**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 fiberreinforced 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


#### **Table 1.**

*Distribution of fibers in different part of reinforced specimen.*

*Experimental Investigation of Glass Fiber Reinforced Clayey Soil for Its Possible Application… DOI: http://dx.doi.org/10.5772/intechopen.102802*

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.

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.

**Figure 3.** *Effect of fiber content on compaction curve.*

**Figure 4.** *Variation of compaction parameters (OMC and MDU) with fiber content.*

**Figure 5.** *Effect of fiber content on stress–strain response.*

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

### *Experimental Investigation of Glass Fiber Reinforced Clayey Soil for Its Possible Application… DOI: http://dx.doi.org/10.5772/intechopen.102802*

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 [21]. 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 multishear 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.

**Figure 6.** *Effect of fiber content on UCS and peak strain.*

**Figure 7.**

*Effect of fiber inclusion on specimen failure mode: (a) fc = 0%; (b) fc = 0.25%; (c) fc = 0.5%; (d) fc = 0.75%; (e) fc = 1%.*

### **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 [48], ensuring enhanced load carrying capacity of the reinforced soil. Tang et al. [21] told that randomly distributed fibers perform as a three-dimensional

**Figure 8.** *Effect of fiber content on load-penetration response under unsoaked condition.*

*Experimental Investigation of Glass Fiber Reinforced Clayey Soil for Its Possible Application… DOI: http://dx.doi.org/10.5772/intechopen.102802*

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 [49] 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 [50], 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 [51], 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 [52], Ranjan et al. [35] and Estabragh et al. [22].

**Figure 9.** *Effect of fiber inclusion on CBR value under both soaked and unsoaked conditions.*

**Figure 10.** *Effect of fiber content on deviator stress-axial strain response.*

**Figure 11.** *Effect of fiber content on pore pressure response.*

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. [35] and Estabragh et al. [22] where the initial stiffness of fiber reinforced

### *Experimental Investigation of Glass Fiber Reinforced Clayey Soil for Its Possible Application… DOI: http://dx.doi.org/10.5772/intechopen.102802*

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.

**Figure 12.** *Effect of fiber content on stiffness modulus response.*

**Figure 13.** *Effect of fiber content on strength ratio.*

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 (*SR*) similar to that of Estabragh et al. [22], Haeri et al. [53] and Zhang et al. [54]. Strength ratio is the ratio of deviator stress of reinforced soil at failure (σ *dr* ) to that of deviator stress of unreinforced soil at failure (σ*du* ).

$$SR = \frac{\sigma\_{dv}}{\sigma\_{du}}\tag{1}$$

The influence of fiber content on *SR* under varying confinement is shown in **Figure 13**. For any fiber content, the strength ratio decreased with increasing confining pressure, indicating that the effect of fiber decreased with increasing confining pressure. It can also be noted that *SR* increased with fiber content up to 0.75% at any confining pressure and then decreased for 1% fiber content.
