Behaviour of plastic-fibre-reinforced sand

Geotextiles and Geomembranes 13 (1994) 555-565 1994 Elsevier Science Limited Printed in Ireland. 0266- I 144/94/$7.00

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ELSEVIER

Behaviour of Plastic-Fibre-Reinforced Sand

Gopal Ranjan, R. M. Vasan & H. D. Charan Department of Civil Engineering,University of Roorkee, Roorkee-247667,India (Received 20 December 1993; accepted 4 February 1994)

ABSTRACT Stress-deformation behaviour of sand reinforced with discrete randomly distributed fibres depends on the properties of the fibres and sand-fibre surface friction. This paper presents the results of triaxial compression tests, performed to determine stress-strain behaviour of fine sand reinforced with discrete, randomly distributed plastic fibres, and to observe the influence of fibre properties (i.e. weight fraction and aspect ratio) and confining stress on shear strength of reinforced sand. The results indicate that the sand-fibre composites have a curved or a bilinear failure envelope, with the break occurring at a certain confining stress, called the 'critical confining stress'. The magnitude of the critical confining stress decreases with increase in aspect ratio of the fibres. The shear strength of plasticfibre-reinforced sand increases with increase in fibre content and aspect ratio.

1 INTRODUCTION Reinforced soil construction is an effective and reliable technique for improving the strength and stability of soils. The technique is used in a variety of applications, ranging from retaining structures and embankments to subgrade stabilisation beneath footings and pavements. In traditional methods of reinforced soil construction, the inclusions (e.g. strips, sheets, fabrics, bars or grids) are normally oriented in a preferred direction and are introduced sequentially in alternating layers. But, fibrereinforced soil called 'poly-soil' (McGown et al., 1978), is similar to 555

556

Gopal Ranjan, R. M. Vasan, H. D. Charan

stabilisation by admixtures, in its preparation. Essentially, discrete fibres are simply added and mixed with soil, in much the same way as cemel~t, lime or other additives. Experimental results reported by various researchers (McGown et al., 1978; Hoare, 1979; Gray & Ohashi, 1983; Gray & A1-Refeai, 1986; Setty & Rao, 1987; Gray & Maher, 1989; Maher & Gray, 1990; Setty & Murthy, 1990; AI-Refeai, 1991) showed that the fibre-reinforced soil is a potential composite material which can be advantageously employed in improving the structural behaviour of soils. One of the main advantages of randomly distributed fibres is the maintenance of strength isotropy and the absence of potential planes of weakness that can develop parallel to the oriented reinforcement. The fibre reinforcement causes significant improvement in shear strength of sand. More importantly it exhibits greater extensibility and small loss of post-peak strength (i.e. greater ductility in the composite material) as compared to sand alone or to sand reinforced with high modulus inclusions. The main objective of this study has been to investigate the stressdeformation behaviour of plastic-fibre-reinforced fine sand and to study the effect of confining stress on the failure envelope of reinforced sand. The effect of fibre parameters, e.g. fibre content and aspect ratio on the shear strength has also been investigated.

2 MATERIALS AND E X P E R I M E N T A L P R O C E D U R E 2.1 Materials Reinforcement

Plastic fibres used in the present investigation were cut to length from locally available continuous fibres. The physical properties of the fibres are given in Table 1. The coefficient of skin friction has been assumed on the basis of the published data (Potyondy, 1961) for different materials in contact with sand. So// Soil used in the present investigation was locally available sand. It is classified as poorly graded fine sand (SP-SM) as per Unified Soil Classification (USC) system. The grain size distribution curve and properties of the sand are shown in Fig. 1 and Table 2 respectively.

Behaviour of plastic-fibre-rein[brced sand

557

Table 1 Properties of Fibre Reinforcement

Type of .fibre

Plastic-I Plastic-2 Plastic-3 Plastic-4

Diameter (d ) (mm)

Aspect ratio

Spec(fic gravity

(l/d)

Gt

0-3 0.3 0.3 0-5

60 90 120 75

0-92 0.92 0-92 0.92

Tensile Tensile -strength modulus (kPa) (kPa) 3 3 3 3

× × × ×

104 104 104 104

lo PARTICLE SIZE (ram)

ol

2 2 2 2

× × × ×

106 106 106 106

Skin friction angle 6o 21 21 21 21

100 9C 8C

70 OC Ill

6C

Z

,7 5C

,., 40

2C 1C 0 10

001

Fig. i. Particle size distribution curve of fine sand.

2.2 Sample preparation and testing Fibre-reinforced sand samples were prepared at maximum dry unit weight and optimum moisture content, obtained by conducting a standard Proctor test on unreinforced sand. The quantity of fibres to be added to the sand was taken as a percentage weight of the soil solids. The quantity of fibres and sand required to fill a standard mould of size 38 mm × 76 mm at the desired density was taken together with water of known quantity (i.e. the optimum moisture content). Fibres were mixed thoroughly by hand to

558

Gopal Ranjan, R. M. Vasan, H. D. Charan Table 2 Properties of Fine Sand

Effective grain size D io (ram)

Median grain size Dso ( mm )

Coefficient of uniformity C~

Specific gravity of solids G~

Maximum void ratio e,,,ax

Minimum void ratio emin

0.14

0.29

2.28

2.60

1.08

0.495

c-c~ parameters a c (kPa)

(9 (degree)

10.5

34 °

"c-~b parameters were determined by triaxial compression tests on samples prepared at the optimum moisture content of 15% and maximum unit weight of 16.2kN/m 3, obtained from a standard Proctor test.

achieve a uniform mix. If fibres are mixed in dry sand segregation and/or floating of fibres tends to occur. The moist sand and fibre mixture was transferred to the mould in four layers and compacted by light tamping of successive layers to achieve a uniform density throughout the height of the sample. Consolidated undrained triaxial tests were conducted on partially saturated reinforced samples at an axial displacement rate of 1.25mm/ min. Samples were tested at a confining stress of 50-400 kPa with varying fibre content and aspect ratio.

3 TEST RESULTS Data from triaxial compression tests have been analysed to study the stress-deformation and failure behaviour of fibre-reinforced sand. 3.1 Stress-strain behaviour Figure 2 shows a typical axial strain-deviator stress plot from triaxial tests at a confining pressure of 300 kPa, Four different samples with a fibre content ranging from 1 to 4% were tested in the triaxial state. For the sake of comparison, tests were also carried out on unreinforced sand. Figure 2 indicates that the stress-strain behaviour of the reinforced sand is very much different from that of the unreinforced sand. Unreinforced sand attains a peak stress at around 10% axial strain which then remains practically constant up to 20% axial strain. However, fibre-reinforced sand samples do not exhibit any peak stress. The stress-strain curves of reinforced sand indicate an increasing trend even at axial strain of 20%. Also, as the fibre content is increased, the increase in stress for the same magnitude of increase in strain is much higher.

559

Behaviour of plastic-fibre-reinforced sand 220C CONFINING

2000

STRESS :. 300 kPa

ASPECT RATIO

: 75

Wf : 4 '/,

J J

Wt : 3 % 1800 1600

Wt : 2 %

o i~001

~

Wf= 1"1,

120O 1000

,~

800 Wt : 0'0

a

600 40O 200 01r 0

I

4

I

8

I I 12 16 A X I A L S T R A I N (*1,)

1 20

I 24

Fig. 2. Stress-strain behaviour of fibre-reinforced sand.

As the reinforced sample is subjected to strain, friction between sand and fibres comes into play resulting in development of tensile stress in the fibres. The tensile stress developed in the fibres is responsible for enhanced stress in fibre-reinforced sand. Thus, at strains beyond the peak strength of sand alone, although the strength of unreinforced sand may remain unchanged or may be slightly reduced, the reinforced sand tends to gain strength. The failure condition in such situations is generally defined by the permissible a m o u n t of deformation. Usually failure stress is taken at a corresponding strain of 15 or 20%. Thus, in the present analysis, the failure condition has been defined as the stress corresponding to the peak stress condition or at 20% axial strain whichever is earlier. Here, shear strength of fibre-reinforced sand has been denoted in terms of major principal stress at failure (trlr). Table 3 shows the increase in shear strength of fibre-reinforced sand at axial strains of 20 and 10%. At 20% axial strain, the increase in strength ranges from 43 to 130% with increase in fibre content from 1 to 4% respectively. Also, even at an axial strain of 10% (i.e. half of axial strain at

Gopal Ranjan, R. M. Vasan, H. D. Charan

560

Table 3 Increase in S h e a r S t r e n g t h o f F i b r e - R e i n f o r c e d S a n d ( C o n f i n i n g Stress, a3 = 300 k P a ) Fibre content wt ( % )

0.0 1.0 2.0 3.0 4.0

Axial strain = 20%

Axial strain = 10%

Major principal stress (kPa)

% Increase in stress

Major principal stress (kPa)

% Increase in stress

1 040 1 490 1 892 2 190 2 395

-43 81 111 130

1 058 1 320 1 520 1 730 1 840

-25 44 64 74

failure) the increase in strength is quite significant e.g. 25-74% with fibre content 1-4%. 3.2 Soil-fibre interaction: effect of confining stress Figure 3 shows strength envelopes of fibre-reinforced sand for a fibre content of 1%, and aspect ratios (l/d) of 60, 90 and 120. The strength envelope of unreinforced sand has also been plotted for comparison. Examination of these strength envelopes indicates a marked difference in strength envelopes of unreinforced and reinforced sand. The strength envelopes for fibre-reinforced sand are either curvilinear or bi-linear having a change of slope at a certain confining stress. The confining stress corresponding to the break in failure envelope is termed the 'critical confining stress' (O'crit). The existence of a critical confining stress is very significant in understanding the mechanism of strength increase in fibre-reinforced sand. The increase in strength is attributed to sand-fibre surface friction and tensile stress developed in the fibres. To mobilise sand-fibre surface friction, a certain amount of confining stress is required, below which the fibres tend to be pulled out. At confining stresses less than the critical (0"3 ~ 0.crit), the fibres slip during deformation, and at confining stresses higher than the critical (03 > 0.crit), they stretch or yield. The level of critical confining stress is very much influenced by the aspect ratio of the fibres. It is evident from Fig. 3 that the critical confining stress decreases with increase in aspect ratio. The magnitude of critical confining stress for lid = 60 is about 150-200 kPa, whereas this drops to about 100 kPa for lid = 90, 120. This trend may possibly be due to the fact that in case of lower aspect ratio, say I/d <~60, the length of fibre available to mobilise surface frictional resistance is small and thus it requires high

Behaviour

of plastic-fibre-reinforced sand

561

2400

2200 ASPECT RATIO ,

FIBRE CONTENT Wf = 1 "1. BY Wt. 2000

1800 Q. q~:~ 1600 I1: -~ Iz,O0 u.

o'l tar) IJJ

1200

~ooo & z or. ix.

800

o

60C

t, OQ

200

01 0

I S0

I 100 CONFINING

I 200 STRESS,~

I 300

I ~*00

(kpa)

Fig. 3. Effect of aspect ratio on critical confining stress. confining stress for mobilising frictional resistance. On the other hand, at higher aspect ratio, the confining stress required to mobilise frictional resistance is small, i.e. lower value of critical confining stress. 3.3 Strength of fibre-reinforced sand As mentioned earlier, the shear strength of fibre-reinforced sand is affected by the fibre content, aspect ratio and confining stress. The strength of reinforced sand increases with an increase in fibre content (Fig. 4). The rate of increase is higher at lower fibre content (i.e. wr ~< 2%). At higher fibre

Gopal Ranjan, R. M. Vasan, H. D. Charan

562

3000

ASPECT

2800 = 300 k Po

2600 EL

=

2400

~ 2200 2000 0~3:200 k Pu

1800 16oo

1

4

0

0

~

<~ 1200

O~: 100k pc=

EL

~

g ,7

1000

8ooi 600

IE

400~ 200

0

I

0"5

I

I

I

I

I

I

1"0 1"5 2.0 2'5 3'0 3.5 FIBRE CONTENT~W! {'/o) BY WEIGHT

I

4'0

I

I

4.5 5'0

Fig. 4. Effect of fibre content on strength of reinforced sand at low and high confining stresses. content (i.e. wf > 2%), the relative gain in strength is small. This may be because of the fact that fibres, of specific gravity -~0.92, occupy a relatively large volume in the composite (a fibre content of 2% by weight is approx. equal to 4% by volume of the composite). Thus, with higher fibre content, the quantity of soil matrix available for holding the fibres is insufficient to develop an effective bond between fibre and sand. Also, for fibre content beyond 2% (by weight), the mixing of fibres and soil is impracticable as bailing up of fibres takes place and a uniform distribution cannot be obtained. The resistance to compaction also increases with increase in fibre content and heavy compaction is required to attain a constant density. The strength of fibre-reinforced sand increases with an increase in aspect ratio (l/d) as shown in Fig. 5. The rate of increase in strength of

Behaviour of plastic-fibre-reinforced sand

563

2200 2000 O~ k p a

(1. W f = 1 *1.

1800

_

/~"~

1-1

7~ 1600 t.IJ n,.-= 140,

s~

~ 120 rY p-

"S I

~

100(

,~s S 800

Q.

60C

z cL

40C

o "~

20,

~E

I

30

45

I

I

60

75

ASPECT

I

I

90

RATIO(Lid

105

I

120

)

Fig. 5. Effect of aspect ratio on strength of reinforced sand. reinforced sand is higher for lid <<. 90; thereafter the rate of increase is lower. At higher aspect ratios (i.e. lid > 90), the fibre may not remain straight and thus the effective length of fibre available to mobilise shear strength reduces. At low aspect ratios (i.e. lid < 60), the length of fibre is insufficient to develop tensile stress along it. It is evident from Fig. 5 that the rate of increase in strength of fibre-reinforced sand is higher at higher confining stress (i.e. a3 > Gcrit) at the same aspect ratio. Figure 6 shows a plot of fibre content, in terms of percentage by weight, and ratio of strength of reinforced sand to the strength of unreinforced sand, Si at confining stresses ranging from 50 to 400 kPa. It is evident that, for the range of confining stresses 50-400 kPa, the strength of reinforced sand over unreinforced sand increases practically linearly with increase in the fibre content. However, closer study of the trend reveals that the increase is more pronounced up to a fibre content of 2% beyond which the increase is not in the same proportion.

4 CONCLUSIONS A series of triaxial compression tests was conducted to study the stressstrain behaviour of plastic-fibre-reinforced sand and increase in shear

30[

564

2 c

Gopal Ranjan, R. M. Vasan, H. D. Charan

03"=

SO- 400 kPa 0

t/d = 75

r',, tlJ U IZ

U, (Z 0 U,.

~.~

i

~0.5

II

0 0

I 0"5

I 1.0

I 1"5

FIBRE CONTENT ( W f ) - -

I 2.0 %

I 2'5

I 3,0

I 3.5

4.0

BY WEIGHT

Fig. 6. Effect of fibre content on increase in strength of reinforced sand.

strength of sand due to fibre inclusions. Test results show that fibre reinforcement increases shear strength and modifies stress-strain behaviour of sand significantly. The following conclusions emerged from the present study: 1. Relatively low-modulus, plastic-fibre reinforcements do not rupture during shear. The main role of fibres is to increase peak shear strength and to reduce the loss of post-peak stress. Thus," residual strength of fibre-reinforced sand is higher as compared to that of unreinforced sand. 2. The principal stress envelopes for fibre-reinforced sand are bilinear having a break at a confining stress, called critical confining stress (O'crit), below which the fibres tend to slip or pull out. 3. An increase in fibre aspect ratio, (l/d), results in a lower critical confining stress and higher contribution to shear strength. 4. Shear strength increases approximately linearly with increasing fibre content up to 2% by weight, beyond which the gain in strength is not appreciable. The findings of this study have practical significance as a ground improvement technique, with respect to embankment, subgrade and other

Behaviour of plastic-fibre-reinJbrced sand

565

such problems. Further experimental work is in progress to find optimum fibre content and aspect ratio for a particular stress environment and soil granulometry.

REFERENCES AI-Refeai, T. (1991). Behaviour of granular soils reinforced with discrete randomly oriented inclusions. J. Geotext. Geomembr., 10, 319-33. Bishop, A. W. & Henkel, D. J. (1969). Measurement of Soil Properties in Triaxial Test. E.L.B.S., London. Fatani, M. N., et al. (1991). Reinforcing soil with aligned and randomly oriented metallic fibres. Geotech. Testing J., 14, 78-87. Gray, D. H. & A1-Fefeai, T. (1986). Behaviour of fabric versus fibre-reinforced sand. J. Geotech. Engng, ASCE, 112, 804-20. Gray, D. H. & Maher, M. H. (1989). Admixture stabilization of sand with discrete, randomly distributed fibres. Proc. XIIth Int. Conf. on SMFE, Rio de Janeiro, Brazil, pp. 1363-6. Gray, D. H. & Ohashi, H. (1983). Mechanics of fibre-reinforcement in sand. J. Geotech. Engng, ASCE, 109, 335-53. Hoare, D. J. (1979). Laboratory study of granular soils reinforced with randomly oriented discrete fibres. Proc. Int. Conf. on use of Fabrics in Geotech, Vol. I. Paris, France, pp. 47-52. Maher, M. H. & Gray, D. H. (1990). Static response of sands reinforced with randomly distributed fibres. J. Geotech. Engng, ASCE, 116, 1661-77. McGown, A., et al. (1978). Effect of inclusion properties on the behaviour of sand. Geotechnique, 28, 327-46. Setty, K. R. N. S. & Rao, S. V. G. (1987). Characteristics of fibre reinforced lateritic soil. IGC (87), Bangalore, Vol. 1, pp. 329-33. Setty, K. R. N. S. & Murthy, A. T. A. (1990). Behaviour of fibre-reinforced black cotton soil. IGC (1990), Bombay, pp. 45-9.