume change and it has got enough tensile strength with reinforcing.

9. Fiber reinforcement significantly reduced the extent and distribution of

cracks due to desiccation as observed by the reduced number, depth and

width of cracks. These results show that it can be used for covering waste

material in containments and also can be used for canal slopes.

10. Hydraulic conductivity is increasing with fiber content up to particular

limit.

11. Centrifuge modeling gives a clear idea about the performance of the fiber

reinforced soil and it points to the vast scope of this method of reinforcing

soil with waste plastic fibers.

12. The most important point is the environmental concern regarding the

effects of waste plastic in soil and the problems and threats that is related

with their excessive usage and disposal. This gives an effective solution to

waste treatment with the advent of soil reinforcement.

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5. REFERENCES

1. Carol J. Miller and Sami Rifai, (2004), “Fiber Reinforcement for Waste

Containment Soil Liners”, (ASCE) Journal,(1-5).

2. Behzad Kalantari, Bujang B.K. Huat and Arun Prasad, (2010) ,” Effect of

Polypropylene Fibers on the California Bearing Ratio of Air Cured Stabilized

Tropical Peat Soil ” , American J. of Engineering and Applied Sciences,(1-6).

3. Mahmood R. Abdi, Ali Parsapajouh, and Mohammad A. Arjomand,(2008),”

Effects of Random Fiber Inclusion on Consolidation, Hydraulic Conductivity,

Swelling, Shrinkage Limit and Desiccation Cracking of Clays”, International

Journal of Civil Engineering, Vol. 6, No. 4, (284-292).

4. S. A. Naeini and S. M. Sadjadi ,(2008) ,” Effect of Waste Polymer Materials on

Shear Strength of Unsaturated Clays”, EJGE Journal, Vol 13, Bund k,(1-12).

5. Dr. D S V Prasad, Dr. G V R Prasada Raju and M Anjan Kumar,

(2009),“Utilization of Industrial Waste in Flexible Pavement Construction”,EJGE

Journal,vol 13,Bund d,(1-12)

6. Pradip D. Jadhao and P.B.Nagarnaik, (2008),” Performance Evaluation of Fiber

Reinforced Soil- Fly Ash Mixtures”, The 12th International Conference of

International Association for Computer Methods and Advances in Geomechanics

(IACMAG). Goa, India,(1-10)

7.Dr. K R Arora ,”soil mechanics and foundation engineering”, published by Standard

Publishers Distributors , Delhi.

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fiber reinforced soil-report

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