Distribution of fibers in different part of reinforced specimen.
A clayey soil reinforced with commercially obtainable20 mm glass fiber of varying fiber content (fc = 0.25 to 1% by soil dry weight) was investigated in lab for its possible application as road pavement material. Standard proctor compaction, unconfined compression strength (UCS), California Bearing Ratio (CBR) and undrained triaxial compression tests were conduction on compacted soil-fiber specimens as per ASTM standard. From the fiber mixing process it has been observed that fiber can be uniformly mixed into clayey soil only up to some optimum fiber content. Laboratory test results predicted that UCS, CBR and shear strength value of clayey soil enhanced significantly with fiber content up to some optimum value of 0.75% fiber content. The UCS increases maximum up to two fold, CBR by 2.8 times and shear strength by around 1.75 times than that of clayey soil alone. The inclusion of glass fibers enhances the ductility of clayey soil and modifies its failure pattern from brittle to ductile. It has been found that the glass fiber reinforced clayey soil can be used significantly as a subgrade material for low volume flexible road pavement.
- glass fiber
- UCS test
- CBR test
- shear strength
- subgrade material
The increasing need of construction areas for infrastructure facilities like roads, rails, etc. that spread over large spaces confines the choice of neglecting those locations where poor soil is available. This open the scope of strengthening the available soil by using some ground improvement method. Ground improvement methods address many ground conditions problem and help in modifying the engineering aspects of available soils as per the requirements. These techniques also help in obtaining economical and environmental friendly solutions to mitigate the issues related to soil for construction purpose. Some basic ground improvement techniques including densification, dewatering and use of admixtures and reinforcement are being adopted from ancient times.
1.1 Ground improvement methods
Several ground improvement techniques are currently in use to improve the engineering properties of soils. These modification techniques have been divided into several categories .
Mechanical modification: This includes physical modification of soil and can be carried out by means of controlled densification either by placement and compaction of soil or in-situ methods of soil improvement for deeper application. This includes static and dynamic compaction, and vibro-compaction. This method is most suitable for granular soils.
Hydraulic modification: This involves the modification of flow, seepage and drainage characteristics of soil. This is done by lowering water table, decreasing or increasing soil permeability, consolidation and preconsolidation by using vertical drains to minimize settlement and compressibility and increasing overall strength.
Physical and chemical modification: This deals with the stabilization of soil by physiochemical changes of the soil structure. This includes physical mixing of some chemical or additive material like cement, lime, industrial wastes (fly ash, ground granulated blast furnace slag etc.), injection of grouting materials, bioremediation of soil, thermal treatment.
Modification by inclusion, confinement and reinforcement: This includes application of some other manufactured materials within the soil mass. This involves use of reinforcement as tension resisting element in different forms known as soil-reinforcement. This also includes soil nailing, soil anchor and inclusion of stone column.
The above mentioned soil modification techniques are not limited to any particular type of soil. It can be adopted for any soil depending on their suitability and ease of field applicability, economic constraints along with the availability of resources for their implementation on any particular site.
1.2 Soil reinforcement
Among several ground improvement methods, soil reinforcement is an effective and dependable method for upgrading the strength and stability of various civil engineering construction practice including pavement, embankment, retaining structures, foundations and slopes. Reinforced soil is a composite mass in which tension resisting elements in different forms (geosynthetics, fibers etc.) are embedded to increase the strength, stiffness, compressibility and permeability of soils. After the earliest reinforcement in the form of galvanized steel strips of high tensile modulus, use of synthetic materials named as geosynthetics in different forms (geogrid, geotextile, geocomposite etc.), and of natural products (bamboo, jute, and coir) are being adopted in the form of sheets or meshes. In most applications, the conventional method of soil reinforcement is in a continuous planer form introduced within the soil mass in a definite pattern, resulting in the systematically reinforced soil . The one-dimensional orientation of reinforcement is installed sequentially in alternating layers as per the design requirements of the structure.
1.3 Fiber-reinforced soil
Fiber-reinforced soil has gained popularity in around last 35–40 years  where flexible, discrete fibers are being mixed within soil mass. Fibers act like tension resisting element which cause significant amendment in the various engineering aspects of soil including strength, stiffness, compressibility, permeability. Unlike conventional soil reinforcement methods, fiber-reinforced soil maintain strength uniformity within the soil mass by evading the generation of any weak plane during field placement. Fibers are available in abundance in natural and waste form, and also manufactured in desired properties known as synthetic fibers. Utilization of waste fibers (tyre derived fibers, plastic waste fibers etc.) for civil engineering work can help in solving disposal problems which will be cost effective and also help in enriching the environment.
The method of fiber reinforcement in soil is being used from ancient times where natural fibers in the form of straw were mixed in the soil brick to provide integrity by arresting the crack development . The curiosity of fiber-reinforced soil in last century started by Waldron  when he investigated the effect of roots of plant and tree on the earth slope stability. With increasing attention, fiber reinforced soil is increasingly providing an option of its use behind retaining structure as backfill material , construction of embankments [5, 6, 7], slopes stabilization , earth retaining constructions  and clay liners .
The use of fibers in natural and synthetic form like coir, jute, wool, steel, nylon, polyester, polypropylene, and fiber glass as tension elements for various soil have been reported by other investigators by means of unconfined compression, CBR, direct shear and triaxial compression tests in the last 35–40 years . However, the preliminary works was largely on fiber-reinforced sand where the influence of the key aspects such as fiber concentration, fiber aspect ratio, soil compaction level and testing environments on the overall performance of fiber-reinforced sand was studies [11, 12, 13, 14, 15].
The effects of fiber inclusion on clayey soils have been explored by direct shear tests [16, 17, 18], triaxial compression tests [19, 20, 21, 22], unconfined compression tests [23, 24, 25, 26, 27, 28, 29], tensile strength tests , fiber pullout tests  and CBR tests [32, 33, 34]. The common findings of the past investigations on fiber-reinforced soil are that the fiber inclusion increases the stress–strain responses, UCS, soil ductility and CBR, and modify the post-peak strength reduction of soil. The inducement of shear strength happens up to some controlling fiber concentration and fiber length.
The fiber benefits depend on the bond strength and surficial interaction between soil and fiber . The soil particle size also influences the shear strength of fiber reinforced soil . Fiber reinforcement had also effectively reduced the amount and degree of desiccation and tension cracks development, suppressed the swelling potential, and increased the permeability of clay soils [36, 37, 38]. It has been noted that the compressive strength of fiber-reinforced soil is highly controlled by the size of specimen  and compaction state [28, 29].
In this present study, an attempt has been made to investigate the effect of glass fiber inclusion on the strength aspect of a clayey soil for its possible suitability for road pavement construction. The investigation has been carried out by conducting compaction, UCS, CBR and triaxial compression test by varying fiber content.
2. Materials and methodology
Locally available clayey soil was found form the nearby hill slope in the outskirt of Guwahati city of Assam state in India. The particle size distribution curve of the tested soil is presented in Figure 1. The soil confined 25%, 54% and 21%, sand, silt and clay size particles, respectively. The soil had 46% liquid limit, 25% plastic limit value. As per Unified Soil Classification System (USCS) according to ASTM D2487 , the soil was classified as low plastic clay (CL). The coefficient of uniformity and the coefficient of curvature based on the gradation curve were 12.5 and 3.125, respectively. The optimum moisture content (OMC) and maximum dry unit weight (MDU) values of the soil were 19.4% and 16.8 kN/m3, respectively as per ASTM D698 .
Glass fiber of 20 mm length nd 0.15 mm average diameter was used as reinforcement (Figure 2). The glass fiber has specific gravity and water absorption capacity as 2.57 and zero respectively. The modulus of elasticity, tensile strength and elongation at break of the glass fiber were 112.3 GN/m3, 1.53 GN/m2, and 1.8%, respectively. As glass fiber has higher stiffness, strength, high ratio of surface area to weight, dimensional stability , and is ready available and non-biodegradable , it can be more valuable for long-term soil remediation. Glass fiber has also been found to retain its elastic modulus and tensile strength at 70–75% of that of raw fibers even under 450°C temperature  and thus will be suitable for the country where environmental temperature becomes high in the range of 50° in summer.
2.3 Specimen preparation
Designated weight of dry soil, fiber and water was taken and mixed in a steel tray. At first, the dry soil was mixed only with water, and then fiber was added with moist soil in small increments manually taking proper care. Thereafter, the soil-fiber homogeneous mix was shifted to a poly bags and reserved in a desiccators for 24 hrs to confirm its moisture steadiness. Afterward, the soil-fiber mixture was compacted in a cylindrical mold of 38 mm inner diameter having detachable collars at both ends for UCS and triaxial test sample. The whole amount of moist soil-fiber mix was shifted into the mold from either end, after fixing the collar at the other end. Subsequently, compaction was done from both ends by giving simultaneous equal rotation to the collars till the specimen length of 76 mm was attained. For CBR test, the specimen was compacted in CBR mold using standard proctor compaction energy as per ASTM D 698 .
It was decided not to go for fiber content above 1% as with 1% fiber content, homogeneous mixing of fibers was difficult due to formation of soil-fiber lumps. 20 mm fiber of different fiber doses (
2.4 Testing programme
The standard compaction tests were performed for unreinforced and glass fiber-reinforced soil according to ASTM D698  to obtain the OMC and MDU value of various mixes. Unconfined compressive Strength (UCS) test were performed as per ASTM D 2166/D 2166 M  with 1.25 mm/min axial strain rate for all specimens. Consolidated undrained (CU) triaxial tests were performed according to ASTM D4767  with an axial strain rate of 0.12 mm/min for different soil fiber mixes under varying confining pressure ranging from 100 to 400 kPa. Load, axial deformation and pore pressure during triaxial test were electronically measured and recorded by load cell of a capacity of 10 kN with a sensibility of 0.01kN, LVDT of capacity ±20 mm with a sensibility of 0.01 mm and pore pressure transducers, respectively. The CBR tests were performed as per ASTM D 1883  under both unsoaked and soaked conditions for all soil-fiber mixes.
3. Results and discussions
3.1 Mixing efficiency of fiber
The effectiveness of fiber within soil depends on its mixing efficiency. To investigate the distribution of fibers along the height of reinforced specimen, several fiber-reinforced specimens were disintegrated along its height. Three individual specimens were prepared for each fiber length and fiber content, and each specimen was cut into three equal pieces along the specimen height and the weight of fiber in each piece was calculated. At the time of specimen cutting along diameter, it was noticed that most of the fibers within specimen were aligned in the near horizontal direction perpendicular to the specimen height. Further, the fibers were noted to be uniformly distributed in the cutting plane of each specimen.
For segregating the fibers from the soil-fiber mix, each piece was crushed separately and the crushed soil-fiber was washed through a net of sieves of size 2 mm, 0.425 mm and 0.075 mm. All the soil particles were completely washed away from the 2 mm sieve to the 0.425 and 0.075 mm sieves, whereas most of the fibers were retained on 2 mm sieve. Further, the retained materials on 0.425 and 0.075 mm sieves were transferred to a bucket containing water. Then the water was stirred which settled the soil particles and fibers were accumulated on the water surface. In this way, the fibers were completely separated from the soil-fiber mix from each individual piece. The collected fibers of individual piece were oven dried and weighted. The percentage of fiber in each piece of individual specimen was then evaluated based on total weight of fiber mixed in that specimen.
Typical values of measured fiber content in three different parts of specimens of UCS test for different soil-fiber mixes are given in Table 1 along with their standard deviation. The percentage of distributed fibers within three different parts of any reinforced specimen is relatively close. Therefore, it can be inferred that the mixing efficiency of fibers is uniform along the height of the specimen to some extent, and fibers can be considered to be distributed homogeneously in the specimen. The standard deviation of fiber distribution is varying between ±0.16 to ±4.01%, and the values are found to be higher at higher fiber content indicating that the fiber mixing efficiency decreases at higher content.
|1/3 Top||1/3 Middle||1/3 Bottom|
|Fiber distribution, %||Standard deviation, ± %||Fiber distribution, %||Standard deviation, ± %||Fiber distribution, %||Standard deviation, ± %|
However, during field application, ensuring the uniformity of fiber in the large soil-fiber mass will be very challenging, especially for higher fiber dose. Therefore, for maintaining the uniformity of fibers within soil mass for large scale applications, it is important to use better mixing technique.
3.2 Compaction test results
The compaction curves of all specimens, with different combinations of fiber content are depicted in Figure 3, and their respective OMC and MDU values are shown in Figure 4. The OMC and MDU of the unreinforced soil are found as 19.4% and 16.80 kN/m3, respectively. As the fiber content increases, there is a minor enhancement in OMC from 19.4% to 19.7% and a small decrease in MDU from 16.80 to 16.57 kN/m3. As the OMC and MDU variation is marginal, for specimen preparation of either unreinforced or fiber-reinforced soil, the specimens were compacted at the OMC and MDU value of unreinforced soil.
3.3 UCS test results
Figure 5 presents unconfined compression test curve showing the effect of fiber content for all reinforced specimens. As fibers are added to the soil, the stress–strain behavior has modified appreciably in terms of both peak stress and strain improvement. This is followed by decrease of post-peak stress loss, showing stimulation of plastic nature to the soil and the brittleness nature transforms gradually to ductile. The maximum stress is found for the specimen with 0.75% fibers, and addition of additional fiber of 1% results in strength reduction. This shows that there is an optimal fiber content where advantage of reinforcement is the maximum. As the fiber content increases further to 1%, the number of fibers in soil increases such that the availability of soil matrix quantity for holding the fibers may not be that adequate to develop optimum bond among all soil-fiber interfaces. Consequently, the tensile strength of all fibers is not mobilized completely causing in peak strength drop at 1% fiber. However, the UCS of specimen reinforced with 1% fibers is higher than that of with 0.5% fibers. Fiber reinforcement advantage is mainly subjective to the bond strength and friction between soil particles and fibers . It was also noted that at the time of soil-fiber mixing with 1% fibers, uniform mixing of fibers was difficult and development of fiber lumps started to become visible which hindered the specimen uniformity.
The peak UCS and corresponding axial strain of all tested samples are represented in Figure 6. It has been found that with increasing fiber content the peak axial strain is increasing continuously indicating the more ductility in the soil specimen with added glass fibers. The peak axial strain of unreinforced soil was 2.65% which has increases maximum to 10.85% at 1% fiber content indicating around four time increment of peak axial strain. The UCS value is noted to be 137 kPa for unreinforced soil which improved to 181 kPa, 238 kPa, 279 kPa and 239 kPa for 0.25%, 0.5%, 0.75% and 1% fiber content, respectively showing around a maximum two fold increment of UCS value with 0.75% fiber content.
Figure 7 depicts the failure patterns of unreinforced and reinforced specimens. The unreinforced soil specimen (Figure 7a), showing a single shear plane across the specimen indicating its brittle behavior. This brittleness of unreinforced soil can also be observed from the stress–strain curve (Figure 5), where a sudden drop in stress is noted after peak. For specimen reinforced with 0.25% and 0.5% fiber, some dissimilar multi-shear planes in some portion of the sample are noted to develop (Figure 7b and c). Whereas, with 0.75% and 1% higher fiber dose, the specimens undergone largely bulging with the development of minor fissures around the sample (Figure 7d and e). The bridging effect of the fibers restricted the progress of shear planes or fissures, causing reallocation of stresses within the reinforced sample. It has also been noted in stress–strain response that the specimen fails at gradually higher axial strain with high fiber content (Figure 5), reflecting the inducement of ductility.
3.4 CBR test results
The load-penetration responses of the CBR tests on unreinforced and reinforced soil samples with varying fiber content are presented in Figure 8 for unsoaked condition. The load carrying capability of the samples increases with fiber content up to 0.75%, signifying that fibers can improve the load-penetration behavior. The bearing capacity of the specimens improves continuously with penetration depth up to 15 mm for all fiber contents, representing clearly that the specimen peak strength has not been attained even at 15 mm deformation, and that the fibers have not been pullout or rupture and are still in tension. At higher penetration, the curve slope decreases signifying that the rate of bearing capacity enhancement is diminishing.
The fiber indentations due to the soil particles permit to develop adhesion within soil and fiber , ensuring enhanced load carrying capacity of the reinforced soil. Tang et al.  told that randomly distributed fibers perform as a three-dimensional arrangement which interlocks soil grains, and restricts the movement of soil, improving the stretching resistance between soil and fibers, ensuing strength inducement. Also, the tensile restraint in the fibers imparts supplementary soil confinement  and results in enhancement of specimen strength.
The CBR values under both soaked and unsosked condition are shown in Figure 9. Maximum enrichment in CBR for soaking condition is with 0.75% fiber. The maximum enhancement of CBR is from 6.45% to 18.94% under unsoaked condition and 2.89% to 8.23% under soaked condition with 0.75% fiber. For use in field, the determination of optimal soil-fiber mixture is important. For 4 days soaked condition, the CBR of the parent soil is 2.89%, and the maximum CBR of 8.23% is obtained with 0.75% fibers. Therefore, according to IRC: SP: 72 , the unreinforced soil is of very poor quality subgrade material (soaked CBR less than 3%), which can be upgraded to good quality subgrade material (soaked CBR between 7% and 9%). However, according to IRC: 37 , a minimum soaked CBR value of 6% is essential for subgrade layer of low-volume flexible pavements. Thus, the clayey soil mixed with 0.5, 0.75% and 1% glass fibers having CBR values of 6.89%, 8.23% and 7.62%, respectively can be used in subgrade layer of low-volume flexible pavements.
3.5 Triaxial test results
The effect of fiber content on stress–strain and pore water pressure-strain behavior for all specimens sheared under 100 kPa confinement, are shown in Figure 10 and Figure 11, respectively. The deviator stress-axial strain response was found to enhance continuously with fiber content only up to 0.75% and then remain close to 0.75% fibers with 1% fiber. No peak appears till 20% strain for any specimen tested (Figure 10). Similar stress–strain response on fiber reinforced soil where no clear peak was observed, even at an axial strain of 20% was noted by Andersland and Khattak , Ranjan et al.  and Estabragh et al. .
As fiber content increases, number of fiber increases within specimen which provide additional surficial friction between soil and fiber. Consequently additional mobilization of fiber tensile strength occurs with fiber content, which ultimately increases the overall strength of specimen. The initial stiffness at smaller strain (< 1%) of specimen was found to decrease with fiber content which was different from that of Ranjan et al.  and Estabragh et al.  where the initial stiffness of fiber reinforced soil was improved with fiber content. The decrease in initial stiffness with fiber content is due to the fact that the fiber within compacted specimen remains in compression at the start of shearing under confining pressure. With increasing axial strain during shearing of specimen, the fiber gets stretched by surficial interaction with soil particles and mobilizes its tensile strength resulting in improvement of strength and stiffness of the specimen.
The contraction or dilation behavior of specimen particles can be related with the generated pore water pressure during shearing and can be found by inspecting the slope of pore pressure response. The positive slope specifies the contraction behavior while negative slope indicates specimen dilation. The generated pore pressure generation was found to be positive for both unreinforced and reinforced specimens indicating contractive behavior (Figure 11). The positive pore water pressure generation increased with fiber content, indicating that that the increase of fiber content increased the contractive behavior of specimen by uniformly distributing the stresses within the specimen.
Stiffness is a measure of resistance offered by a material against its deformation under external applied load. Stiffness of specimen can be expressed in terms of stiffness modulus which is the ratio of stress to the corresponding axial strain. The effect of fiber content on stiffness modulus is shown in Figure 12 under 100 kPa confining pressure. The initial stiffness of soil at smaller axial strain (<1%) is found to decrease with increasing fiber content, while at higher axial strain (> 1%) the stiffness modulus can be noted to increase with fiber content up to 0.75%. The decrease in stiffness at lower axial strain is due to the fact that reinforcement needs some stretching to mobilize its tensile strength. At smaller axial stain level as soil particles move, it try to stretch the fiber and after some deformation the fiber start to work. In this case the limiting value of that point is noted around 1%. Nevertheless, stiffness modulus remains much higher than that of unreinforced specimen with 1% fiber content. For any fiber content stiffness modulus was noted to be higher at small axial strain and it progressively decreased with increasing axial strain. The stiffness modulus reduction rate decreased at higher axial strain.
Effect of fiber benefit on strength of soil during undrained shearing has been presented in terms of a parameter called strength ratio (
The influence of fiber content on
Following conclusions have been drawn from the experimental investigation of glass fiber-reinforced clayey soil:
Mixing efficiency of fibers within soil mass decreases with increasing fiber content.
Addition of glass fibers marginally changes the compaction parameters (OMC and MDU) of clayey soil.
The UCS, CBR and shear strength of clayey soil increases with glass fiber content up to a limiting value of 0.75%.
The addition of glass fibers enhances the UCS of clayey soil by around two fold, CBR by 2.8 times and shear strength by around 1.75 times that of unreinforced soil.
The glass fiber inclusion continuously increases ductility of clayey soil.
The inclusion of glass fibers decreases the stiffness modulus of clayey soil at smaller axial strain and then increases the stiffness at higher axial strain. The boundary of axial strain which changes the stiffness behavior is noted to be around 1%.
The strength ratio of clayey soil decreases with increasing confining pressure for any fiber content.
The 20 mm glass fibers of 0.5%, 0.75% and 1% is found to be used expressively in the subgrade layer of low-volume flexible pavement.
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