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Full Paper Proc. of Int. Conf. on Advances in Civil Engineering 2012

Top Soil Erosion Control Using Geojute Abdul Jabbar Khan1 and Tarik Habib Binoy2 1

Professor, Bangladesh University of Engineering and Technology (BUET)/Department of Civil Engineering, Dhaka, Bangladesh.

Email: ajkhan66@gmail.com.

2 Research Student, Bangladesh University of Engineering and Technology (BUET)/Department of Civil Engineering, Dhaka, Bangladesh.

Email: tarikhabib.ce@gmail.com the rain drop impact energy, reduce surface runoff and reduce total soil loss. It may be emphasized that the vegetations should be correctly selected, i.e. they must be environment friendly, fast growing, bushy type and deep and widely spread rooted. 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.

Geojute is an open mesh type of jute geotextiles manufactured abundantly in the mills of Bangladesh and in India. Usually, 350 gsm, 500 gsm and 700 gsm geojutes of 1.2m width are available in the production line. Geojutes have a typical ground cover ratio of 50%~60% and an absorption capacity of 2.5~3.5 times of their air dry weight [1]. They can usually survive two monsoons without biodegradation.

Abstract—Top soil erosion of side slopes of road embankments and hill slopes is very common in the tropical regions. Most of the top soil erosion occurs when the surface of the slope is denuded or deforested due to man-made activities. The impact energy of the rain drops loosens the top soil and infiltration of water into the top soil layer makes it heavier to slide down the slope. Such erosion does hardly occur when the slope surface is covered with vegetation canopy. But, it takes 6~9 months time for the vegetation cover to grow on a barren surface. For the interim period, geojute, an open mesh type of naturally biodegradable jute geotextile, may be used in order to inhibit the top soil erosion. By the time the geojute overlay decomposes and become nutrient for accelerated vegetation growth, vegetation canopy gets established. Eventually, it becomes a sustainable and eco-friendly solution for reducing top soil erosion of the exposed slope surface. In this paper, some case studies undertaken in the sub-continent have been cited and a laboratory simulation test setup established at the Bangladesh University of Engineering & Technology (BUET) has been presented. The test results show how geojute overlay may improve overall stability of a slope and significantly reduce volume of erosion and surface runoff.

II. BACKGROUND Jute geotextiles have been used extensively in India since 1987 and gaining acceptance in Bangladesh recently. The major areas of application of jute geotextiles are separation, filtration, initial reinforcement and top soil erosion control.

Geojute was first used for mine spoil stabilization in 1987 at Uttaranchal in India and soil erosion was reduced to 0.8 kg/ m2. By 1990, entire area was stabilized. Hill slopes were also protected at Darjeeling by Dept of forest, Govt. of West Bengal in 1988. After that geojute was used in sand dune stabilization at Digha sea beach, West Bengal in 1988; river bank protection at Nayachar, West Bengal in 1989; control of top soil erosion at Assam in 1995; land slide management at Sikkim in 2004; bridge approach at Raidighi, West Bengal in 2007; road embankment at Gujrat in 1998; railway embankment at Assam in 1997 and hundreds of other projects in India [2].

In Bangladesh also, hill slopes and road side slopes have been stabilized using geojute aided vegetation establishment system [3]. Furthermore, [4] & [5] also performed some field trials on application of geojute in India for topsoil erosion control.

To date, no laboratory simulation test has been undertaken in the sub-continent in order to systematically identify the efficacy of geojute in reducing soil loss, surface runoff and improving overall stability of slopes. Recently, an attempt has been made at the laboratory of BUET to assess and evaluate these features. Details of the laboratory test setup, test results and materials used are presented in the following sections.

Index Terms— Geojute, Rain drop, Soil erosion, Surface runoff, Overall stability.

I. INTRODUCTION In road construction, the side slopes of a road embankment are often cladded with thick layer of clay in order to inhibit raincut erosion of the erodible dredge fill soils of embankment.

However, the additional amount of the clay soil required for this purpose is becoming increasingly difficult, expensive and detrimental to eco balance.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 barren 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. For the interim period, a geojute overlay may be used to withstand

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Full Paper Proc. of Int. Conf. on Advances in Civil Engineering 2012 III. EXPERIMENTAL SETUP

IV. MATERIAL PROPERTIES

A. Container for Preparing Soil Slope The soil slope was prepared in an 8mm thick transparent 1.22m×0.91m×0.91m Perspex container. The skeleton of the box was constructed with 25mm×25mm×3mm mild steel angles and the Perspex container was inserted into the frame. The angles were connected to each other by necessary bolting and welding to make the frame. A 0.91m face of the container contains an opening which is associated with toe of slope to help removing the runoff water. This opening was made 0.15m high from bottom and 0.91m wide.

A. Properties of Soil Sandy soil of FM 1.18 was used for the simulation test.

D10, D15, D50 and D90 of the soil were 0.12mm, 0.15mm, 0.22mm, 0.40mm and 0.48mm, respectively. The peak angle of friction of the soil, obtained from direct shear test, was 350.

B. Properties of Geojute The geojute (soil saver) used for the simulation test was procured from Bangladesh Jute Mills Corporation (BJMC).

Geojute used for this test was an open mesh type of jute geotextile. The aperture of geojute was 25mm×20mm. The weight and thickness of the geojute were 590gsm and 4.90mm, respectively. The wide width tensile strength of the sample was 16kN/m. Ground cover ratio and absorption capacity of the geojute were determined to be 55% and 2.9 times the air dry weight, respectively.

B. Container for Collecting Eroded Soil and Runoff A 2.44m×0.30m×0.61m container was used to collect runoff water. Runoff water along with eroded soil was drained through the opening at bottom of a 0.91m face of container for preparing soil slope and separated here using a synthetic geotextile filter for measuring weight of eroded soil and volume of runoff water. The runoff water volume was found by multiplying the height of water with length and width of this container. Eroded soil was air dried after separation and weighed for necessary calculations.

V. TEST METHODOLOGY The experiment was performed inside the laboratory under controlled environment. First, soil slope was prepared in a container. Characteristics of this slope are described in section III. Approximately, 500kg sand was compacted to obtain same volume, shape and slope for all tests to make sure same level of compactness of sand slopes used for both bare soil and geojute covered soil. Soil bed was compacted thoroughly until it gets a shape shown in Fig. 1. Then rainfall of approximate intensity of 120mm/hour was distributed uniformly over the soil surface from a constant height to maintain constant rainfall energy for all tests throughout the time. The volume and intensity of rainfall was checked at every ten minute interval. Simultaneously, the soil washed away with runoff water was separated with filter and weighed after air drying. Both runoff water volume and eroded soil mass were measured at ten minute interval. This process was repeated several times for bare soil to obtain a representative result.

After the completion of these experiments on bare soil this process was repeated for slope covered with geojute under similar conditions using same gradation of soil. As before, in this case also all the readings were taken at ten minutes interval and the graphs were plotted to compare obtained values.

C. Rainfall Distribution System A steel tray rainfall distribution system was used to simulate uniform rainfall from a constant height at a constant rate. This tray looks like a grating of 2mm opening with 25.4mm center to center spacing. It was associated with an orifice meter to measure the intensity of simulated rainfall.

D. Slope Characteristics Tests were carried out on 1:1.5 slope of compacted fine sand with 0.15m horizontal support of same sand on each side of the slope. Length of slope used was 1.22m excluding 0.15m on each side. A schematic diagram of slope is shown in Fig. 1. Compaction effect was so maintained that the moist density of sand during compaction was approximately 9.9 KN/m3.

E. Rainfall Characteristics Average rainfall intensity of Bangladesh is 50-100 mm/hr.

The rainfall intensity used in this model test was between 100 mm/hour & 125mm/hour.

VI. TEST RESULTS A. Bare Soil In case of bare soil, block or slide failure occurred within 1 hour. The amount of soil washed out in one hour for rainfall intensity of 118mm/hour was 15.74% of total mass of soil used. Runoff volume was 0.0779m3 for 0.133m3 water supply.

Conditions of soil surface before rainfall and after one hour rainfall for bare soil are shown in Fig. 2. & Fig. 3.

Fig. 1. Schematic diagram of soil slope prepared at Laboratory

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Full Paper Proc. of Int. Conf. on Advances in Civil Engineering 2012

Fig. 2. Condition of slope of bare soil before rainfall.

Fig. 4. Condition of slope of geojute covered soil before rainfall

Fig. 3. Condition of slope of bare soil after 1 hour rainfall

Fig. 5. Condition of slope of geojute covered soil after 1 hour rainfall.

B. Soil Covered with Geojute No slide or block failure occurred within first four hours of continuous rainfall when geojute cover was used. Instead, top soil was slowly washed out with runoff water. Soil became more stable as impact energy of rainfall was reduced due to a cover of geojute. For geojute covered soil after one hour of 122mm/hour rainfall soil erosion was 1.64 % of total mass of soil used. Runoff volume was 0.0651m3 for 0.139m3 water supply. The soil surfaces of geojute covered soil before rainfall; one hour later & four hours later are shown in Fig. 4, Fig. 5 & Fig. 6, respectively. Amount of soil eroded at different times for geojute covered soil is shown in Fig. 7. In both cases, bare soil and geojute covered soil, soil erosion started from the toe of slope and the volume of erosion increased exponentially with time.

Fig. 6. Condition of slope of geojute covered soil after four hours rainfall.

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Full Paper Proc. of Int. Conf. on Advances in Civil Engineering 2012

Fig. 7. Eroded soil collected at different times for geojute covered soil.

Graph. 1. Cumulative percent soil erosion Vs Time graph.

After 1 hour of rainfall of approximately same intensity on equal mass, grading & density of soil at similar conditions total erosion for bare soil is around 16% of total mass of soil whereas it is slightly below 2% for geojute covered soil. Hence total reduction of erosion is almost 95% of total erosion.

C. Graphs Comparative results of soil erosion without and with geojute cover can be seen from the following graphs.

Cumulative percentage of soil erosion for one hour for both bare soil and geojute covered soil, cumulative percentage of soil erosion of bare soil for 1 hour & for geojute covered soil for 4 hours and comparison of runoff volume against time are shown in following graphs. Graph I shows the variation of cumulative percent erosion with time for bare soil and soil covered with geojute. It may be seen from the graph that cumulative erosion after one hour rainfall on bare soil is significantly high (16% of total soil mass used for the test) compared to that of (bellow 2% of total soil mass used for the test) geojute covered soil.

Graph II shows that the cumulative erosion for geojute covered soil after four hours (approximately 15%) is almost equal to the cumulative erosion after one hour rainfall on bare soil (16%). This indicates that the total reduction in erosion due to application of geojute is almost 95% of total erosion. Graph III shows that the cumulative runoff is almost same in both cases. Geojute can absorb water 2.5~3.5 times of its own weight but geojute used for this tests were of very low rate compared to amount of water supplied and hence it could not show significant result in runoff reduction. Also, the runoff was almost equal to rainfall after the saturation of soil. Geojute can reduce erosion by decreasing impact energy of rainfall as it works as a shade and by providing obstacle in the way of flow of washed out soil particle with runoff water.

It also works as a reinforcing material for top surface of soil.

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Graph. 2. Cumulative Percent Soil Erosion Vs Time Graph Showing 4 Hours Erosion for Geojute Covered Soil is Less Than 1 Hour Erosion for Bare soil.

At similar conditions total erosion for bare soil after 60 minutes is around 16% of total mass of soil whereas it is around 15% for geojute covered soil after 240 minutes.

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Full Paper Proc. of Int. Conf. on Advances in Civil Engineering 2012 top soil to be eroded. Rapid growing plants with long, strong and wide spread root system are appreciable for plantation with suitable spacing.

CONCLUDING REMARKS Geojute may be used as an interim overlay on the exposed slopes for reducing total erosion of the soil mass due to rainfall. It increases stability of a slope for a longer duration of rainfall in absence of vegetation cover. Amount of top soil erosion may be reduced by about 95% as obtained from thisstudy in the laboratory. Field applications in India and Bangladesh suggest that geojute (soil saver) may be successfully implemented for the purpose described earlier.

It may be noted that selection of designed vegetation is another important aspect of this bio-engineered approach to the solution of top soil erosion control. More rigorous field trials and laboratory simulations studies should be undertaken in order to establish a more sound understanding.

Graph. 3. Cumulative Runoff Vs Time Graph

Total runoff with time for both cases gives almost similar results

REFERENCES [1] GOI (2008) – A Manual on Use of Jute Geotextiles in Civil Engineering-2008. Jute manufacturers Development Council (JMDC).

[2] GOI (2007) – Performance Evaluation of Jute Geotextile. A Publication under IJIRA-JMDC Project on Promotion of Jute Geotextile.

[3] Rahman M. M. and Khan A. J. (2009) – Raindrop Erosion Control with Geojute and Vegetation. Proc. Of Bangladesh Geotechnical Conference. p-208-217.

[4] Sanyal T, Verma S and Choudhury P. K. (2003) – Jute Geotextile in Hill Slope Management – Case Studies in Sikkim and Meghalaya – Proc. National Seminar on Disaster Management with Specific Reference to Land Slides and Avalanches, Vigyan Bhawan, New Delhi – 29th October, 2003.

[5] Ghosh S. N, Chatterjee P.K, Ghosh Satya N. and Palit S. (1993) – Controlling Soil Erosion by Geojute Application; Textile Trend, May, pp. 93-97.

VII. METHOD OF FIELD APPLICATION Geojute cover might be used at any site having rain cut erosion prone top soil or unstable slope for temporary protection. Surface should be prepared at first to level any form of undulation. Then open mesh type geojute should be laid on and anchored with horseshoe steel hook to keep geojute in place. The quality, expressed in terms of gsm, of geojute should be selected for the extent of present erosion condition using expertise and availability. Immediately after the placement of geojute, plantation is necessary as untreated geojute is going to be decomposed within two monsoons which will work as fertilizer for better growth of plants. Meanwhile, plants will be grown enough to prevent

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