Proceedings of Indian Geotechnical Conference December 15-17,2011, Kochi (Paper No.J-342)

A CASE STUDY ON SOIL BIOENGINEERING APPLICATION USING STITCHED

EROSION CONTROL BLANKETS FROM COIR FIBRE

K. Balan, Professor in Civil Engineering, College of Engineering,Trivandrum, drkbalan@gmail.com

Sibi Joy, P.G Student, Civil Engineering Department, College of Engineering,Trivandrum, joy.sibi@gmail.com

ABSTRACT: Stabilization of a steep slope of 1: 1.2 having a slope length of 6 m over a conventional retaining wall of 9 m

height using erosion control blanket (ECB) made of coir fiber is presented in the paper. The soil in the site, Wayanad in

Kerala, is silty sand with clay. The soil is stiff in dry condition whereas will become slushy during rainy season. Being a

cricket stadium, at high altitude, paving of the slope with rock boulders is not recommended considering the aesthetic and

to maintain the natural appearance, hence a soil – bioengineering solution was adopted. The work was executed during the

beginning of September 2010 and the slope fully covered with vegetation and also survived two rainy seasons of November

2010 –January 2011 and June 2011.

INTRODUCTION

A high altitude cricket stadium was planned by the District

Cricket Association of Wayanad District in Kerala. The

land proposed consists of highly undulating ground surface

with hillocks and deep valleys. The required ground area

has been developed by cutting the hillocks in the northern

area and filling the valley in southern side. To achieve a

level ground, a gravity retaining wall of 9 m height was

constructed with rubble masonry. Above the retaining wall,

a further filling of 5 m is needed. The exposed slope of the

filled soil has to be protected against erosion, and at the

same time it should not hamper the aesthetic appearance of

the stadium, since the exposed surface of the fill is facing

one of the main entrances to the stadium [1].

Slope protection against surface runoff and rainwater splash

is done with Erosion Control Blanket made of coir fiber in

conjunction with Bermuda grass [2, 3]. This paper details

the methodology adopted for laying the geotextiles and the

performance of the system since execution.

GEOGRAPHICAL DETAILS OF THE AREA

Wayanad district of Kerala is situated in an elevated

picturesque mountainous plateau in Western Ghats. It lies

between north latitude 11º 26' 28'' and 11º 48' 22'' and east

longitude 75º 46' 38'' and 76º 26' 11''. Placed on the

southern tip of the Deccan Plateau, its prime glory is the

majestic Western Ghats with lofty ridges interspersed with

magnificent forests, tangled jungles and deep valleys. In the

centre of the district, hills are of lower in height, while the

northern area has high hills.

Wayanad is situated at a height of 700 to 2100 m above sea

level. Temperature varies from 12 to 25 degrees centigrade.

The mean average rainfall in this district is 2322 mm. High

velocity winds are common during the south west monsoon

and dry winds blow in March - April. The nearest area to

the proposed ground, Ambalavayal, the maximum and

minimum temperature for the last five years was 29º C and

18º C respectively. This place experiences a high relative

humidity which goes even up to 95 percent during the south

west monsoon period.

THE PROPOSED STADIUM

The southern side of the proposed stadium has a retaining

wall to retain the filled up soil. The height of the retaining

wall is 9 m. Above the retaining wall; a further filling of 5

m is needed. The problem was referred to the Authors at

this stage. Further filling was made with the locally

available soil of silty sand with clay. The slope of the filled

embankment was 1:1.2. In the natural condition, the soil at

the site is very intact, but the excavated soil will become

more slushy as the water content increases. Hence proper

drainage of the surface of the ground is required.

The finished surface of the stadium will have a marginal

slope to all the sides from the centre potion, so that rain

water will not be stagnated in the stadium. Concrete edge

drain to collect the rain water from the surface of the play

area is constructed all around the stadium. The slope of the

drain has been made in such a way that the storm water is

disposed off safely, on either side of high retaining wall,

without coming on the top of the sloping face of the filled

up soil. Edge drains are to be constructed at about 2 m away

from the top edge of the filled up slope. Protective nettings

will be provided at about 1.50 m from the top edge of the

slope, i.e., 0.50 m away from the outer edge of the drain.

Owing to the above procedure, the filled up slope will

experience only the direct downpour of the rain on the

slope surface during rainy season. In order to protect the

loosening of the surface soil along the slope during direct

impact of rain water and also to prevent the subsequent

downward movement of soil particles, it has been decided

to use Erosion Control Blankets (ECB’s) made of coir

fibers. Coir Erosion Control Blankets will protect the soil

along the slope surface against splash and surface erosion

till well rooted and thick canopied vegetation establishes

along the slope.

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SLOPE SURFACE PROTECTION AND

STABILISATION

Ground filling above the retaining wall was made with

proper quality control. The edge of the filled up soil was

trimmed off, removing the uncompacted portion along the

slope. A schematic diagram of the proposed slope

protection is shown in Figure 1. Construction of the slope

above the retaining wall is shown in Figure 2.

Fig. 1 Schematic diagram of the proposed slope protection

measures

Fig. 2 Construction of fill over the retaining wall is in progress. Since the slope is to be protected against the rain water splash erosion, conventional woven or mesh coir mattings are not effective and hence it has been decided to use CCM Coir Blankets (Non woven) of 450 gram per square meter. The same blanket has been used as a filter media along the trench at the base of the slope, i.e., at the top of the retaining wall. The prepared slope surface is shown in Figure 3. Laying of Coir ECB along the slope is shown in Figure 4.

Fig. 3 Prepared slope surface above retaining wall.

Fig. 4 Laying and fixing of the coir erosion control blanket. Coir ECB used at the site consists of non woven coir

stitched with HDPE monofilaments, UV stabilized and

supported on the top and bottom with thin PP netting. The

properties of the coir ECB used are given in Table 1.

Non-woven coir geotextile erosion control blankets (Coir ECB) are fixed at the top of the slope, by the side of the concrete edge drains of the stadium. It is anchored properly in a trench of 30 cm x 30 cm x 30 cm using iron “J” clips made from 8mm diameter tore steel and refilled with soil.

9 m

Filled up area

Coir Erosion Control

Blanket (Non-woven)

Drain of 30 cm x 30 cm

With 20 – 40 mm aggregates

covered with Coir ECB

as filter media.

5 m

2 m

Edge

drain

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A case study on soil bioengineering application using stitched erosion control blankets from coir fibre

Coir geotextiles are rolled down along the slope and properly anchored using iron “J” clips [4]. CCM coir blankets are having a width of 2.40 m. Hence overlapping of the geotextiles is required for extension of the work along the length direction of the slope. An overlap of 30 cm is provided and these junctions are properly anchored (Fig. 5).

Table 1 Properties of Coir Erosion Control Blanket

Description Values

Fiber 100% coir

Width & length 2.4 m x 42 m

Unit weight 450 gsm

Top and bottom netting 14 x 14 mm; 4.8 gsm

Stitching thread HDPE monofilament, UV stabilized

Tensile Strength ISO - 10319 104 N/m Maximum permissible shear stress

1.10 gsm

A trench, similar to the one provided at the top is provided at the base of the slope, i.e., just at the top level of the retaining wall. Coir geotextiles are extended to these trenches, properly anchored and usually back filled with soil. However, in order to avoid the overflow of surface water coming from the slope surface over the retaining wall, the bottom trench has been filled with aggregate of size ranging from 20 mm to 40 mm, to act as drain covered with coir geotextiles. Good quality stones from the cutting area graded to the above size are used for this purpose.

The slope coming over the retaining wall area was treated with coir erosion control blankets. Area where the slope length is comparatively less and which is directly starting from the natural ground, surface of the slope has not been treated with coir erosion control blanket and vegetation alone has been done in these area.

Fig. 5 Slope above the retaining wall is treated with coir

erosion control blanket.

OBSERVATIONS

Slope treated with coir erosion control blanket has been

further treated with planting of Bermuda grass. Since the

slope is visible from the entry road itself, it was decided to

have ornamental grass rather than thickly growing bush type

plants. The work was executed during September 2010 and

during execution rain also started in Wayanad District. The

grass planted over the geotextile developed fully within

short time and helped in preventing the surface erosion. The

site experienced the monsoon rain of 2011 and is performing

well, with more vigorous growth of the grass over the slope.

Figure 6 shows the fully vegetated slope during February

2011. A comparison of the vegetation growth along the

slope where coir erosion control blankets was used and that

where vegetation alone was executed after 5 months of the

trial is shown in Figure 7.

Fig. 6 Vegetation is in full along the slope after 5 months.

Fig. 7 Comparison of the growth of vegetation in areas with

or without the use of geotextiles

CONCLUSION The slope protection works against surface runoff and rain

splash erosion has been done using CCM Coir Erosion

Control Blankets of 450 gsm. The work was completed in

September 2010 and the erosion control blankets assisted in

the speedy growth of vegetation on the treated area. Slopes

which are not treated with coir geotextile and only

vegetation has been done, the growth of vegetation was

found to be very less even after 5 months of the completion

of the project. The protection work has by now undergone

two rainy seasons and performing well.

REFERENCES

1. Subha Vishnudas, Hubert H.G. Savenije, Pieter van

der Zaag, K. R. Anil and Balan, K.(2008),

Participatory research using coir geotextiles in

watershed management – A case study in south India,

Intl. Journal of Physics and Chemistry of the Earth,

Vol (33), 1-2, 41-47.

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K.Balan & Sibi Joy

2. Balan, K. (2004), Coir Geotextiles in Watershed

Management and Soil Conservation – Case studies,

Geosynthetics – New Horizons, Asian Books Private

Limited, New Delhi, 371-376.

3. Balan,K and Rao.G.V. (1996), Erosion control with

natural geotextiles, Proc. Int. Seminar and

Technomeet on Envrionmental Geotechnology with

Geosynthetics, Asian Society for Environmental

Geotechnology, New Delhi, 317-334.

4. Venkatappa Rao, G, Balan, K, Prakash, G.K, and

Prasad, S.R.D, (1994), Erosion control measures in

plantation areas using coir geotexiles, Proc. Int. Symp.

on Biocomposites and Blendson Jute allied Fibres,

UNDP/IJIRA, New Delhi, 219-230.

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