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1ST BANGLADESH CIVIL ENGINEERING SUMMIT
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BUET-ANWAR ISPAT 1st Bangladesh Civil Engineering SUMMIT 2016
BUET, Dhaka, Bangladesh
www.cesummitbd.com
Investigation of Vetiver Root Growth in Sandy Soil
Mohammad S. Islam1, Md. Z. U. Arif2, Faria F. Badhon3, Soumik Mallick4, Tanzila Islam5
Department of Civil Engineering, Bangladesh University of Engineering and Technology, Dhaka
1msharifulbd@gmail.com,2mzuarif@gmail.com,3 fariafahim_badhon@yahoo.com,
4soumik081@gmail.com,5ti.cebuet187@yahoo.com
ABSTRACT
Slope stabilization of embankments is a great challenge for a developing country like Bangladesh. Present
practices for slope stabilization are the use of synthetic geo-textile, cement-concrete block, retaining wall
etc. These systems are costly and not environment-friendly. In contrast, bio-engineering is being used for
slope stabilization in many countries of the world. Vetiver is one of the species which is used as bio-
engineering for its many advantages like adaptation ability in adverse environment, fast growth of the
root, high tensile strength of its root, deep root etc. The present study aims to investigate the root
morphology of vetiver grass in sandy soil. To determine the root architecture of vetiver grass a wooden
frame of 2.7m×1.83m×0.8m was filled by dredged sand and 97 bottles (100 mm diameter) were placed at
the top of the wooden frame with one vetiver grass planted in each bottle. Then root length was measured
and root morphology was determined after certain time interval. Shear strength of vetiver rooted sand has
also been determined by conducting direct shear test. It is found that vetiver system can grow very well in
sandy soil and vetiver root can enhance the shear strength of sand. Thus vetiver root network can be
effective to protect the sandy soil from erosion.
KEYWORDS: Bio-engineering; root architecture; sand; slope stabilization; vetiver.
INTRODUCTION
Population pressures are increasing in most of the countries today and will certainly accelerate in the
future. These pressures have resulted in rapid urbanization and development. The increase in urban
population will require considerable expansion of urban boundaries. As a consequence of this urban
expansion, housing development and the construction of industrial structures, urban transportation
facilities and communications systems will disturb large volumes of geological materials. Along with the
development of homes in residential subdivisions comes the entire fabric of infrastructure, such as streets,
sidewalks, water and sewer lines and utility lines (Schwab et al., 2005). Such facilities require large
amounts of grading and excavation. All of these modifications may contribute to slope instability.
Particularly in developing nations, this pattern is being repeated, but with even more serious
consequences. So, it is clear that earth slopes should be protected to minimize the losses that occur every
year.
Protection of slope and embankment from erosion has become an important issue. The construction of
strong structures requires large capital, integrated designing, high maintenance cost. Moreover, strong
structural measures have negative impact on the environment. Plantation of vetiver system along the
slopes is an alternative solution. Verhagen et al. (2008) conducted different laboratory and model tests on
vetiver grass to realize the use of it in coastal engineering and showed that vetiver grass is able to
establish a full-stop to bank erosion caused by rapid drawdown. Vetiver grass is a sustainable and
innovative solution for the protection of banks. Most important thing is that vetiver grass is an
ISBN: 978-984-34-1512-7
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economically attractive and eco-friendly solution and has aesthetic value. Bangladesh Water
Development Board, BWDB (2000) found that vetiver grass is very common in about 40% of the total
land area of Bangladesh. Vetiver grass has wider applications due to its unique morphological,
physiological and ecological characteristics that highlight its adaptability to a wide range of
environmental and soil conditions.
The most impressive characteristic of the vetiver grass is its root system. The roots of vetiver grass are
fibrous and reported to reach depths up to 3m thus being able to stabilize the soil. Islam (2003) studied
the performance of vetiver grass on eighteen coastal polders over eighty-seven kilometers of earthen
coastal embankment of Bangladesh during the period from September 2000 to October 2001. He provided
some guide lines on vetiver application which is helpful for better performance. He achieved successful
cases where initial protection and watering could be ensured. Arifuzzaman (2011) studied the
performance of vetiver grass in protecting coastal embankment in Bangladesh. Islam and Arifuzzaman
(2010) and Islam et al. (2010) investigated the in-situ shear strength of vetiver rooted soil matrix and
bared soil.
Islam et al. (2012) compared the cost of vetiver with other traditional practices used for slope protection
and analyzed the factor of safety for bare soil and vetiver rooted soil. Islam (2013) used vegetation and
geo-jute for slope protection in different regions of Bangladesh. It was showed that this method is
effective in protecting slopes for different geological conditions.
To evaluate the actual performance of vetiver grass for protection of embankments, it is necessary to
estimate the factor of safety against the natural forces. Different tests were conducted by different
researchers (e.g., Hengchaovanich, 1998; Ke et al., 2003; Verhagen et al., 2008) to know the strength of
vetiver roots for the analysis of stability of slopes. Islam et al. (2013) conducted in-situ test and also
conducted direct shear test on laboratory reconstituted soil samples at different root content to know the
shear strength of vetiver grass. The aim of this paper is to investigate the root morphology of
vetiver for particular time interval in sandy soil.
METHODOLOGY
Test was conducted at BUET campus in BUET-JIDPUS premises. Temperature and humidity of this
area ranged between 14°C and 34°C and between 45% and 79%, respectively during the time of
investigation. Average annual rainfall was 1875mm. Figure 1 shows the location of study area.
Vetiver plants (obtained by splitting full grown vetiver plants and as far as possible, washing out the
material between the root) were planted on 14 August, 2016 into a 183cm×275cm×61cm wooden
frame, one plant in each bottle (10cm dia, 25 cm height). The stems of the vetiver grass were cut off
at a length of around 30 cm, while the roots were cut off at a length of 4 cm. Figure 2 shows some
pictures of vetiver plantation. Each stem of vetiver grass has several leaves and each leaf was
measured individually from the cut to the top, once a week for a period of 21 days. Temperature
fluctuated with weather and plants irrigated every day after plantation.
For determining the growth of vetiver in sand, a single vetiver tiller was carefully uprooted in every
week after five weeks of plantation had passed. The length of root and shoot and diameter of root
were carefully measured during the monitoring process. With a view to observing the root matrix
carefully, after uprooting, the sand in between the root system was carefully washed. After the
monitoring process was done, the tiller was replanted in the same location again.
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Figure 1: Location of experiment area for investigating the growth of vetiver root in sand
(a) (b) (c )
(d) (e)
Figure 2: Photographs of different stages of the experimental set-up: (a) collection of sand, (b) wooden
frame, (c) wooden frame filled with sand, (d) stem of vetiver just after plantation and (e) fully grown
vetiver after 39 days
LABORATORY SHEAR STRENGTH TEST
Direct shear test was conducted in Consolidated Undrained (CU) condition on reconstituted sand samples.
Tests were conducted on both bared and root mixed composite fine sand samples. Samples were prepared
with 2.54 cm long vetiver root at 15% water content. Root content was 3% of dry weight of sand. At first,
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roots were collected and then preserved in a refrigerator (at 4ºC) with arbitrary moisture content to keep
the roots fresh. Roots were chopped off to the desired length (2.54 cm). Chopped vetiver roots were
randomly mixed with the wet sand. The sand was then compacted by a wooden rod inside a probing ring
of the size 63.5 mm in diameter and 25.4 mm in height from a falling height of 100 mm. The compaction
was done in three layers where 25 blows were applied in each layer. The prepared samples were kept in a
desiccator to keep the moisture content unchanged. Direct shear test was conducted on those prepared
specimens according to ASTM standards.
The remolded soil sample was placed carefully in the shear box from the ring. Then the desired normal
load was applied. Normal stresses were arbitrarily selected as 15.49 kPa and 30.98 kPa. Vertical
displacement dial gauge was attached to record the vertical deformation with respect to time. The shear
force was applied to the soil sample with a constant strain rate of 0.75 to 1.25 mm/min. The lateral
deformation was recorded by same strain rate. The lateral deformation was recorded by a lateral
displacement dial gauge of 25 mm capacity. The applied shear force was recorded by a load dial gauge of
2.22 kN capacity.
RESULT AND DISCUSSIONS
The soil was collected from Buriganga river bed by dredging. Specific gravity of the soil was 2.74. Its
grain size distribution curve is shown in Figure 3. From the graph, it has been found that the soil is poorly
graded having coefficient of uniformity cu=1.86 and coefficient of curvature cc=1.1. All results are based
upon the observations with respect to length increase of root and shoot in time. Figure 4a, 4b and 4c
shows the stage of growth of root at approximately one week time interval and a schematic diagram of
bushy vetiver root network has also been shown here. Figure 4d shows the schematic diagram of the root
matrix. After 51 days, root has grown to 86 cm. Figure 5 shows some pictures of vetiver root matrixes.
Figure 6 shows the close view of new stem. The root network of the vetiver grown in sandy soil is found
to be massive. Vetiver grasses were planted with 4 cm long root. After 90 days, roots have grown up to a
length of 1m in sandy soil. Summary of the investigation has been presented in Table 1.
Figure 3: Grain size distribution curve of the sand collected from river bed
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Figure 4: Root after (a) 39 days, (b) 46 days, (c) 51 days and (d) schematic view of vetiver root
(a) (b) (c)
Figure 5: (a) Vetiver root matrix, (b) Rizome in vetiver at 39 days and (c) root at 46 days
Old Root
New Root
86 cm
(a)
(b)
(c)
(d)
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Figure 6: (a) New pink stem has grown after 39 days and (b) totally new stem after 39 days
Table 1: Growth of vetiver root and shoot with time
Parameters
39 Days
46 Days
51 Days
90 Days
Overall range of shoot (cm)
23-117
23-125
5-121
11-125
Single root diameter (cm)
0.11
0.12
0.12
0.13
Sample maximum root length
(cm)
58.42
72.39
86.36
96.52
Sample minimum root length
(cm)
8.89
6.35
5.08
10.16
Root matrix diameter (cm)
12.7
15.24
10.16
17.78
Islam et. al (2013) studied the growth of vetiver grass in tropical region for slope protection. From this
study, it was found that the root grew up to 25.4cm and shoot grew up to 80cm in 6 months in barind tract
zone in Rajshahi (sandy silt). At Keraniganj site (soil was sandy silt), length of root grew up to about 30
cm. But in the present study, it was found that root grew up to 97 cm in pure sand in 3 months only which
indicates that vetiver root grows very fast in sand than that of other cases. This may occur due to the
heavy rainfall during the growth time.
SHEAR STRENGTH OF ROOTED SOIL
Direct shear test result of reconstituted sandy samples has been presented in Table 2. From Figure 7, It is
seen that angle of internal friction of rooted sand (46.79º) is 2.5% higher than that of bare sand (45.63º). It
means that root increases the shear strength and friction slightly. However, root matrix will be effective in
binding the soils to protect against erosion and wave action.
Table 2: Comparison of shear strength and shear deformation of bared and rooted sandy soil
σn (kPa)
Bare sand
Rooted sand
τmax (kPa)
δhf (mm)
τmax (kPa)
δhf (kPa)
15.49
18.59
15
19.99
15
30.98
30.29
15
31.23
6.28
New pink
stem
New stem
(a)
(b)
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Note: σn =Normal Stress; τmax = Peak Shear stress; δhf = Horizontal Failure Deformation
Figure 7: Peak shear stress vs normal stress for rooted and bare sand
CONCLUSIONS
Bio-engineering technology using vetiver system is effective in protecting earth slopes. An attempt has
been made to determine the root morphology of vetiver grass in sandy soil. It has been found that vetiver
root has grown up to 97 cm length with massive root networks only in 3 months. Presence of vetiver root
also increases the shear strength of sandy soil. So this long and bushy vetiver root network will be able to
retain the soil from erosion and thus vetiver system can be effective in protecting earth slopes constructed
with sandy soil. As most of the road embankments in Bangladesh are constructed with river bed sand,
vetiver plantation is a low-cost maintenance free alternative green solution to protect embankments from
erosion.
ACKNOWLEDGEMENTS
The authors are grateful to the Department of Civil Engineering, Bangladesh University of Engineering
and Technology (BUET), Dhaka, Bangladesh for providing the necessary laboratory facilities and
financial supports for conducting this research.
REFERENCES
Bangladesh Water Development Board, BWDB, (2000), The Dampara Water Management Project, A
Joint Project by Bangladesh Water Development Board and Canadian International Agency
Hengchaovanich, D., (1998), Vetiver grass for slope stabilization and erosion control, Tech. Bull,
No.1998/2, PRVN/ORDPB, Bangkok, Thailand
Islam M. N., (2003), Role of vetiver in controlling water-borne erosion with particular reference to
Bangladesh coastal region, Proceedings of the Third International Conference on Vetiver and Exhibition,
Guangzhou, China
Islam, M. S., and Hossain, M. S. (2013), Reinforcing effect of vetiver (Vetiveria zizanioides) root in
geotechnical structures- experiments and analyses, Geomechanics and Engineering, Vol. 5, No. 4, pp.
313-329
Islam, M. S., (2011), Riverbank erosion and sustainable protection strategies, J. of Engineering Science,
Vol. 2, pp. 63-72.
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Islam, M. S., Arifuzzaman and Nasrin, S., (2010), In-situ shear strength of vetiver grass rooted soil, Proc.
of Bangladesh Geotechnical Conf. Natural Hazards and Countermeasure in Geotechnical Engineering,
Dhaka, Bangladesh, pp.274-279
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embankment slope protection: Bangladesh perspective, International Journal of Geotechnical
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Islam, M. S., Nasrin, S., Islam M. S., and Moury, F. R., (2013), Use of vegetation and geo-jute in erosion
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