FIBER REINFORCED SOIL

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                        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.


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 mm-2)
Water absorption                     none
Softening point                       160° C


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 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 m-3.  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 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.

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.


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.


Fig.4 CBR (%) values of undisturbed peat and different percentage of OPC and polypropylene fibers for the stabilized peat soil cured for 90 days
                                         (S. A. Naeini et al., 2008)
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
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
(S. A. Naeini et al., 2008)
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

 
Table 3: Physical and engineering properties of fibers
(S. A. Naeini et al., 2008)
Type                       polymer
Length (mm)         7-12 mm
Cross-section         rectangular
Thickness (mm)     0.25 mm
Width (mm)           0.35 mm
Density (μg/m3)     1.15
 






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

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


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

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.

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 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.
                                                   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.

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.
(Mahmood R. Abdi et al., 2008)

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.


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.
(Mahmood R. Abdi et al., 2008)



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
(Mahmood R. Abdi et al., 2008)

            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.
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.

                                                         
 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 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°.


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.

 

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.



(a) Unreinforced                                                        (b) Reinforced
Figure 19 Variation of potentiometer reading with time.
(Dushyant Kumar Bhardwaj et al., 2008)
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°.
(Dushyant Kumar Bhardwaj et al., 2008)
*g = Earth’s gravity; Re = Effective radius; ω = Angular velocity; N = Scale factor

Table 9 Maximum vertical displacements obtained from centrifuge tests.



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.


 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.
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.