SHEAR STRENGTH OF MEDIUM PLASTIC EXPANSIVE SOIL …

SHEAR STRENGTH OF MEDIUM PLASTIC EXPANSIVE SOIL REINFORCED WITH POLYESTER FIBERS 1

SHEAR STRENGTH OF MEDIUM PLASTIC EXPANSIVE SOIL REINFORCED WITH POLYESTER FIBERS

Niraj Singh PARIHAR2, Rajesh Prasad SHUKLA1*, Ashok Kumar GUPTA2

Address

1 Department of Earthquake Engineering, IIT Roorkee, Roorkee, India

2 Department of Civil Eng., Jaypee University of Information Technology, Waknaghat, India

* Corresponding author: [email protected]

Abstract

This Expansive soils are very problematic as they are prone to substantial settlements, heave, and possess low bearing capac-ity. These soils cover more than 20% of the land cover of India and cause obstructions in the development of road networks, railways, and various other construction activities. They make soil stabilization essential. An investigation was carried out to determine the effect of various proportions of randomly-orient-ed polyester fibers on the shear strength of expansive soil. The unconfined compressive strength of reinforced soil was deter-mined by incorporating four fiber contents, i.e., 0.25%, 0.50%, 0.75%, and 1%, with varying aspect ratios. The effect of vari-ous aspect ratios of 20, 40, and 60 were studied in the present work. The stress-strain relationship for different aspect ratios and fiber contents is also presented in the study. The optimum quantity of fibers was found to be 0.75% of the total weight of the soil. A maximum enhancement in the strength of the soil was achieved with fibers with an aspect ratio of 40. The effect of the aspect ratio is significant with a fiber content of 0.50 to 0.75%. The peak strength of untreated soil is found at strain levels of 6-8%, whereas it increases to 10-12% in reinforced soil. A statistical analysis was also performed to develop a regres-sion equation to predict the improvement in the strength of the medium plastic expansive soil used in the study.

Key words

● Fibers, ● Expansive soil, ● Strength, ● Aspect ratio, ● Improvement.

Slovak Journal of Civil Engineering Vol. 26, 2018, No. 2, 1 – 8

DOI: 10.2478/sjce-2018-0007

INTRODUCTION

Increases in urbanization and industrial areas have led to a scarci-ty of suitable land for construction. In these circumstances, engineers are only left with the option of using the available lands with a low bearing capacity. In India, expansive soils cover a considerable area, especially the central part and some western and southern parts of the country (Shukla et al., 2014). These soils are locally called ‘Kali Mitti’ due to their black colour. Madhya Pradesh is a state situated in the central part of India. The Madhya Pradesh land report shows that

expansive soil is spread over more than 60% of the area of the state. These soils are recognized as a source of potential hazards that may damage civil engineering structures if not treated effectively. Over the last few years, various soil reinforcement and soil stabilization techniques have been developed and practiced frequently to achieve the improvements desired in the geotechnical properties of expansive soil. In general, stabilization can be achieved either through the inclu-sion of reinforcing elements or by the addition of cementing agents. Both reinforced and agglomerated soil behave like a composite mate-rial, and the best results can be obtained from the incorporation of an

Slovak Journal of Civil Engineering

SHEAR STRENGTH OF MEDIUM PLASTIC EXPANSIVE SOIL REINFORCED WITH POLYESTER FIBERS2

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optimum quantity of reinforcing and stabilizing material. A number of additives and reinforcing materials have been

identified by various research works that simultaneously reduce the hazardous nature of the soil and increase its strength. Hejazi et al. (2009) presented a very comprehensive review of soil reinforcement through the incorporation of synthetic and natural fibers. The review stated that the availability of the fibers, the ability to use them, in all weather conditions, the ease of working with them, and the economic benefits are some advantages that make fibers a very good material for reinforcing soil. The review also established the applicability of fibers in various areas of civil engineering, such as road construction, earthquake resistance design, retaining walls, railway embankments, slope protection, and soil-foundation engineering.

Guangxin et al. (1995) found that the shear strength, plastici-ty and toughness of cohesive soil significantly increased after the incorporation of discrete polypropylene fiber. Loehr et al. (2000) observed that the one-dimensional free swelling of expansive soil was significantly reduced with the incorporation of a small quantity of polypropylene fibers. Puppala and Musenda (2000) determined that the unconfined compressive strength of expansive soil increased with the addition of randomly-oriented discrete polypropylene fib-ers. Although the swelling pressure and the volumetric shrinkage of the soil decreased, the vertical free swelling of clay increased with increases in the quantity of polypropylene fibers in the soil samples. Cai et al. (2006) found that the addition of polypropylene fibers to soil not only reduces the swelling of soil but also increases its shear strength. Akhras et al. (2008) noted that the swelling pressure and the swelling potential of soil reduced with the addition of fibers and that an aspect ratio of 25 was more effective than an aspect ratio of 100. Viswanadham et al. (2009) used two fiber contents (0.25% and 0.50%) with different aspect ratios (15, 30 and 45) for soil reinforce-ment. It was determined that the swelling pressure and one-dimen-sional swelling of remoulded expansive soil was reduced with fiber reinforcements and that a maximum benefit was obtained at an aspect ratio of 45. Malekzadeh and Bilsel (2012) found that the maximum dry density and shrinkage settlements of soil were reduced with the addition of fibers in the soil, while the optimum water content of the soil remained unchanged. The unconfined compressive strength (UCS) also increased by 1.5 times with a fiber content of 1%. Es-tabragh et al. (2014) determined that the reduction in the swelling of soil increased with increases in the width of tape fibers. A few studies also used a combination of polypropylene fibers and fly ash to stabilize expansive soil; a consequent reduction in the shrinkage and swelling of the soil was observed (Punthutaecha et al., 2006 and Olgun, 2013). Rokade et al. (2017) observed an improvement in the California bearing ratio (CBR) of expansive soil with the incorpora-tion of nylon fibers.

From a review of the literature, it was observed that most of the studies have focused on the effect of fibers on the swelling character-istics of soils. A few studies have also considered the effect of poly-ester fibers on the strength of soil. However, the effect of the aspect ratio has not been considered in previous studies. Although the soil used in the present study covers more than 25% of the expansive soils found in India, no study in the past has been carried out on this soil using polyester fiber. The literature reviews also show that fiber reduces the swelling potential and swelling pressure of soils; there-fore, these soil characteristics have not been considered in the present study. This study is focused on a determination of the effect of the aspect ratio and the optimum quantity of the fibers on the strength of expansive soil. For this purpose, various fiber contents (0.25%, 0.50%, 0.75% and 1%) with different aspect ratios (20, 40 and 60) were used in the study; finally, the optimum values of the aspect ratio and fiber content were determined.

MATERIALS USED IN THE TESTING

The soils used in the present study were collected from the city of Guna as shown in Fig.1. The fine content in the soil is more than 65%, and its colour is black-grey. The results of laboratory testing conducted on the virgin expansive soil are shown in Table 1. Fig. 2 shows the result of a sieve analysis carried out on the expansive soil. As per NAVFAC 7.02, soil can be classified based on its unconfined compressive strength; accordingly, this soil is classified as a soft soil, as the unconfined compressive strength of untreated soil samples var-ies between 24 and 47.88 kPa.

Tab. 1 Laboratory testing results conducted on Soils

Description Value

Unit weight (γ) 15.02-15.67 kN/m3

Plastic limit (PL) 27.67-31.2%

Liquid limit (LL) 49.6-56.8 %

Plasticity index 23.7-27.9%

Shrinkage limit (SL) 12.4-13.2 %

UCS 43.5-47.5 kPa

Specific gravity 2.41-2.44

OMC 21-23.5%

Free swell index 72.8%

As per the Indian standards and USCS, this soil is classified as MH. The soil samples were collected from three nearby locations in the city; very little variations in their soil properties were observed. The soil samples from different locations are represented as A, B and C to record the variations in the behaviour of the soil. The differential free swell of the plain soil lies between 70 to 80%, which shows that the collected soil belongs to a highly expansive group (Mohan and Goel, 1959).

Fig. 1 Location of sampling and distribution of different expansive soils in the state Madhya Prasesh, India



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Polyester fibers that have different aspect ratios were used in the analysis. The fibers used in the study are mostly used by furniture industries. These fibers have very good resistance to acids and alkalis. The soil samples and polyester fibers used in the present study are shown in Fig. 3.

Tab. 2 Properties of fibers used in the study

Properties Value

Unit weight 0.90 g/cm3

Diameter (average) 0.037 mm

Fusion point 1650C

Modulus of elasticity 3.5 x 106 kPa

Tensile strength 350 kPa

TESTING PROCESS

Firstly, the field density, water content and all the other index properties of the soil samples were determined in the laboratory. The specific gravity, water content, grain size distribution and compaction parameters, and the curves of the soil samples were determined as per IS: 2720 (Part III)-1980, IS: 2720 (Part II)-1973, IS: 2720 (Part IV)-1975 and IS: 2720 (Part VII)-1980 respectively. The differential free swelling of the collected soil samples was determined as per the procedure given in the Indian standard code, IS 2720-40 (1977).

The unconfined compression test has been widely used in earli-er studies to determine the approximate strength of cohesive soil as it is relatively fast. Unconfined compression tests were conducted on

strain-controlled apparatus. Earlier studies have found that adding more than 1% fibers was not beneficial when enhancing soil proper-ties as the fibers started forming clumps (Brandon et al., 2009). So, the maximum quantity of the fibers was fixed to 1% of the soil’s weight.

Unconfined compressive strength (UCS) tests, with and without the inclusion of any reinforcement, were performed as per IS: 1943 (Part X)-1981. The specimens for the determination of UCS were pre-pared with the help of a metallic split mould with a 38 mm diameter and a 76 mm height and a detachable collar. The detachable collar is attached to the end of the mould, which remains orthogonal with the vertical axis of the mould. Water equal to the optimum moisture con-tent (OMC) of the soil was added to the soil to prepare a uniform mix of soil and fibers. Fig. 4 shows the mixing of the fibers in the expan-sive soil. The samples were prepared and extracted with the mentioned dimensions and then tested for their unconfined compressive strength to evaluate the effect of different proportions of fiber content and the aspect ratio (AR) of fibers. A large number of unconfined compression tests were performed on the fiber-reinforced expansive soil. Detailed information on the testing programme is presented in Table 3.

Table 3. Details of testing programme

S. No. Soil sample designation

Fibre content (%)

Aspect ratio(L/B)

Number of tests

1 A, B and C 0.00 - 03

2 A, B and C 0.25 20, 40 and 60 09

3 A, B and C 0.50 20, 40 and 60 09

4 A, B and C 0.75 20, 40 and 60 09

5 A, B and C 1.00 20, 40 and 60 09

Total number of tests 39

RESULTS AND DISCUSSION

The behaviour of reinforced soil is determined by focusing on two aspects of fibers, i.e., the aspect ratio and the quantity of the fib-ers. A large number of tests were performed, but only a few typi-cal curves are presented to explain the effects of the fiber content (FC) and aspect ratio (AR) on the strength of the soil. Soil samples

Fig. 2 Sieve analysis results of expansive soils

Fig. 3 Material used in the study: (a) Expansive soil, (b) polyester fibers

Fig. 4 Mixing of fibers in expansive soil

a) b)



0.75% shows a higher peak as compared to soil reinforced with other fiber contents (i.e., 0.25%, 50% or 1%). The strains required to reach the peak stress also increased with the increases in the fiber content. In most of the cases, the soil samples reinforced with a higher fiber content indicates a more ductile behaviour.

The effect of the addition of fibers on the unconfined compres-sion strength of soil samples collected from three different locations for various fiber contents and AR is presented in Fig. 8. It is very clear from Fig. 8 (a-c) that the unconfined compressive strength (peak stress) of soils improved up to a fiber content of 0.75%, whereas ad-ditional fibers decreased the strength of the soil. Sharma et al. (2015) also determined that the optimum quantity of fiber contents varies between 0.5 to 1% of the weight of soil in the case of sandy clay. Soil-fiber bonds are strong at a fiber content of 0.50-0.75% due to

after failure under an unconfined condition are presented in Fig. 5. The stress-strain relationship for the A, B and C soils is presented in Fig. 6. It shows that there is a small difference in the peak stress of the A, B and C soils.

Effect of the fiber content

Fig. 7 shows the typical stress-strain behaviour of expansive soil reinforced with different quantities of fiber contents having an aspect ratio of 40. Similar behaviour is evident in the case of soils reinforced with other aspect ratios of fibers (AR=20 or 60). It has been observed that the stress-strain behaviour of soil improves with the addition of fibers in the expansive soil. Soil reinforced with a fiber content of

Fig. 5 Soil samples after failure in UCS testing Fig. 6 Stress-strain relationship for untreated soils

Fig. 7 Effect of fiber content on the stress-strain relationship of soil with an aspect ratio of 40; (a) Soil A, (b) Soil B and (c) Soil C

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Fig. 8 Effect of fiber content on the peak strength of soil; (a) soil A, (b) soil B and (c) soil C

Fig. 9 Effect of aspect ratio on the stress-strain behaviour of soils; (a) Soil A, (b) Soil B and (c) soil C



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the better interaction between the fibers and soil particles; therefore, a maximum enhancement in strength can be observed. The addition of more fibers than the optimum fiber content results in the formation of fibre lumps and a smaller contact area between the soil particles and fiber material (Cai et al., 2006). In the case of fibers with an as-pect ratio of 40, the improvement in the peak strength of the soil is relatively higher when compared to other fiber contents. The effect of the fiber contents on strength is relatively nominal in the case of fibers with aspect ratios of 20 or 60 as compared to an aspect ratio of 40. Based on comparison of present study result with Shukla and Singh (2016), it can be stated that polyester fibers are less effective than micro-fine slag. The reason can be attributed to the higher specific gravity and reactivity of the slag.

Effect of the aspect ratio of fibers

The strength of reinforced expansive soil is found to vary with variations in the aspect ratio of the fibers. Fig. 9 shows the typical stress-strain behaviour of reinforced expansive soil for a 0.75% fiber content with different aspect ratios (20, 40 or 60). Soil reinforced with fibers with an aspect ratio of 40 showed a better stress-strain behaviour as compared to soil reinforced with fibers with an aspect ratio of 20 or 60. Similar behaviour was observed in cases of other fiber contents. As the index properties of soils collected from three different locations are approximately the same, a similar stress-strain relationship was observed in all three soils. The initial stiffness of reinforced soil was found to improve with the addition of fibers with an aspect ratio of 40. A post peak loss of strength is also at a minimum in the case of soil samples mixed with fibres with an aspect ratio of 40. This indicates that the pre and post-failure behaviour of the soil

samples improved relatively better in this case.The effect of various aspect ratios (20, 40 or 60) on the unconfined

compression strength of soil samples with different fiber contents is presented in Fig. 10. The unconfined compressive strength increased when the AR is increased from 20 to 40, but a further increase in the AR resulted in a decrease in the strength of the soil. Although the strength of the soil was reduced significantly when the AR increased from 40 to 60, it always remained more than the soils reinforced with fibers with an aspect ratio of 20. The effect of the aspect ratio is com-paratively significant in the case of fiber contents of 0.5% and 0.75%. The effect of the aspect ratio on the unconfined compression strength is relatively low for a fiber content of 0.25%.

Akhras et al. (2008) and Viswanadham et al. (2009) noted that the swelling characteristics of expansive soil improved significantly with fibers with an aspect ratio of 25 or 45. Although the present study is limited to the unconfined strength of soil and has not considered the swelling characteristics of soil, the optimum aspect ratio of fibers is very close to the findings of Viswanadham et al. (2009). This varia-tion in the optimum aspect ratio of fibers derived in various studies may be due to three causes. Firstly, the characteristics of the expan-sive soil and fibers used in these studies are different. Secondly, the ranges of the aspect ratios of the fibres used are also very different, and thirdly, the quantity of fibers used in the studies is also dissimilar.

Similar to the observations made by Prabakar and Sridhar (2002) and Brandon et al. (2009), it has been observed in the present study that increasing the fiber content in soil increases the strength of the soil, but maximum benefits can only be achieved by adding the op-timum quantity of fibers; in the present study, this value has been found to be 0.75%. Adding more than the optimum content of fib-ers decreases the efficiency of the soil reinforcement. This optimum

Fig. 10 Effect of the aspect ratio on the peak strength of soils: (a) Soil A, (b) Soil B and (c) Soil C



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quantity of fibers can be dependent on the characteristics of the soil and fibers and the quantity of fibers added to the soil.

Statistical Analysis

A statistical analysis was also carried out to develop an equation in order to determine the improvement in the unconfined compressive strength of soil with respect to the aspect ratio and fiber contents. It can be observed from Figs. 8 and 10 that the relationship between the shear strength (strength ratio), the aspect ratio, and the fiber contents is nonlinear and that the nonlinearity is not uniform. This makes the regression analysis very difficult and lengthy. However, the regres-sion equation developed was found to be good in determining chang-es in the strength of expansive soil with changes in the fiber contents and aspect ratio. Initially, it was assumed that changes in the strength only depended on the aspect ratio and fiber contents. Subsequently, 14 other variables were assumed that could affect the strength of rein-forced soil. These 14 variables were assumed to be a function of the aspect ratio and fiber contents. However, it was observed that only 10 variables, including the aspect ratio and fiber contents, were affecting the strength of the soil. The R2 value reduced from 0.978 to 0.927 when the number of variables was reduced from 14 to 10. Equation 1 has been proposed to determine the strength ratio (SR), which is defined as the ratio of the unconfined compressive strength of rein-forced soil to the unconfined compressive strength of unreinforced soil. A comparison of the strength ratio predicted with the strength ratio determined is presented in Fig. 11. This regression equation is limited to expansive soils (the properties shown in Table 1) found in the central part of India with medium plasticity. Further extensive studies are required to check the rationality of the proposed equation for medium plastic expansive soils found in other regions.

(1)

where, SR is the strength ratio; UCSr is the unconfined compres-sive strength of reinforced soil; UCSu is the unconfined compressive strength of unreinforced soil; AR is the aspect ratio of the fibers; and FC is the fiber content.

CONCLUSIONS

The fibers were mixed with highly plastic silt of an expansive nature. The addition of fibers in untreated expansive soils increases the unconfined compressive strength of the soil. The pre-failure and post-failure behaviour of soil also improves with the incorporation of fibers. The improvement in the strength of the soil depends on the fiber contents and the aspect ratio of the fibers. The optimum quantity of fibers to obtain the optimum enhancement in the soil’s strength was found to be 0.75% of the weight of the soil. In almost all the cases, maximum improvement was achieved for fibers with an aspect ratio of 40. The rate of the increase in the soil strength is relatively large in soil samples reinforced with fibers with an aspect ratio of 40 as compared to aspect ratios of 20 or 60. For a fiber content of 0.75%, the unconfined strength increased by approximately 1.5, 2, and 1.7 times the strength of the unreinforced soil for fibers of aspect ratios 20, 40 and 60 respectively. The strain level at the failure of the rein-forced soil samples increases with an increase in the fiber contents. For a fiber content of 0.75%, the strain at failure increases by 1.5 times compared to that of unreinforced soil. The effect of the aspect ratio is relatively more significant in fiber contents of 0.50 and 0.75%. The developed equation has been found to be reasonably good at pre-dicting increases in the unconfined strength of the medium plastic expansive soil used in the present study.

Acknowledgements

Authors are grateful to the Editorial board of the Journal for thor-ough linguistic correction, which has improved the flow and readabil-ity of the manuscript.

Fig. 11 Comparison of the predicted strength ratio to the determined strength ratio



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REFERENCES

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SHEAR STRENGTH OF MEDIUM PLASTIC EXPANSIVE SOIL …