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Department of Civil Engineering Seminar2010
1 IES College of Engineering, Chittilappilly
STUDY ON THE ENGINEERING PROPERTIES OF SOILS
REINFORCED BY SHORT DISCRETE POLY PROPYLENE
FIBRE
Seminar Report Submitted by
IRSHAD IBRAHIM N.P.K
In Partial Fulfillment of the
Requirements for the Award of the Degree of
Bachelor of Technology
in
Civil Engineering
Department of Civil Engineering
IES COLLEGE OF ENGINEERING
Thrissur, Kerala, India 680 551
(Approved by AICTE and affiliated to University of Calicut)
2010

STUDY ON THE ENGINEERING PROPERTIES OF SOILS REINFORCED BY
SHORT DISCRETE POLY PROPYLENE FIBRE
ABSTRACT
In order to understand the engineering properties of soil reinforced by short
polypropylene fibre, a series of tests were carried out to study the effect of fibre content and
fibre length on the strength of the fibre-reinforced soil, as well as the effect of aggregate
size and fibre additives on the engineering properties of the fibre-reinforced soil. It was
shown from test results that the unconfined compressive strength (UCS), cohesion and
internal friction angle of fibre-reinforced soil were greater than those of the parent soil;
the UCS, cohesion and internal friction angle of fibre-reinforced soil exhibited an initial
increase followed by a rapid decrease with increasing fibre content and fibre length and
hence the optimal fibre content and fibre length were found as 0.3% by weight of the parent
soil and 15mm, respectively in this investigation. Similar trends were found in the parent
soil and the fibre-reinforced soil that the strength declined with an increase in aggregate
size and there was a critical size for aggregate breakage between 3.5 and 7.5 mm in
average diameter; the presence of polypropylene fibre could effectively contribute to the
increases in the strength and stability of the parent soil.

1.0 INTRODUCTION
Soil reinforcement is an effective and reliable technique for improving strength and
stability of soils. In conventional methods of reinforced-soil construction, the inclusions of
strips, fabrics, bars, grids etc. are normally oriented in a preferred direction and are
introduced sequentially in alternating layers. When some fibre (or geotextile) of high
tensile strength is admixed (or laid) in soil, the engineering properties of soil are improved
and the reinforced soil is known as fibre-reinforced soil. In the ancient time, some natural
materials including wood, bamboo, reeds, wheat straw and rice straw were used to improve
the strength of soil. With the advent of synthetic fibre and its rapid development, plenty of
synthetic fibres have been employed in many fields as innovative engineering materials, as
well as main reinforcement agents for ground improvement. At present, the common types
of synthetic fibre are: carbon, steel, glass, asbestos, polyester, polyethylene, polypropylene,
nylon, polyacrylonitrile (PAN) and high-elastic-modulus polyvinyl alcohol (PVA) fibre. In
these artificial synthetic fibres, mass long fibre is laid in the soil to improve the engineering
properties of soil as geogrid or geotextile while a small quantity of short fibre dispersed
into concrete or asphalt as an additive to improve their strength and other properties (Shi et
al 2007).
In order to improve the anti-deformation behaviour, strength and uniformity of soil, some
experimental studies were performed on the soil reinforced with short fibre and a number of
significant findings were obtained. Experts reported that the addition of a small amount of
synthetic fibre increases the failure stress of the composite. Researchers carried on a series
of triaxial compression test, concluded that the increase in strength of fibre reinforced soil
is a function of fibre weight fraction. Researches based on their test results, indicated that
fibre inclusion reduced crack formation and hydraulic conductivity of compacted clay soil.
Experts found that the strength of soil was improved when shreds of plants such as jute,
sisal and coconut shells were mixed into soil. Taking the biological degradation of plant
into account, some scholars attempted to adopt artificial synthetic fibre to improve the
engineering properties of soil. Researchers conducted an experimental study on California
bearing ratio of soil and indicated that the presence of fibre in sand fill caused an
appreciable increase in the peak piston load. The test results of experimentalists showed

that fibre reinforcement could significantly improve the shear strength, tensile strength and
anti-hydraulic-fracture behaviour of clayey soil. The work of experts indicated that the
presence of fibre greatly improved the dynamic strength of silty clay samples under
dynamic tension, as well as the toughness and plasticity of soil. Experimentalists carried
out the centrifuge model test and found that the failure of the untreated slope was often
sudden while the failure of the fibre-reinforced slope was progressive: the formation and
development of cracks could be observed, which confirmed that fibre reinforcement could
effectively enhance the stability of slopes. Researchers reported that the admixture of fibre
and cement in soil could greatly improve the toughness and strength of pure cement-treated
soil.
The research results indicated that fibre-cement-fly ash stabilized soil of higher strength
and toughness was obtained by adding a little glass fibre into cement-fly ash treated soil
and that its strength increased with increasing content and length of glass fibre.
Experimentalists conducted a series of experiments on fibre-reinforced filled soil and found
that the presence of fibre could markedly improve the toughness of filled soil and change
the failure characteristics of soil. Experts studied the effect of polypropylene fibre and lime
admixture on engineering properties of clay soil. Experts analyzed the strength and
mechanical behaviour of fibre-cement treated clayey soil based on some experiments.
Due to access of cheap polypropylene fibre material, short discrete polypropylene fibre was
employed to prepare the fibre-reinforced soil samples in this investigation. A series of tests
were carried out to analyze the variation of engineering properties of soil reinforced by
polypropylene fibre and some significant findings are presented here.
2.0 MATERIALS AND EXPERIMENTAL PROGRAMME
2.1 MATERIALS
2.1.1 Reinforcement
Both synthetic and natural fibers were used in present investigation to study the effect of
fiber inclusion on the strength of fine sand. Table 1 is a summary of the properties of the
different types of fibers that were used in the study.

Table1: Fiber Characteristics
Polypropylene fibres are high-intensity fascicular single fibres of stable chemical behaviour
and good resistance to ageing and corrosion. Fig. 1 illustrates the fasciculus and singles of
polypropylene fibre. Physical, chemical and mechanical characteristics of the
polypropylene fibre used in this investigation are presented in Table 2. As a new kind of
reinforcement material used in geotechnical engineering, polypropylene fibre has the
following unique advantages: (i) It is of high intensity; (ii) Like lime and cement, it can be
dispersed easily into soil and the reinforced soil samples take on isotropic strength
characteristics; (iii) The presence of short discrete polypropylene fibre in soil can prevent
the occurrence of potential weak structural planes which usually form due to the laying
direction of geotextile and laying distance of geogrid. The parent soil used in this
investigation was Xiashu clayey soil, a typical clayey soil extensively distributed in
Nanjing region, China. Its engineering properties were determined as follows: liquid limit
is 34.2%; plasticity index 17.3; optimum moisture content 15.7% and maximum dry density
is 1.70 g/cm3.
Fig.1 (A): Polypropylene Fibres (Shiet Al, 1997)

Fig.1 (B): Polypropylene Fibre Single Fibre
Table2: Physical, chemical and mechanical characteristics of polypropylene fibers
studied
2.1.2 Soil Characteristics
Five different types of soil were used in the present investigation. The selection of cohesion
less soils for testing was made in such a way that it covered a relatively broad range of
particle size diameters, Le. Medium - grained sand to non plastic silt. However, all the soils
tested were uniformly graded, having coefficient of uniformity, C. = 2.28 - 2.38. Table 3 is
a summary of the properties of the soils.

Table3: Properties of Solis Used
"Detailed triaxial tastings’ have been carried out on soils, SI, S2. And S3 to develop a
mathematical model, while selected tests have been conducted on soils, S4 and S5 for
verification of the model.
c- parameters were determined by triaxial compression tests on samples prepared at an
optimum moisture content and maximum dry density of respective soils, obtained from
standard Proctor's tests.
2.2 EXPERIMENTAL PROGRAMME
In order to figure out the influence of fibre characteristics and soil nature on engineering
properties of fibre-reinforced soil, two experimental plans were proposed: Plan A
concerning fibre characteristics variation and Plan B with interest in soil nature change.
Four different fibre lengths (i.e. 10 mm, 15 mm, 20 mm and 25 mm) and four different
percentages of fibre content (i.e. 0.1%, 0.2%, 0.3% and 0.4% by weight of the parent soil)
were chosen to prepare a series of fibre-soil admixtures for tests designed in Plan A.
Considering the effect of aggregate size and distribution on the engineering properties of
the remolded soil (Shi et al., 1995, 1998, 2007), Plan B was designed to analyze the
influence of aggregate size on the engineering properties of fibre-reinforced soil. Four
groups of aggregates were obtained by having the air-dried parent soil simultaneously pass
through four different sieves and the four aggregate sizes are illustrated in Fig. 2: (i) less
than 1 mm; (ii) 1 to 2 mm; (iii) 2 to 5 mm and (iv) 5 to 10mm. Subsequently, each group of
aggregates was mixed with 0.4 % fibre by weight of the parent soil to prepare different
fibre-soil admixtures required for Plan B. Besides the fibre-soil samples, one unreinforced

sample for Plan A and four ones for Plan B were prepared as reference to understand the
reinforcement effects. Thus, there are 17 groups of soil samples for Plan A and 8 groups for
Plan B in total.
All test samples were prepared with static compaction method and, the moisture content
and dry density of the prepared samples were 15.4% and 1.70 g/cm3, respectively. It should
be pointed out that the four-layered compaction was adopted to keep the uniformity of test
samples for USC tests and two-layered compaction for direst shear tests. USC tests were
performed on samples with the diameter of 39.1mm and height of 80mm at the strain rate
of 2.4 mm/min until samples failed. Direct shear tests were carried out on the samples,
61.8mm in diameter and 40mm in height, at the strain rate of 0.8 mm/min under the normal
pressures of 50, 100, 200 and 300 kPa. Additionally, considering the influence of
experimental conditions and random samples on the measurements, each test in the
investigation was perform on samples in triplicate.
Fig.2: The aggregates with different sizes
3.0 STATISTICAL ANALYSIS OF TEST RESULTS
The test results of the investigation are given in Table 4 and Table 5. In order to validate
the reliability of the test results, the coefficient of variation (Cv) was calculated for every
single triplicate test and the values of Cv are presented in the three tables above. It is shown
that Cv values range from 16.67% to 0.95% in the investigation and most of those are less
than 10%. Such Cv values mean that the distribution of test results is of low dispersion (or
variance), which makes the results from the triplicate test reliable. The average of the three
measured results was, therefore, used for the later analysis.

3.1 SHEAR STRENGTH OF FIBER-REINFORCED SOIL
Following fig. shows the typical plots of deviator stress-axial strain for plastic fiber-
reinforced fine sand, having fiber content of 0, 1, 2, 3, and 4% at aspect ratio 75. The stress-
strain behavior of fiber-reinforced sand is much different from that of unreinforced sand
(Fig. 3). Unreinforced sand attains a peak: stress at around 10% axial strain which then
remains practically constant even up to 20% axial strain, whereas fiber 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%. The failure in such situations is generally
defined in terms of serviceability by specifying a permissible amount of deformation.
Usually failure stress is taken corresponding to a strain of 15 or 20%. Thus, in the present
analysis, the failure has been defined as the stress corresponding to the peak: stress
condition or at 20% axial strain, whichever is earlier. Here, shear strength has been denoted
in terms of major principal stress at failure.
Fig.3: Deviator Stress-Axial Strain Plot of Plastic Fiber Reinforced Fine Sand

3.2 EFFECT OF FIBRE CHARACTERISTICS AND SOIL NATURE ON THE
ENGINEERING PROPERTIES OF FIBER-REINFORCED SOIL
3.2.1 Effect of Fibre Content And Fibre Length on Strength of Reinforced Soil
3.2.1.1 Effect of Fibre Content And Length on UCS
It is obviously seen from Table 4 that the UCS values of fibre-reinforced samples are
greater than those of unreinforced samples. From Fig. 4, it is seen that the same trends are
found in four groups of samples differing in fibre length: with an increase in fibre content,
the UCS experienced an initial increase followed by a decrease and the maximum value of
the strength is found at the fibre content of 0.3%. Fig. 5 shows that for any fibre content
studied, the UCS increased gently and then decreased rapidly with increasing fibre length
and the maximum value is observed at the fibre length of 15 mm. In addition, fibres less
than 15 mm long contribute to better reinforcement effect in strength than does longer
fibres greater than 15 mm.
Table 4: Unconfined Compressive Strength of Samples Tested

Fig.4: Effect of fibre content on unconfined compressive strength of fibre-reinforced
soil
Fig.5: Effect of fibre length on unconfined compressive strength of fibre-reinforced
soil
3.2.1.2 Effect of Fibre Content And Length on Cohesion And Internal Friction Angle
It is indicated from Table 5 that both cohesion and internal friction angle of fibre-reinforced
samples are greater than those of unreinforced ones and hence the presence of fibre benefits
increases in cohesion and internal friction angle of soil. Furthermore, it is also found that
the improvement of cohesion and internal friction angle of fibre-reinforced samples has
some relation with both the length and content of fibre.

Table 5: Shear Strength Parameters of Soil Samples Tested
It is seen from Fig. 6 that for any particular fibre length, the cohesion of fibre-reinforced
samples increases initially and then decreases with increasing fibre content and the
maximum value is observed at a fibre content of 0.3%.

Fig.6: Effect of Fibre Content on Cohesion of Fibre - Reinforced Soil
Fig. 7 shows that for any fibre content, the cohesion of fibre-reinforced samples increases
gently followed by a rapid decrease with an increase in fibre length, and the maximum is
found at a fibre length of 15 mm. This demonstrates that to improve cohesion, it is better to
use short fibres with a length not more than 15 mm for reinforcement.
Fig.7: Effect of fibre length on cohesion of fibre-reinforced soil
It is clearly indicated from Fig. 8 that with an increase in fibre content, the internal friction
angles of three groups of samples respectively reinforced by 10, 15 and 20 mm fibre show
an initial increase followed by a decrease and the maximum values are observed at the fibre
content of 0.3%. The angle of internal friction of the 25 mm fibre-reinforced samples
remained constant.

Fig.8. Effect of fibre content on internal friction angle of fibre-reinforced soil
Fig. 9 shows that the internal friction angles of fibre-reinforced samples all increased gently
at first and then decreased gradually with increasing fibre length and maximum values are
found at the fibre length of 15 mm. The small difference in the internal friction angles of
fibre-reinforced samples, ranging from 27 to 33 degrees, shows that the fibre content and
length has more significant influence on cohesion than on the angle of internal friction of
soil.
Fig.9: Effect of fibre length on internal friction angle of fibre-reinforced soil

3.2.1.3 Effect of aggregate size on the strength of fibre-reinforced soil
The graphs below illustrate the effect of aggregate size on cohesion and angle of internal
friction, respectively. It is clearly indicated that when average aggregate size is up to 3.5
mm, with an increase in aggregate size, cohesion of both unreinforced and reinforced soil
decreased while internal friction angles of the two types of soil increased. However, when
average aggregate size is greater than 3.5 mm, the opposite trends were found in cohesion
and internal friction angle of both unreinforced and reinforced soil. Moreover, the variation
of cohesion and internal friction angle of unreinforced soil with aggregate size was much
more dramatic than that of reinforced soil.
Fig.10: The Variation of Cohesion of Samples with Aggregate Size
Fig.11: The Variation of Internal Friction Angle of Samples with Aggregate Size

Following Figure represents graphically the variation of UCS of the two types of soil with
aggregate size. It is seen that the UCS of both unreinforced and reinforced soil decreases
with increasing aggregate size.
Fig.12: The variation of unconfined compressive strength of samples with aggregate
size
4.0 DISCUSSIONS ON MECHANISMS FOR STRENGTH PROPERTY
VARIATIONS
It is indicated from in the investigation that both fibre content and length influence UCS,
cohesion and internal friction angle to different extents. The fibre reinforcement mechanism
can be described as a physical interaction between fibre and soil particles. In the case of a
particular fibre length, with an increase in fibre content of samples, the interface between
fibre and soil particles increases and hence the friction between fibre and soil particles
increases, which makes it difficult for soil particles surrounding fibres to change in position
from one point to another and thereby improves the bonding force between soil particles,
i.e. cohesion of soil. Moreover, when local cracks appear in soil, fibres across the cracks
will take on the tension in the soil with fibre-soil friction, which effectively impedes further
development of cracks and improves the resistance of soil to the force applied, i.e. UCS.
An increase in fibre length can result in the two impacts: on the one hand, the interface
between fibre and soil particles increases and thereby the friction between them increase;
but on the other hand, in the case of a certain percentage of fibre, the number of single

longer fibres decrease, which reduces the entire reinforcement effect. With the combination
of these two aspects, the fibre length has a little influence on the compressive strength and
shear strength of fibre-reinforced soil. Owing to the physical interaction which has little
effect on the soil fabric, the fibre reinforcement hardly give rise to the improvement of
interlocking force between fibre and soil particles, which explains indistinct variations in
internal friction angle of soil with increasing fibre content and length. However, if the
amount of fibre admixed into soil is too much, the fibres will adhere to each other to form
lumps, which leads to an uneven distribution of fibre in soil and deficient contact of fibre
with soil particles. This consequently reduces the effectiveness of improvement in
compressive strength and shear strength parameters. Furthermore, too long fibre added can
reduce the reinforcement effect of fibre, inasmuch as it is difficult to fully mix long fibre
with soil and thereby the uneven distribution of fibre in soil impacts the homogeneity of
fibre-reinforced soil. Therefore, the optimal fibre content and length for the investigation
are considered as 0.3% by weight of the parent soil and about 15 mm, respectively. As far
as the influence of aggregate size is concerned, it is obvious that there is a turning point
observed at the average aggregate size of 3.5 mm in the variation curves of cohesion and
internal friction angle of samples with aggregate size. As opposite variation trends are
found either side of the turning point, the mechanism of the variation of cohesion and
internal friction angle are discussed in two cases.
When average aggregate size is less than 3.5 mm, the cohesion of unreinforced clayey soil
is mainly derived from the bonding force between adjacent aggregates, the viscous force of
the hydrated membrane and surface tension of capillary water. With an increase in
aggregate size, the interface between adjacent aggregates in unit area gradually decreases
and thereby the entire adhesion at contact sections declines, which leads to a decrease in
cohesion of unreinforced soil. The cohesion of fibre-reinforced soil is composed of the
cohesion of unreinforced soil and the friction between fibre and soil particles. When
aggregate size increases, the number of aggregates contacting with fibre of unit length will
decrease and hence the friction between fibre and aggregates decrease. Due to the same
variation trends found in cohesion of unreinforced soil and fibre-aggregate friction, the
cohesion of fibre-reinforced soil, therefore, decreases with increasing aggregate size.
Moreover, the angle of internal friction of both unreinforced and reinforced soil increased

with increasing aggregate size. A possible explanation is that, with an increase in aggregate
size, the potential energy required to move an aggregate from one position to another
during shearing increases, thus the sliding friction of aggregates correspondingly increases,
which in turn increases the angle of internal friction.
When average aggregates size exceeds 3.5 mm, the cohesion of both unreinforced and
reinforced soil gradually increased while the angle of internal friction decreased. Moreover,
the internal friction angle of unreinforced soil varied greatly while those of reinforced soil
did a little. This may be due to many big aggregates in unreinforced samples being crushed
into small ones during the preparation of soil samples, which makes the compressed
samples take on similar mechanical characteristics (i.e. high cohesion and small internal
friction angle) to those of samples with average aggregate size less than 3.5 mm. Due to the
higher toughness of fibre-reinforced soil, the broken aggregates in reinforced soil are much
smaller than those in unreinforced soil under the same load applied, which contributes to
the smaller variation of internal friction angle of reinforced soil. Therefore, it could be
inferred that there exists a critical size for aggregate breakage between 3.5 and 7.5 mm. If
the aggregate size is beyond the critical size, the aggregates will begin to be crushed into
fine grains under the compaction density applied in the investigation. However, as far as
single aggregate is concerned, the strength has an adverse relation with the size due to the
subordinate structures inside aggregates and consequently the entire strength of soil
decreases during shearing while aggregate size increasing, which explains that the UCS of
both unreinforced and reinforced soil decreases with an increase in aggregate size, as
shown in Fig. 12.
5.0 CONCLUSIONS
1. The effect of short discrete polypropylene fibre on the engineering properties of soil
studied in investigations and indicate that the UCS, cohesion and internal friction
angle of fibre-reinforced soil were greater than those of the parent soil
2. The UCS, cohesion and internal friction angle of fibre-reinforced soil experienced
an initial increase followed by a decrease with increasing fibre content and hence
the optimal fibre content was reported to be 0.3% by weight of the parent soil in the
investigation.

3. With an increase in fibre length, the UCS, cohesion and internal friction angle of
fibre-reinforced soil gently increases at first and then rapidly decreases and the
optimal fibre length for the study was observed at about 15 mm.
4. The strength of both unreinforced and reinforced soil decreases while aggregate size
increasing.
5. But with an increase in aggregate size, the cohesion of the two types of soil
experiences an initial decrease followed by an increase and, the lowest value is
found when the average aggregate size is around 3.5 mm.
6. The internal friction angle takes on an experience opposite to that happens to the
cohesion and the peak value is also found at the average aggregate size of about 3.5
mm.
7. There is a critical size for aggregate breakage between 3.5 and 7.5 mm average
diameter.
8. The presence of polypropylene fibre could effectively contribute to the
improvement in the strength and stability of the parent soil.
6. REFERENCES
1. Shi, B., Liu, Z.B., Cai, Y., Zhang, X.P.,( 2008). Micropore structure of aggregates
in treated soils. Journal of Materials in Civil Engineering, ASCE, 19(1): 99-104.
2. Gopal Ranjan, R. M. Vasan , and H. D. Charan(1996), Probabilistic Analysis of
Randomly Distributed Fiber-Reinforced Soil, Journal of Geotechnical Engineering,
Vol. 122, Paper No. 9546.

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