Mohammad Shariful Islam 1, Abdul Jabbar Khan 1, Abu Siddique 1, Asif Mohammad Saleh 2 and Shamima Nasrin 3

Transcription

1 CONTROL OF EROSION OF HILL SLOPE TOP SOIL USING GEOJUTE AND VEGETATION Mohammad Shariful Islam 1, Abdul Jabbar Khan 1, Abu Siddique 1, Asif Mohammad Saleh 2 and Shamima Nasrin 3 Abstract Possibility of using JGT in hill slope management was preliminarily assessed for five sites. Among these, three sites along Alikadam-Thanchi road in Bandarban have been finally selected for JGT trial. Two of these sites are downhill (10.8 km and 11.7 km points) and the other one is in the uphill (14.75 km point). The soil of the hill slope was silty clay (liquid limit: 32-40; plasticity index: 15-21). Percent fine # 200 sieve (0.074 mm) and clay content of the collected two soil samples varied between 92%-94% and 18%-19%, respectively.700 gsm grade of open mesh type JGT was used for topsoil erosion control of these slopes for an interim period, i.e. until the vegetation canopy grew up. JGT was applied in all three sites in April, After laying of JGT, a local plant called Gyama seeds were sown just before the monsoon (end of May, 2013 to mid of September, 2013) started. JGT performed well during the first rainy season after JGT application. No significant soil loss or erosion was visible. Meanwhile, Gyama grew well. The growth rate of roots had been found to be excellent. The roots of Gyama were found to be 203 mm, 1219 mm, 1372 mm and mm after 1 month, 2 month, 3 month and 1 year of plantation, respectively. The roots were found to have spread in all directions more or less of same length. Concentration of roots was found to be good enough to serve the purpose. Height of Gyama plant was found to be 3048 mm to 3658 mm at the age of 3 months and 3962 mm after 1 year. Being bushy in nature, it demonstrated good canopy action on the soil. Performance showed that JGT (soil saver) can be effectively used for hill slope management. Key Words : Soil saver, erosion, hill slope management 1Professor, Department of Civil Engineering, Bangladesh University of Engineering and Technology (BUET), Dhaka-1000, Bangladesh 2Major, SWO (Chittagong), Bangladesh Army 3Project Engineer, Development and Application of Potentially Important Jute Geo-textiles (CFC/IJSC/21) Project, BUET Part Jute Geo-textiles Project 1

2 Introduction Biotechnical stabilization system provides an attractive and environmentally compatible means to protect slopes against surficial erosion and shallow mass movement. This system includes the use of nets, mats, or other types of structural/mechanical reinforcement to improve the establishment and performance of grass-cover on slopes in conjunction with the vegetation roots to provide sufficient mechanical strength. Bangladesh being a major producer of jute fibres and exporter of geojute (commercial name soil saver ), biotechnical stabilization system of slopes may be envisaged as a simple and cost effective solution for the rain induced erosion control problems (Rahman and Khan, 2009). In the hilly areas, naturally balanced hill slopes often get disturbed either due to road construction activities or by the characteristic native farming method in which they burn trees and vegetations of the slopes in order to get a bare piece of land. Due to these man-made activities, the slopes become highly vulnerable to top soil erosion. Other than the disruption of the road network activities, the soils of the hills lose a lot of nutrients every year. This leaves serious impact on the growth of native agriculture. In order to address such erosion of top soil, a sustainable, eco-friendly and aesthetic solution would be ensured by landscaped vegetation cover. However, it takes about 6-9 months for the vegetation cover to grow. During the interim period, a geojute overlay may be used to withstand the rain drop impact energy, reduce surface runoff and reduce total soil loss. It may be emphasized that the vegetations should be selected correctly, i.e., they must be environment friendly, fast growing, bushy type and deep and have widely spread roots. The plants/trees must grow up as canopy so that they cover most of the ground. Such vegetations may be appreciated to hold grounds via a natural soil nailing action (Khan and Binoy, 2012). Geojute is an open mesh type of jute geotextiles manufactured abundantly in the mills of Bangladesh and India. Usually, 350 gsm, 500 gsm and 700 gsm geojutes of 1.2m width are available in the production lines. Geojutes have a typical ground cover ratio of 50%-60% and an absorption capacity of times of their air-dry weight (GoI, 2008). They can usually survive two monsoons without biodegradation. This geojute is produced by different jute mills of Bangladesh Jute Mills Corporation (BJMC) and in few private jute mills as well. Each year more than 7000 metric tones of soil saver is exported worldwide. In this project, possibility of using soil saver in 6 hill slopes has been investigated. Among these, 3 sites have been selected for field trials. Besides this, SRDI has also been conducting research independently on performance of soil saver by using a multi-slot divisor for soil-loss and runoff measurement. Site Selection Three hill slope sites have been selected for soil saver application in Alikadam-Thanchi road, Bandarban. Locations of the sites are: Uphill km point, Downhill-11.7 km point and Downhill km point. Selected sites are located very much in South-East hilly area of Bangladesh, where landslide at selected locations every monsoon causes extreme difficulties for the road operators, i.e., Special Works Organization (SWO) of Armed Forces Division (AFD). Soil type of the slope is predominantly silty clay. Such soils are erosion prone. Application of soil saver in all sites has been completed. A map of Bangladesh with selected hill slope has been shown in Figure 1. Some photographs of selected sites have been shown in Figures 2a-2d. Jute Geo-textiles Project 2

3 Soil and Climatic Condition of Selected Sites Soil sample was collected from side hill and uphill. The samples showed almost the same grain size distribution. The soil of the hill slope is silty clay (liquid limit: 32-40; plasticity index: 15-21). Percent fine # 200 sieve (0.074 mm) and clay content of the collected two soil samples varied between 92%-94% and 18%-19%, respectively. A typical grain size distribution curve has been presented in Figure 3. The average temperature, rainfall, maximum and minimum humidity are 26.2, 3031 mm, 57% and 86%, respectively. Figure 1 : Location of selected sites for hill slope management of Bangladesh map (a) (b) (c) (d) Figure 2 : Condition of selected hill slopes before JGT application in Bandarban Jute Geo-textiles Project 3

4 100 Percent Finer by Weight (%) G s = 2.67 Sand (larger than 0.06 mm) = 11 % Silt (0.002 mm to 0.06 mm) = 70 % Clay (smaller than mm) = 19 % E-3 1E-4 Particle Size (mm) Figure 3 : Typical grain size distribution curve of hill slope soil Properties of Soil Saver Used in Hill Slope Management Selection of the type of geotexile is based on the following considerations (Reynolds 1976, Rickson 1992a, Rickson, 1988, Rickson 1992b, Thomson and Ingold, 1986) : 01. The geotextile which has higher percentage of cover, higher water holding capacity and when wetted, is capable of remaining in contact with the soil surface is more effective for erosion control works. 02. Natural geotextiles have the ability to shape it in keeping with the soil surface contours to establish full contact with the surface, especially when wet, as compared to the more rigid synthetic products. 03. After being saturated, open-weave geotextiles can store runoff water behind their thick weft fibres, which act as mini-dam and so reduces runoff volume. This ponding of water behind the geotextile fibres may protect the soil further from raindrop impact, through a cushioning effect. 04. Jute geotextile is capable of reducing surface runoff by transmitting water downslope through the fibres of this geotextile rather than on the soil surface. The delay in the onset of runoff represents an increase in time for infiltration as the flow is intercepted by the geotextiles. 05. Biodegradable jute geotextiles contribute some organic matter to the soil once they begin to break down, but the quantities are insignificant to effect on ameliorating soil susceptibility to erosion. 06. By absorbing and retaining a great deal of moisture, jute geotextiles reduce the effective rainfall on site because of higher evaporation losses which is crucial in areas where moisture is limited. Considering the above points, 700 gsm (grams per meter square) soil saver was selected for hill slope management. A close view of the selected soil saver is shown in Figure 4. The absorption capacity of the selected JGT is The opening of the mesh is 31 mm 33 mm. The weft yarn is 4.0mm in diameter (Islam et al., 2013). Jute Geo-textiles Project 4

5 Field Trials 40 Slope was prepared in each site. It is necessary to fix the soil saver with the ground. Otherwise, when the soil saver is wet and becomes heavy, it will slide down to the bottom of the slope and moreover, contact with the surface, will be lost and efficiency of soil saver will be reduced. U shaped fixing pins were used to ensure firm contact of soil saver with the slope surface. The picture of U hook is shown in Figure 5. A gap of 50 cm between two U hooks in the lateral (direction of road) and of 60 cm in the longitudinal (in the direction of slope) direction are to be maintained while pegging down those. U hook was prepared by using 6 mm MS rod. Two rolls of JGT are to be laid with 10 cm superimposed. At the top and bottom, two lines of anchorage trench are to be prepared in order to fix the JGT ends sufficiently inside the anchorage trenches filled with other in situ materials. A toe wall of 60 cm height is to be constructed at the bottom of uphill slope located at Uphill km point. Details of Geo-jute overlay is shown in Figure 6. A grass locally named Gyma was used in slope to take over the role of geojute once it is decomposed. It is a bushy type grass. The growth rate of roots is very fast and extended in all direction. Root system of Gyma plant is shown in Figure 7. Some picture of site preparation is shown in Figures 8a-8b. Figure 4 : Close view of selected soil saver Figure 5: U hook used for JGT application Figure 6: Details of Geojute overlay on hill Figure 7: Root system of Gyma plant Jute Geo-textiles Project 5

6 Monitoring a. Downhill-10.8 km point b. Uphill km point Figure 8 : Preparation of the sites Initially monitoring is done by SWO. Monitoring is done in 1st week, 2nd week, 4th week and after 6 months of application of soil saver. Some pictures of monitoring at 1st week and after 6 months of soil saver implementation are shown in Figures 9a 9c and Figures 10a 10c. Results and Discussions No significant soil loss or erosion was visible in the selected site after 1year of application of soil saver and Gyma plantation. The roots of Gyama were found to be 203 mm, 1219 mm, 1372 mm and mm respectively after 1 month, 2 month, 3 month and 1 year of plantation. The roots were found to have spread in all directions more or less having same length. Concentration of roots was found to be good enough to serve the purpose. Height of Gyama plant was found to be 3048 mm to 3658 mm at the age of 3 months and 3962 mm after 1 year. Conclusion Among the six investigated hill slope sites three sites were selected for the application of soil saver. Application of soil saver was completed in April, 2013 and the seeds of Gyma plant sown in end of May, 2013 to mid of September, From the field trails after one year of application of soil saver, it is found that the proposed method is effective for erosion control of hill slope in Bangladesh. a. Downhill-10.8 km point Jute Geo-textiles Project 6

7 b. Downhill-11.7 km point c. Uphill km point Figure 9 : Picture of selected sites after 1 st week of implementation a. Downhill-10.8 km point b. Downhill-11.7 km point c. Uphill km point Acknowledgements Figure 10 : Picture of selected sites after 6 months of implementation The authors are thankful to Jute Diversification Promotion Center (JDPC) for co-ordination and corporation. The authors are also greatful to Common Fund for Commodities (CFC) for fund, Special Works Organization (SWO) of Armed Forces Division for implementing the project and Bangladesh Jute Mills Corporation (BJMC), Janata Jute Mills Limited for supplying the soil saver. Jute Geo-textiles Project 7

8 Reference [1] Rahman, M.M. and Khan, A.J. (2009). Raindrop Erosion Control with Geojute and Vegetation, Proc. of Bangladesh Geotechnical Conference 2009, pp [2] Khan, A.J. and Binoy., T.H. (2012). Top Soil Erosion Control Using Geojute, Proc. of Int. Conf. on Advances in Civil Engineering 2012, pp [3] GOI (2008). A Manual on Use of Jute Geotextiles in Civil Engineering Jute manufacturers Development Council (JMDC). [4] Reynolds, K.C. (1976), Synthetic Meshes for Soil Conservation Use on Black Earths, Soil Conservation Journal, New South Wales, Australia, pp 34, [5] Rickson, R.J. (1988), The Use of Geotextiles in the Soil Erosion Control: Comparison of Performance on Two Soils in Land Conservation for Future Generations, (Editor S. Rimwanich), Department of Land Development, Ministry of Agriculture and Cooperatives, Bangkok, Thailand, pp [6] Rickson, R.J. (1992a), Control of Sediment Production Using Geotextiles: Results of Experimental Testing Using Simulated Rainfall and Runoff, in The Environment of Our Future, Proceedings of the XXIII IECA Annual Conference, Reno, Nevada, International Erosion Control Association, Steamboat Springs, CO, pp [7] Rickson, R.J. (1992b), The Application of Geotextiles in the Protection of Grassed Waterways, in Erosion, Conservation and Small-Scale Farming, (Editors H. Hurni and Kebede Tato), Geographica Bernensia, Bern, pp [8] Thomson, J.C. and Ingold, T.S. (1986), Use of Jute Fabrics in Erosion Control, Report to the Jute Market Promotion (Western Europe) Project, International Jute Organization (IJO), International Trade Centre, UNCTAD/GATT, Project No. RAS/77/04. [9] Islam, M.S., Nasrin, S.,Moury, F. R. and Islam M. S. (2013). Use of Vegetation and Geo-Jute in Erosion Control of Slopes in a Sub tropical Climate, Journal of World Academy of Science, Engineering and Technology, Vol. 73, pp Jute Geo-textiles Project 8