IJERD – International Journal of Environmental and Rural Development (2018) 9-1

Research article
erdStrength Characteristics of Stabilized Soils Containing
Bamboo Fiber Extracted by Steam Explosion
FUTOSHI KANAWA*
Faculty of Regional Environment Science, Tokyo University of Agriculture, Tokyo, Japan
Email: fk205262@nodai.ac.jp
Received 10 February 2018

Accepted 18 May 2018(*Corresponding Author)

Abstract Construction work and materials depend mostly on earth resources. A sustainable
recycling-oriented society becomes possible only when technologies for recycling as well
as for energy conservation in construction are disseminated and when untapped natural
resources are effectively utilized. In this paper, applicability of natural fibers for the soil
stabilization technique was examined. Especially, this paper focuses on bamboo fiber
extracted by steam explosion, which technique is capable to produce thin and flexible fibers
from raw bamboo. In order to understanding the mechanical properties of soil mixed withbamboo fiber produced by steam explosion, unconfined compression tests were conducted.
As a result of unconfined compression tests, following findings are obtained. (1) The
addition of bamboo fiber to cohesive soil helps to increase the unconfined compression
strength of the soil. Moreover, the addition of bamboo fiber to soil helps to increase thetoughness of the soil. (2) An increase in the mix ratio of bamboo fiber results in a change inthe failure mode of test specimens. In addition, at a mix ratio by weight of 0.4% or higher,
specimens did not occur shear failure, but instead broke such as to become barrel-shaped.
(3) The deformation coefficient of the soil mixed with bamboo fiber is similar to thedeformation coefficient of cohesive soil containing no bamboo fiber. The addition ofbamboo fiber does not affect deformation properties.
Keywords bamboo fiber, steam explosion, soil stabilization, unconfined compressive test

INTRODUCTION
Construction work and materials depend mostly on earth resources. A sustainable recyclingoriented society becomes possible only when technologies for recycling as well as for energy
conservation in construction are disseminated and when untapped natural resources are effectivelyutilized. The effective utilization of soft soil in situ and of soil generated as a byproduct ofconstruction is realized by adding short-fiber materials to such soil in order to improve its
mechanical properties (Gowthaman, 2018). This soil stabilization technique differs from those thatutilize solidifying material, in that the addition of short-fiber materials has little effect on drainage
and plant growth. Thus, this technique is expected to be applied to vegetation beds (Miki, 1994;
Masuysms, 2012). It is known that mixing of polyester, polypropylene, polyethylene or vinylon
fibers with soil helps to improve the toughness, strength and erosion resistance of the soil. Those
reinforced soils have been already applied to materials that cover embankment slopes and toroadbeds on soft ground. For example, Sato et al. (1998) examined the applicability of vinylon
fiber as material for subgrade soil improvement. That research found the following: Adding fiber
materials to subgrade soil increased the unconfined compression strength, California Bearing Ratio
and resilient modulus; the effectiveness of soil improvement varies depending on the type andmoisture content of the subgrade soil as well as on the shape of fibers; and long, thin fibers areeffective when fiber is added to soft subgrade soil. As mentioned above, the majority of theresearch on improving slope stability and bearing capacity addresses chemical fibers, and researchutilizing natural fibers is limited (Dipika, 2016). For example, Otsubo et al. (2014) examined theerosion resistance of a revegetation bed on a slope to which bamboo fiber was applied. TheⒸ ISERD

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findings of that research included the benefits and application methods of bamboo fiber, but nomechanical assessment of bamboo fiber application was conducted.
Bamboo has been utilized as a useful natural resource in Japan. However, because bamboo hasbeen increasingly replaced by plastic, demand for domestic bamboo has significantly declined andmany bamboo forests have been left unmanaged. Bamboo grows fast and is expected to be used asa renewable resource. Techniques for the effective use of bamboo need to be developed. Bamboo isknown for its high tensile strength. In the past, concrete reinforced with bamboo members wasdeveloped. Bamboo-reinforced concrete is drawing renewed attention as a material for aseismicreinforcement (Terai, 2012). The use of bamboo as a construction material has also been examined.
In this paper, applicability of natural fibers for the soil stabilization technique was examined.
Especially, this paper focuses on bamboo fiber extracted by steam explosion, which technique iscapable to produce thin and flexible fibers from the raw bamboo. In order to understanding themechanical properties of soil mixed with bamboo fiber produced by steam explosion, unconfinedcompression tests were conducted.
METHODOLOGY
Production of Bamboo Fiber
Bamboo fiber was extracted by steam explosion. This technique is said to have been developed by
Mason in the 1920s for producing pulp used for wood fiberboard. In this technique, material issteamed by using high-temperature, high-pressure saturated water vapor in a pressurized vessel fora given length of time. When the steamed material is suddenly re-exposed to air at normal
atmospheric pressure, condensed moisture in the material evaporates to cause explosive volumetricexpansion. As a result, the material is crushed. This fiber extraction technique has the following
characteristics: The material structure can be crushed from within; the technique utilizes water
vapor alone and no chemicals, so no wastewater disposal is required; and mechanical crushing andchemical treatment by steaming can be done simultaneously. The steam explosion machine used inthis study is shown in Photo 1. When bamboo fibers are extracted, the properties of the fibers varydepending on the predetermined conditions of steam explosion. The length and flexibility of fibers
need to be considered when bamboo fibers are extracted for mixing with soil. In this study, thesteam pressure was 1.5 MPa, the steaming time was 30 minutes, and the fiber length was 25 mm.
On the basis of the preliminary test results, these steam explosion conditions were determined toensure the production of bamboo fibers flexible enough to be easily mixed with soil. The fiber
length was adjusted by cutting the raw bamboo to a predetermined length. Photo 2 shows thebamboo fiber that was created by steam explosion in this study.

Photo 1 Steam explosion machine

Photo 2 Bamboo fiber obtained by steam explosion

Water Absorption Test for Bamboo Fiber
Soil strength properties vary according to the moisture content of soil. Because bamboo has a highⒸ ISERD

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water absorption rate, the production of specimens for unconfined compression tests needs to takethe water absorption rate into account. For this purpose, water absorption tests of bamboo wereconducted. The water absorption rate was defined as the ratio of the absorbed moisture to the dry
mass of bamboo. In a water absorption test, a specimen was immersed in water for a predetermined
period of time. Then the bamboo surface was wiped with a cloth, and the bamboo mass wasmeasured. This process was repeated until the bamboo mass reached a predetermined value. Whenthe predetermined bamboo mass was obtained, the specimen was oven-dried and its dry mass wasmeasured. The water absorption test was conducted six times. The test results indicate that bamboo
fiber absorbs water immediately upon immersion in water and that prolonged immersion does not
significantly increase the bamboo mass thereafter. The mean value of water absorption rate for thebamboo used in this study was 42.2%, and the variation coefficient was 8.00%. Based on the waterabsorption rate of 42.2%, the water content of the soil used in the tests described below wasadjusted.
Soil Properties and Unconfined Compression Test
The soil for stabilization in the tests is a cohesive soil called “Kanto loam”, which is a kind ofvolcanic ash soil. With the aim of understanding the physical properties of the cohesive soil, a sieve
analysis test, a density test of soil particle, a compaction test, a liquid limit test, and a plastic limit
test were performed. Each of these soil tests was conducted in accordance with The Japanese
Geotechnical Society: Soil Tests (JGS, 2010). The tests found the fine fraction content to be 46.0%,
the soil particle density to be 2.70 g/cm3, the liquid limit to be 68.5 %, and the plastic limit to be56.7%. Fig.1 shows a moisture density curve. The maximum dry density of the cohesive soil ρdmax
is 1.35 g/cm3, and the optimum water content wopt is 34.6%.

Dry Density ρd [g/cm3]

1.40
1.36

Zero void ratio curve

ρdmax

1.32

1.28
1.24wopt

1.20
2530354045
Water content w [%]50Fig. 1 Moisture density curve
The unconfined compression test was conducted by using specimens whose moisture content
was adjusted to be optimum moisture content (wopt). The mix mass ratios of bamboo fibers were0%, 0.3%, 0.4%, and 0.5%. Six specimens were used for each mix ratio. The unconfinedcompression test was performed with reference to The Japanese Geotechnical Society: Soil Tests(JGS, 2010).
RESULTS AND DISCUSSION
Figure 2 shows the test results in terms of compressive stress-strain curves. These curves
demonstrate that after a unconfined compressive strength has been attained, compressive stress ismaintained for soil that contains bamboo fiber, while that is not the case for soil containing nobamboo fiber. This result suggests that the addition of bamboo fiber to soil helps to increase theⒸ ISERD

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200

150

No.1

No.2

No.3

No.4

No.5

No.6

100
5000.02.0

4.06.08.0
Compressive Strain [%]Compressive Stress [kN/m2]

Compressive Stress [kN/m2]

toughness of the soil. It is also seen that the toughness of soil increases with increase in the mix
ratio of bamboo fiber.
200

No.1

No.2

No.3

No.4

No.5

No.6

500
2.0

4.0
6.0
8.0
Compressive Strain [%]

No.6

4.0
6.0
8.0
Compressive Strain [%]

10.0

500
0.0

2.0(b) Mix ratio of 0.3%

1000.0

No.4

No.5

100

10.0

10.0

Compressive Stress [kN/m2]

Compressive Stress [kN/m2]

150

No.2

No.3150(a) Mix ratio of 0.3%200

No.1

200

No.1
No.3
No.5

150

No.2
No.4
No.6

100
500
0.0

2.0

4.0
6.0
8.0
Compressive Strain [%]10.0(c) Mix ratio of 0.4%(d) Mix ratio of 0.5%
Fig. 2 Compressive stress-strain curves
Table 1 shows the test results in terms of the unconfined compressive strength anddeformation modulus. As shown in Table 1, the unconfined compressive strength increases fromthe addition of bamboo fiber to soil. The variation coefficient of unconfined compression strengthis 11.0% when the mix ratio is 0.0%, while the variation coefficient at other mix ratios is 18.0%.
This result likely owes to the heterogeneous property of the soil mixed with bamboo fiber, which
points to the need for examining methods of mixing bamboo fiber with soil more uniformly. The
deformation coefficients calculated by using the data shown in Fig.2. As shown in Table 1, thedeformation coefficient for the soil mixed with bamboo fiber is similar to the deformation
coefficient for the original soil containing no bamboo fiber. This result suggests that the uniformity
in mechanistic deformation is ensured in the soil mixed with bamboo fiber. Thus, it is not necessary
to give special consideration to the possible effects of bamboo fiber on deformation when
designing the earth structure.
Table 1 The results of unconfined compression tests
Mix mass ratios of bamboo fibers [%]2

The unconfined compressive strength qu [kN/m ]Failure strain [%]Deformation modulus E50 [MN/m2]Ⓒ ISERD

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0.0

0.3

0.4

0.5

94.6
2.4
4.9

129.1
3.3
4.9

125.2
3.2
4.8

137.8
3.3
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IJERD – International Journal of Environmental and Rural Development (2018) 9-1

Photo 3 shows the failures of the specimens after the tests. The specimens whose bamboofiber mix ratio was 0.0% or 0.3% were occurred shear failure, and specimens whose bamboo fibermix ratio was 0.4% or 0.5% were occurred breakage that produced barrel-shaped specimens. The
shear failure surface of a specimen containing bamboo fiber at a mix ratio of 0.3% is shown in Phot.
4. In the photograph, a bamboo fiber spans the shear failure surface. No bamboo fibers were broken
during the tests. Thus, it is said that the pull-out resistance of bamboo fibers works against shear
failure and helps to prevent brittle failure. In the case of higher bamboo fiber content (e.g., 0.4% or0.5%), it is presumed that the large failure resistance caused the failure mode to be barrel-shaped
failure instead of shear failure. This suggests that the toughness of subgrade soil increases from theaddition of bamboo fiber. It can be said that soil mixed with bamboo fiber is adequate as a soil
improvement material that is applicable to pavement subgrade which needs to have good bearing
capacity and durability.

bamboo fiber(a) Shear failure(b) Barrel-shaped failure

Photo 3 Failure of specimens after
the compression test

Photo 4 Shear failure surface of a specimen
containing bamboo fiber

CONCLUSION
In this study, applicability of bamboo fibers extracted by steam explosion for the soil stabilization
technique was examined. In order to understanding the mechanical properties of soil mixed withbamboo fiber produced by steam explosion, unconfined compression tests were conducted. The
major findings of this study are as described below.
(1) That the addition of bamboo fiber to cohesive soil helps to increase the unconfined compression
strength of the soil. Moreover, the addition of bamboo fiber to soil helps to increase the toughnessof the soil.
(2) An increase in the mix ratio of bamboo fiber results in a change in the failure mode of test
specimens. Then, at a mix ratio by weight of 0.4% or higher, specimens did not occur shear failure,but instead broke such as to become barrel-shaped.
(3) The deformation coefficient of the soil mixed with bamboo fiber is similar to the deformation
coefficient of cohesive soil containing no bamboo fiber. The addition of bamboo fiber does notaffect deformation properties. Thus, uniformity of mechanical deformation is ensured.
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