Rod Smith[11]
United Kingdom
1 BACKGROUND AND APPLICATIONS
What are geotextiles? They are textiles (fabrics) used in or near the ground to enhance the ground's characteristics. Applications are usually in the field of civil engineering and environmental engineering and consequently the design of these applications is often closely associated with geotechnical engineering.
Figure 1: Geotextile materials and forms
Natural fibre geotextiles have been used for thousands of years and references are found in the Bible. However, since the 1960's a drop in the market for synthetic textile clothing and carpets together with the importation of cheaper textiles caused textile manufacturers in Europe and North America to seek other applications. Various materials, both natural fibre and synthetic, are used in geotextile manufacture. The final product may be in various forms, including, textile sheets (woven and non-woven), nets, grids, strips, grids woven from strips, sheets woven from strips and strips made of narrow grids as indicated in Figure 1.
Figure 2: Selected geotextile applications
The use of industrial fabrics in the civil engineering industry was identified as the new, and potentially very large, market in which to sell synthetic textiles. Figure 2 shows some of these applications for textiles. Originally the main use was as a filter between soils of different particle sizes and the general name used for textiles in civil engineering was "filter fabric". Only later did the term "geotextile" gain usage[12]. The required properties and function of the geotextile is related to its application. The Geotextiles Manual[13] identifies the relative importance of its function as shown in Figure 3.
Figure 3: Importance of geotextile functions and properties related to applications
The main uses of geotextiles are as separators and in asphalt overlays followed by drainage with erosion control accounting for about 8 percent of all uses[14] in 1997. (Figure 4) More recently other authors[15] have estimated erosion control applications as using some 170 million sq.m of geotextiles which would equate to about 12 percent of total usage in 2000.
There are three possible areas to explore for sisal geotextiles:
The last of these will probably be the most difficult unless the manufacturers identify a unique property of sisal which is not possessed by other geotextiles. It will be useful to explore the second area to review if sisal can compete with synthetics, but many of these applications are for medium or long term periods. However the first area is where sisal manufacturers are likely to begin to make headway into the existing market for natural fibre geotextiles. Here the characteristics of sisal will be pitted against those of coir and jute and the sisal geotextile manufacturers against a long established and functioning supply system of other natural fibre geotextiles.
2 MAIN USES OF NATURAL FIBRE GEOTEXTILES
As a pointer to the potential of sisal it may be helpful to examine the requirements made of the geotextile in the current area of use of other natural fibre geotextiles. There are many requirements for the applications shown in Table 1, but strength, low extension in service and durability are usually of prime importance.
Figure 4: Use of geotextiles by application
Designers of soil erosion control systems usually only need the geotextile to provide ground protection and to create a micro-climate for the seedlings until vegetation is established which is often for one or two growing seasons. Thus the durability of even low weight jute geotextiles is adequate in most cases and sisal would be expected to outlast jute. In cases of river banks or extreme applications where plant growth is expected to take longer then sisal would probably have an advantage.
The requirement of a mulch is to provide tight ground cover to suppress weed growth and to enhance the crop by providing a beneficial micro-climate around the plant. Thus there is generally no need for longevity and strength is needed only for reasons of handle-ability. Here the sisal fabric would need to be tight and lightweight.
Table 1: Requirements of geotextiles in selected applications
|
|
Required Characteristics |
||
|
Strength |
Low Extension |
Durability |
|
|
Erosion Control |
Ö |
|
Ö to ÖÖ |
|
Agro-mulching |
|
|
Ö |
|
Un-paved Roads |
ÖÖ |
ÖÖ |
ÖÖÖ |
KEY: Important: (ÖÖÖ); Less important (Ö); Not important ().As seen in Figure 4 above, a large tonnage of geotextiles is used as separators between the road sub-base and the soil formation below and as reinforcement below sub-base layers of non-paved roads and temporary roads. The design of many such roads will probably require a geotextile of medium durability or better. This application demands a geotextile of strong tensile capacity which exhibits only a small extension at working loads in order that the geotextile shares some of the load imposed by the traffic and to assist the granular sub-base to remain stable with minimum rutting.
Let us examine each of these applications in turn.
2.1 Soil erosion control
Soil erosion has been occurring for some 450 million years, since the first land plants formed the first soil although it only became a serious problem in recent centuries because of the accelerated erosion. Erosion is often the result of human activity, such as unsuitable cultivation practices and forestry exploitation which leaves the land vulnerable during times of heavy rainfall and high winds. Often slopes are formed either by cuttings or embankment fills when roads or railways are built or when land is developed. For an economical earthwork and to reduce the area of un-productive land, steep slope angles are preferred, but, the steeper the slope the greater the risk of soil erosion. Soil erosion by water and wind is responsible for about 56 percent and 28 percent respectively of world-wide land degradation.[16] The US Army Corp of Engineers has estimated that in the USA alone the damage caused by soil erosion costs at least $200 million annually.
A soil erosion nuisance can become a serious landslide problem causing damage to property and loss of life. The solution is the provision of an erosion control systems such as shown in Figure 5. Figure 5a shows a geotextile laid out over a pre-seeded slope and Figure 5b shows the completed work with the green shoots just starting to grow through the geotextile. Vegetation is well established in Figure 5c.
Figure 5a: Geotextile laid on pre-seeded slope
2.2 Agro-mulching
A ground cover, in the form of a sheet, is required to:
Figure 5b: Green shoots growing through the geotextile
Figure 5c: Vegetation well established through the geotextile
The ground cover used is not usually a geotextile but a lightweight plastic film but some geotextiles, both synthetic and natural fibre, are used. The crop is planted in holes cut through the sheet. The film sheet is usually discarded at the time of harvesting the crop and thus there is no need for long durability.
It is desirable to use a geotextile with a large percentage of cover in order to block out the light and hence deprive weeds of growth potential and thus non-woven fabrics would be the ideal choice. A geotextile is also more effective than a thin film as a temperature insulator and studies have shown that they benefit the crop by reducing high summer peak temperatures and reducing the risk of low night time temperatures.
2.3 Un-paved roads
The principal method is to lay a geotextile on the road formation before the granular sub-base is laid. If the geotextile is strong enough and does not extend too much it contributes to supporting the traffic loads by various actions[17] as illustrated in Figure 6.
Figure 6: Geotextiles in unpaved roads
Another important characteristic is a separation function. The sub-base may be separated from the formation by a geotextile. This can have significant beneficial effects especially on poor clay soils where the granular particles of the sub-base tend to penetrate more and more with increasing passage of wheel traffic causing the sub-base to be "lost" into the underlying clay.
Another less used method to increase the load carrying capacity or longevity of a road is Soil Stabilisation using Fibres which consists of mixing staple fibres into the road surface layer. Short fibres, a few cm long, are either rotavated, in-situ, into the sub-base layer or pre-mixed and laid. Chemical additives such as lime or cement may also be used. Recent work by the University of Birmingham, United Kingdom has demonstrated the beneficial effect of sisal fibres mixed into the road surface layer in laboratory tests. To confirm the effect at full scale, staff of the University of Birmingham, United Kingdom in conjunction with the Ministry of Works, Transport and Communication of Uganda carried out trials with sisal fibre on a laterite formation. This involved the mixing of the sisal fibres into the soil. This method can also be a cost effective repair method for the potholes in roads surfaced with only a thin surface dressing.
3 GEOTEXTILE MARKET SIZE, GROWTH AND GEOGRAPHIC SECTORS
The growth of the geotextile market has been spectacular and not many products have experienced similar growth. From only 10 million sq.m in 1970 it has grown almost exponentially to approximately 1400 million sq.m in 2000[18],[19] as shown in Figure 7.
The majority of geotextiles are used in Western countries having polymer/synthetic textile manufacturing industries and the geographic market share is as shown in Figure 8.
Fig 7: Growth in world use of geotextiles
Fig 8: Geotextiles - geographic distribution of market
Accurate figures are not easy to obtain in some sectors of the market but Figure 9 shows a best estimate for the various market segments.
About half (750 M sq.m) of the total are applications where sisal geotextiles could possibly be considered for use depending on the durability of the fibres. The established market for erosion control geotextiles which does not require product longevity is 170 M sq.m and half of this is provided for by natural fibre geotextiles. While most agro-mulch sheets are currently not geotextiles they amount to an enormous market of some 2600 M sq.m, nearly twice that of all geotextile applications[20].
Figure 9: Geotextiles - world market
4 WHAT PRICE COULD BE OBTAINED FOR SISAL GEOTEXTILES?
A guide to the possible price which could be obtained for sisal geotextiles are the prices paid now by users for other geotextiles in applications where sisal could be considered. The prices below are in US dollars per square metre of geotextile. The range reflects the different requirements in each application and the variety of products available.
It would be expected that sisal erosion control geotextiles could compete with other types and in the applications of un-paved roads/separators sisal's apparent durability may prove to be a great advantage compared with jute and coir and may even oust synthetics in some applications. As the cost of agro-mulch films is very low it is unlikely that a sisal geotextile could be produced at that cost level. However if the technical advantages of a geotextile are promoted by suppliers and are understood by users then they may be willing to pay a small cost premium above the film cost.
5 WHAT IS NEEDED FROM SISAL SUPPLIERS FOR SISAL TO IMPACT ON THE GEOTEXTILE MARKET?
Whilst the first geotextiles used were of natural fibres it was the petro-chemical industry which seized the opportunity to sell large quantities of synthetic textiles into civil engineering projects. The mass market was established by the widespread use of synthetic geotextiles which continue to dominate. Therefore for sisal to impact this market the sisal suppliers will be forced to provide similar services to the users and specifiers at similar costs. If sisal suppliers can improve on the services offered by the manufacturers of synthetics and at a more economic cost then sisal could start to make serious inroads into this mass market which is growing at a spectacular rate.
Table 1: Guide to geotextile prices
|
Application |
Price |
|
US $ per sq metre |
|
|
Erosion Control |
|
|
Synthetic |
1.00 - 3.00 |
|
Woodfibre/Straw |
0.55 - 1.10 |
|
Jute |
0.30 - 1.00 |
|
Coir |
0.90 - 2.20 |
|
Agro-mulching |
|
|
Plastic film (not geotextile) |
0.10 - 0.25 |
|
Un-paved roads/separators |
|
|
Synthetic |
3.00 - 5.00 |
|
Jute |
0.60 - 1.00 |
|
Coir |
0.90 - 2.20 |
Fig 10: Low rainfall and runoff
Sisal suppliers will need to provide reliable data giving technical details of the geotextiles offered and their expected behaviour. Users will have a vast array of information about synthetic geotextiles and will expect to have something similar concerning any sisal geotextiles proposed. Figure10 gives an example of the effectiveness of various synthetic and natural fibre geotextile in erosion control trials. Extensive work at the University of Birmingham, United Kingdom shows that the use of sisal fibres reduces erosion control of soil and improves the strength of a clay road formation (Figure 11). As in any market it is very important that the seller understands the needs of the user and specifier. In this technical market the synthetic manufacturers have found it necessary to use the services of civil engineers in order to liaise satisfactorily with the civil engineer users of the products.
Fig 11: Unconfined Compressive Strength for Clay Soil and 20mm Sisal Fibres
Source: Balamu (1998).6 CONCLUSION
The market for geotextiles is growing at an exceptional rate, and in the year 2000 some 1400 million square metres of geotextiles were sold. New applications are being found and new products developed at great rate. There is an established market for natural fibre geotextiles and in some applications they have characteristics superior to synthetics but they often come second best due to the ability of the synthetic geotextile suppliers in meeting users demands for technical data.
Sisal geotextiles could compete with other natural fibre geotextiles and would be expected to have some properties superior to other natural fibres. Further study of these properties should be carried out in order to establish a good data bank of knowledge and to also focus on those applications where synthetics have the monopoly and where prices paid are high. This work would benefit from the support and sponsorship of all sisal producing countries in order to catch up with the work already produced by the petro-chemical companies who manufacture synthetic geotextiles and to compete in this technical field.
One interesting development[21] in this field is the use of composite geotextiles incorporating different materials. An example is the combination of straw fibres and a synthetic mesh to form an erosion control geotextile - the straw provides the erosion control properties and the mesh gives structural form and strength to the geotextile improving handling characteristics and the product's fixity to the ground. This trend of combining materials is expected to continue and the barriers between synthetic and natural fibre geotextiles will be less rigid. It may be of benefit to sisal producers to form alliances now with the producers of synthetics as it is likely that the producers of other natural fibres will do so.
APPENDIX: SOURCES OF INFORMATION ON GEOTEXTILES
International Geosynthetics Society.
Chapters are established in: Brazil, China, France, Germany, India, Indonesia, Italy, Japan, Korea, Netherlands, North America (Canada & USA), Romania, South Africa, United Kingdom, West Pacific Region (Taiwan).
Here geosynthetics has a broad definition for the purposes of the IGS and includes natural fibre products and other in-soil related products and technologies.
REFERENCES
Balamu P.B., (1998), Reinforcement of Soils with Natural Fibre Sisal, MSc Thesis, Birmingham University, United Kingdom.
CFC & IJO, (1998), Jute Geotextiles Techno-Economic Manual. CFC & IJO.
Giroud J. P. & J. Perfetti, (1977), Classification des textiles et measures de leurs propriété en vue de leur utilisation en géotechnique. Proc. Int. Conf. on Use of Fabrics in Geotechnics.
Ingold T. S., (1994), Geotextiles & Geomembranes Manual. Elsevier.
Smith R. J. H., (1997), Goeotextile Applications, Seminar: Jute Geotextiles, United Nations International Trade Centre, Geneva & London.
Smith R. J. H., (1998a), Back to Nature, Ground Engineering Journal, March. Emap.
Smith R. J. H., (1998b), Geotextiles Applications, Japan Textile Importers Association, Seminar, (unpublished).
Smith R. J. H.,(1998c), Le Jute: Ecologique, Economique, Polyvalent (in French), TUT La Revue Européenne des Utilisateurs de Textiles Techniques, Juin-Aout, Pub Institute Textile de France.
Wibisono G., (2000), The effect of water on the erodibility of a fibre reinforced or stabilised kaolin, MSc Thesis, University of Birmingham, UK.
