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20320130405015

  1. 1. International Journal of Civil Engineering OF CIVIL ENGINEERING AND INTERNATIONAL JOURNAL and Technology (IJCIET), ISSN 0976 – 6308 (Print), ISSN 0976 – 6316(Online) Volume 4, Issue 5, September – October (2013), © IAEME TECHNOLOGY (IJCIET) ISSN 0976 – 6308 (Print) ISSN 0976 – 6316(Online) Volume 4, Issue 5, September – October, pp. 143-151 © IAEME: www.iaeme.com/ijciet.asp Journal Impact Factor (2013): 5.3277 (Calculated by GISI) www.jifactor.com IJCIET ©IAEME VETIVER AS A GREEN AND ECONOMICAL TECHNOLOGY TO PROTECT RIVER BANK IN BANGLADESH Arifuzzaman1, Md. Anisuzzaman2, Md. Mostafizur Rahman3 & Farhana Akhter4 1 2 Lecturer, Department of Civil Engineering, UITS, Dhaka, Bangladesh. Graduate Student, Faculty of Geological Engineering, University of Padjadjaran, Indonesia. 3 Lecturer, Department of Civil Engineering, UITS, Dhaka, Bangladesh. 4 Lecturer, Department of Civil Engineering, UITS, Dhaka, Bangladesh. ABSTRACT River bank failures occur continuously throughout Bangladesh. From a strict economic viewpoint, cost of ramification of these problems is high, and the national budget for such works is never sufficient. It was found that the traditional practices for embankment protections are expensive, not eco-friendly and sometimes not effective due to improper design and construction fault for the designed life. On the other hand, protection of embankment slopes and river bank using vetiver grass (Vetiveria zizanioides) is being used in many countries of the world. A device is developed to determine the in-situ shear strength of the vetiver rooted block soil matrix and the bared block soil. It is found that the cohesion and angle of internal friction of vetiver rooted soil matrix is significantly higher than those of the bared soil. It is found that factor of safety of the embankment protected by vetiver grass is 1.76 to 2.06 times higher than that of embankment without any protection. The cost of slope protection by vetiver grass is significantly lower than the cost of other available slope protection measures. Also, it generates zero carbon-di-oxide. Therefore, it can be said that vetiver grass plantation might be an economical and sustainable green solution for the protection of river banks against natural disasters in Bangladesh. Key words: Bank Protection, Economical, Green, Natural Disaster, Vetiver Plantation. 1. INTRODUCTION Bangladesh is one of the most populated countries in the world having population density more than 850 per square kilometer [1]. Bangladesh, with its repeated cycle of floods, cyclones, and storm surges has proved to be one of the most disaster-prone areas of the world. During the years from 1797 to 2007, Bangladesh has been hit by more than 60 severe cyclones. Bangladesh is a land 143
  2. 2. International Journal of Civil Engineering and Technology (IJCIET), ISSN 0976 – 6308 (Print), ISSN 0976 – 6316(Online) Volume 4, Issue 5, September – October (2013), © IAEME of rivers and has a largest sea beach in the world. River bank and embankment failure occurs each and every year in our country. About 4000 km of coastal embankments and 4600 km of embankments along the bank of big rivers have been constructed to safeguard against inundation, intrusion of saline water and devastation [2]. According to the population census in 2001, some 35 million people live in the coastal region which is 28% of the total population [1]. As the embankments and other hydraulic structures are the first and immediate defense against the storm surge, they face the most severe damages. As for example, cyclone SIDR destroyed fully 362 km and partially 1927 km of coastal embankment, whose damage value is 32 million US$ [3]. But unfortunately, our State budget is never sufficient which confines rigid structural protection measures to the most acute sections, never to the full length of the river bank or coastline and embankment. This bandage approach compounds the problem. The traditional practice for protection of such embankment slopes is to use cement concrete (CC) blocks, stone or wood revetments, geotextile, geobags and improper plantation etc. These are expensive and in many cases not effective to protect them during their designed lives. On the other hand, protection of slopes by long rooted vegetation like vetiver grass (Vetiveria zizanioides) is being used in many other countries efficiently. Many research have been conducted in abroad to know the performance of vetiver grass against climatic change, slope protection, coastal embankment protection erosion control and so on. Hengchaovanich [4] analyzed slope stability based on vetiver root strength. Ke et al. [5] tested vetiver as a bank protection measure on several test sites (in Australia, China, Philippines and Vietnam). Their tests showed promising results for the use of vetiver grass as a bank protection measure. To evaluate the actual performance of vetiver grass for protection of river bank, it is necessary to estimate the factor of safety against the natural forces. In-situ shear strength determination of rooted soil matrix is essential to evaluate the performance of vegetative soil against natural forces. However, no such research has been conducted to know the in-situ strength of vetiver rooted soil matrix for proper understanding and analysis of slope stability. On this goal, the present research investigates the usefulness of vetiver grass in protection of river bank against natural hazards such as erosion, tidal surge and flood. The present study also compares the costs of different slope or bank protection measures used in Bangladesh. 2. CAUSES OF EMBANKMENT OR RIVER BANK FAILURE AND PROTECTIVE MEASURES River Banks are dynamic interface zones involving the meeting of atmosphere, land and river. From the field survey and past studies it was observed that, the most common causes of embankments and river bank failure can be broadly classified into two major groups [6]. a) Natural forces (such as; rainfall impact, wave action, wind action etc) b) Human interference (such as; travel paths for men and cattle, cattle grazing, unplanned forestation of embankment slopes etc) 2.1 Natural forces The natural forces which are responsible for embankment erosion or damage are discussed in this section. a) Rainfall impact Mean annual rainfall varies from 1500 mm in the northwest (Khulna district) to over 3750 mm in the south (Cox’s Bazar) of Bangladesh. The heaviest rainfall occurs in July and ranges from 350 144
  3. 3. International Journal of Civil Engineering and Technology (IJCIET), ISSN 0976 – 6308 (Print), ISSN 0976 – 6316(Online) Volume 4, Issue 5, September – October (2013), © IAEME mm to over 875 mm accordingly. The slope erosion caused by rain runoff is enormous and its speed/force grows exponentially towards the toe. Toe erosion is the combined effect of runoff and wave action. Figure 1.a shows the impact of rainfall, surface runoff and high head of water on the river side. From the Figure 1.a it is seen that due to heavy rainfall and surface runoff holes and gullies are formed on the embankment surface and lead to initiate piping action. b) Wave action Tidal waves cause damage to the embankments located too near to the river. A severe hydraulic load is steadily exerted on the toes and slopes and causes erosion. Cyclonic storms in the coastal zone (occurring repeatedly) act upon the water surface, causing it to advance towards the shore with enormous hydraulic loads. The waves thus formed eventually hit the embankment toe and slopes. The high hydraulic loads exerted on the embankment cause erosion and if there is overtopping, the physical structure of the embankment is destroyed. Photograph of Figure 1.b shows the wave action on a river embankment. Figure 1.b shows failure of an embankment slope due to wave and tidal action, where no protective measures and no vegetation at the slope of embankment are used. From the Figure, it is seen that due to wave and tidal action the slopes of the embankment change its original shape and becomes steeper. Therefore, the factor of safety of embankment becomes lower and lower and the vulnerability of embankment increases. c) Wind action The slow and steady action of wind in the relatively sparse fields and river lines blows away the topsoil of the embankments where it is sandy or a mixture of silt and sand. But wind with the high velocity during cyclone may lead overturning or uprooting of trees on the embankment slope. This may cause severe injury of the coastal embankment. Photograph of Figure 2 shows the failure of embankment slope due to overturning or uprooting of trees during cyclone. It indicates that only plantation can’t protect the embankment and also cause problem to transfer relief material and communication during disaster. Therefore, it is essential to select long rooted trees with suitable vegetation for environment friendly solution and the best performance of embankments. 2.2 Common practices for embankment protection The traditional practice for protection of embankments/river banks in Bangladesh Rainfall Impact Initiate of hole EGL Piping Runoff EGL Firm Base (a) (b) Fig. 1: Reasons of embankment failure: (a) impact of rainfall, surface runoff and high head of water; (b) slope of embankments are eroded or enhanced to fail due to wave action(JSCE Investigation Report, 2008 145
  4. 4. International Journal of Civil Engineering and Technology (IJCIET), ISSN 0976 – 6308 (Print), ISSN 0976 – 6316(Online) Volume 4, Issue 5, September – October (2013), © IAEME Fig. 2: Photograph showing the failure of embankment due to overturning or uprooting of trees during cyclone (a) (b) Fig. 3: Poor performance of the revetment structure: (a) poor performance of sand bags against cyclonic storm surge and (b) poor performance of CC blocks on a newly constructed embankment. is to use cement concrete (CC) blocks, stone or wood revetments, geotextiles, geobags and plantation etc. Usually, cement concrete (CC) blocks are used where storm surge is high. Sand bags and wood revetments are used where flow of water is moderately high. Protection of embankment by plantation is also another practice in our country. But unfortunately, it is also not effective during cyclone because of overturning or uprooting of trees. Photograph of Figures 3.a~3.b shows the poor performance of a revetment structure on a newly constructed embankment. Figure 3.a shows that sand bags were washed away from some portion of the embankment slope due to wave action. Thus it made the embankment slope unprotected and vulnerable at this portion and this weak portion may lead the embankment failure. Figure 3.b shows that protected embankment slopes by cement concrete (CC) blocks were failed. The reasons of this failure may be the lack of proper compaction of embankment slope, existing soft layer(s) below the embankment, lack of proper placement of CC blocks on the embankment slope or high tidal surge make the embankment toe weak and wash away the soil particles below the CC blocks, etc. 146
  5. 5. International Journal of Civil Engineering and Technology (IJCIET), ISSN 0976 – 6308 (Print), ISSN 0976 – 6316(Online) Volume 4, Issue 5, September – October (2013), © IAEME 3. EXPERIMENTAL PROGRAM Field tests were conducted to determine the in-situ shear strength and failure strain of vetiver rooted soil matrix and soil without root at Patuakhali region. Soil samples were also collected in polythene bag during the field test for laboratory investigations. 3.1 In-situ shear strength of block samples In-situ shear strength test was conducted in the field on twenty block samples. Tests were conducted under different normal stresses at different depths. Normal stresses for the in-situ tests were arbitrarily selected in the range between 10.96 kPa and 19.98 kPa. 3.1.1 Apparatus for in-situ shear strength test A device was developed in this study to determine the in-situ shear strength of the vetiver rooted soil and soil without root. The apparatus used for the in-situ shear strength tests were hydraulic jack, pressure gauge, wooden plate, metal plate, metal box (approx. 29×15×19 cm3), normal load and Linear Variable Displacement Transducer (LVDT). The capacity of the pressure gauge used for this in-situ shear strength test was 800 psi and the capacity of LVDT was 50 mm. Both the pressure gauge and LVTD were calibrated before using them in the test. 3.1.2 Preparation of block sample Clump of vetiver grass was cut at the ground level with a sharp knife. Keeping the root position undisturbed a trench of the size (1 m × 1m) was made up to the desired depth. Initially the rooted area was greater than desired block sample size. After that the rooted area was made in desired block sample shape by sharp knife. Photograph of Figures 4.a~4.b shows the preparation of block soil sample for in-situ shear strength test. 3.1.3 Test set-up Block samples (approx. 29×15×19 cm3) were tested at different depths (250 mm to 500 mm) under different normal stresses at the field to know the in-situ strength of the rooted soil and soil without root. After preparing the block sample in the desired shape the metal box (having bottom face open) was smoothly pushed from the top of the block sample. Then normal load was placed on the metal box. It was carefully ensured that, the bottom edge of the metal box could not touch the ground level. Wooden plate was placed between the metal box and the hydraulic jack. The back sides of the hydraulic jack was made hard and smooth enough by placing brick and wooden plate between the jack and edge of the prepared hole. (a) (b) Fig. 4: Photograph showing preparation of block sample: (a) trench surrounding the clump of vetiver grass and (b) prepared block sample of desired shape 147
  6. 6. International Journal of Civil Engineering and Technology (IJCIET), ISSN 0976 – 6308 (Print), ISSN 0976 – 6316(Online) Volume 4, Issue 5, September – October (2013), © IAEME 1 1) Hydraulic jack 2) Wooden plate 3) Metal box (0.29 × 0.15× 0.19 m3) 4 6 3 2 Sample is inside the box 5 4) Normal load 5) Metal plate 6) LVDT Failure line Figure 5: Schematic diagram of the Experimental set-up Then horizontal force was applied to the box from one side by hydraulic jack. Calibrated pressure gauge was used to measure the horizontal force. The block sample was failed at the bottom and the deflection of sample was measured by Linear Variable Displacement Transducer (LVDT) which was fixed to the ground surface by metal plate. For this purpose, Linear Variable Displacement Transducer (LVDT) having capacity of 50 mm was used to determine the horizontal deformation. LVDT was placed on a metal box and fixed it with the metal plate by magnetic stand. The metal plate is placed on the ground and the metal was fixed with the ground by metal clamp. Figure 5 shows the schematic diagram of the test set-up for the in-situ test. 3.2 Laboratory tests A detailed laboratory test was carried out on samples collected from Patuakhaali region. Tests were conducted according to ASTM standards [7]. 4. RESULTS AND DISCUSSIONS 4.1 In-situ shear strength of block samples Figure 6a shows the peak shear stress versus normal stress graph of bared and vetiver rooted soil matrix at Patuakhali region. It is seen that the peak shear stress of vetiver rooted soil matrix is always higher than that of bared soil for a particular normal stress. Strength of vetiver rooted soil is about 1.90 times higher than that of the bared soil. Figures 6b show the increment of peak shear stress versus normal stress of vetiver rooted soil matrix. It is seen from the Figure 6b that the enhanced effective soil cohesion due to vetiver root matrix is 10.4 kPa and the enhanced effective angle of internal friction is 21o, respectively. It means that Vetiver grass root might be able to protect the river bank and embankment slopes from natural forces. 4.2 Stability of slopes Stability of slopes for different heights and different slope angles are estimated by using mass procedure of slope stability analysis. For this analysis it is considered that the density of sol is 18 kN/m3. The cohesion and angle of internal friction of vetiver rooted soil and bared soil is considered 15 kPa and 9 kPa and 35˚ and 18˚, respectively. Stability of slopes for different heights and different slope angles are presented in the Table 1. It is found From the Table 1 that for a particular soil the factor of safety of vetiver rooted soil slope is 1.8 to 2.1 times higher than that of the slope without root. 148
  7. 7. International Journal of Civil Engineering and Technology (IJCIET), ISSN 0976 – 6308 (Print), ISSN 0976 – 6316(Online) Volume 4, Issue 5, September – October (2013), © IAEME 15 (kPa) 40 (kPa) τ max 30 Increment of τ max Peak shear stress, τ Depth from EGL = 300 mm Depth from EGL = 300 mm 20 10 Veriver rooted soil matrix Bared soil 15 20 = 0.38 σ + 10.4 n 10 Enhanced cohesion, c' = 10.4 kPa o Enhanced angle of friction,φ ' = 21 Vetiver rooted soil matrix 0 10 max 5 10 25 Normal stress, σ (kPa) 15 20 Normal stress, σ (kPa) 25 n n (a) (b) Figure 6: (a) Peak shear stress versus normal and (b) Increment of peak shear stress of vetiver rooted soil matrix versus normal stress Table 1: Comparison of factor of safety of embankment slope (by Mass Procedure) Height of embankment, H (m) Side slope Factor of safety of embankment slopes without protection, Fsb Factor of safety of embankment slopes with vetiver grass protection, Fsv Ratio F = sv Fsb (h : v) 2 2:1 2.7 5.4 2.00 3 2:1 2.4 4.3 1.79 3 1:1 1.6 3.0 1.88 4 2:1 1.8 3.8 2.11 4.3 Cost of different slope protection measures According to the rate schedule of Local Government Engineering Department, LGED (July, 2009), the cost of vetiver application per square meter is 22 taka including labour and placement cost. Again, according to the work schedules of Water Development Board of Bangladesh, WDB (Dhaka Division Rate Schedule, 2009-2010) the cost of geotextiles and cement concrete blocks including labour and placement cost per square meter are 151 taka and 5843 taka, respectively. According to the rate schedule, the cost of one kilometer embankment section is estimated. Table 2 shows the cost estimation of the different slope protective measures. From the Table 2, it is seen that the cost of slope protection for one kilometer of embankment by vetiver grass is 2109 US $, by geotextiles is14475 US $ and by CC blocks is 560093 US $. Therefore, it is seen that the cost of slope protection by vetiver grass is significantly lower than the other methods of slope protective measures. 149
  8. 8. International Journal of Civil Engineering and Technology (IJCIET), ISSN 0976 – 6308 (Print), ISSN 0976 – 6316(Online) Volume 4, Issue 5, September – October (2013), © IAEME Table 2: Comparison between costs of different slope protective measures Slope Protective Measures Cost Per Sq. Meter Height Length Width Area Cost (m) (m) (m) (m2) (USD) (USD) Vetiver Grass 3.0 1000 6.71 6710 2109 Geotextiles 2.157 3.0 1000 6.71 6710 14475 CC block 5. 0.314 83.471 3.0 1000 6.71 6710 560093 CONCLUSIONS River bank and embankment failures happen continuously throughout Bangladesh. It is found that the general reasons of embankment failures are erosion due to rain splash, wave action and overtopping of storm surge. Poor maintenance practice, overturning or uprooting of trees also enhance embankment failure. The traditional practice for embankment protection is to use cement concrete blocks, stone or wood revetments, geotextile and plantation etc. These are expensive and not so effective to protect the embankments for the designed life. Protection of embankments by bioengineering process (e.g., using vetiver grass) is being used in many countries efficiently. The special attributes of vetiver grass is its longer life, strong and long finely structured root system and high tolerance of extreme climatic change. It is seen that the enhanced effective soil cohesion due to vetiver root matrix is 10.4 kPa and the enhanced effective angle of internal friction is 21o, respectively. Again vetiver plantation can enhance the factor of safety for a particular soil is 1.8 to 2.1 times higher than that of the slope with bared soil. The cost of slope protection by vetiver grass is significantly lower than the cost of other available slope protection measures. Also, vetiver plantation generates zero carbon-di-oxide. Therefore, it can be said that vetiver grass plantation might be an economical and sustainable green solution for the protection of river banks against natural disasters in Bangladesh. 6. REFERENCES [1] BBS (2007). “Bangladesh Bureau of Statistics”, Statistical Year Book of Bangladesh, 26th Edition, Ministry of Planning, Government of the People’s Republic of Bangladesh. [2] Bangladesh Water Development Board, BWDB (2000). “The Dampara Water Management Project.” A Joint Project by Bangladesh Water Development Board and Canadian International Agency. [3] DMB (2008). “Disaster Management Bureau”, Cyclone Sidr in Bangladesh: damage, loss, and needs assessment for disaster recovery and reconstruction, a report prepared by the Government of Bangladesh assisted by the International Development Community with Financial Support from the European Commission. [4] Hengchaovanich, D. (1998). “Vetiver grass for slope stabilization and erosion control.” Tech. Bull. No. 1998/2, PRVN/ORDPB, Bangkok, Thailand. [5] Ke, C.C., Feng, Z.Y., Wu, X.J., and Tu, F.G. (2003). “Design principles and engineering samples of applying vetiver eco-engineering technology for landslide control and slope stabilization of riverbank.” Proc. of the 3rd International Conference on Vetiver, Guangzhou, China. 150
  9. 9. International Journal of Civil Engineering and Technology (IJCIET), ISSN 0976 – 6308 (Print), ISSN 0976 – 6316(Online) Volume 4, Issue 5, September – October (2013), © IAEME [6] Islam, M.S., and Arifuzzaman. (2010). “Performance of vetiver grass in protecting embankments in Bangladesh coast against cyclonic tidal surge.” Proc. of the 5th National Conference and Expo on Coastal and Estuarine Habitat Restoration, Texas, USA. (http://www.estuaries.org/pdf/2010). [7] ASTM (1989). Annual Book of ASTM Standards, Vol. 04.08, Soil and Rock; Building stones; Geotextiles. [8] Ishwar Chand Sharma and Prof.Naresh Chandra Saxena, “Environmental Hazard and Disaster in Disposing Marble Slurry”, International Journal of Civil Engineering & Technology (IJCIET), Volume 3, Issue 1, 2012, pp. 62 - 66, ISSN Print: 0976 – 6308, ISSN Online: 0976 – 6316. 151

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