Geosynthetics

Introduction

Geosynthetics (geotextiles, geom.embranes and related products) have become widely accepted as essential to the cost effective completion of a wide variety of projects in civil and geotechnical engineering. The selection and correct installation of an appropriate geosynthetic can often result in substantial cost savings compared to alternative methods of construction and indeed in some cases can make the difference between whether or not a project is feasible. Geosynthetics represent the most exciting and rapidly developing field in the area of geotechnical engineering. However, although geosynthetics can produce remarkable results, there is no magic. In all areas of civil and geotechnical engineering, the success of a project requires a good design which makes cost effective use of available materials, a good understanding of material behaviour, and an appreciation of how the design will actually be constructed. In addition there is need for construction specifications which are clear, practical and which will result in the designer’s intention being realized in construction. Finally, it is necessary for those involved in construction supervision to understand the designer’s intentions and to be familiar with the appropriate methods of construction.

The International Geotextile Society (IGS) was formed 10 years ago as a professional society dedicated to the scientific and engineering development of geotextiles, geomembranes and related products; referred to collectively as geosynthetics. While there is a certain degree of overlap and interaction with geotechnical societies (e.g. the International Society for Soil Mechanics and Foundation Engineering) the scope of the IGS goes beyond geotechnical engineering. The development of the discipline has relied upon polymer scientists, textile specialists, chemical and mechanical engineers as well as civil and geotechnical engineers. This interaction is becoming increasingly more important, as geosynthetics are being used in ever larger and more complex applications. There has also been a corresponding major increase in the world wide use of geosynthetics with Asia and Southeast Asia becoming one of the major growth areas. The need for interaction between professionals in a multi-disciplinary field and the need for dissemination of information regarding geosynthetics has resulted in a rapid growth in the membership of the IGS and in the growth of the local chapters. The IGS now has chapters in Japan, the United Kingdom, India, North America, Southeast Asia, China, The Netherlands, Indonesia, Italy, France, Korea, Germany and Latin America and a number of new chapters are in the early stages of formation. Unfortunately, there is as yet no chapter in Australia.

The objective of this present article is to provide a brief summary relating to a selected few of the numerous applications of geosynthetics and to provide readers with an indication as to where they can find additional information. An excellent starting point for any search for more information on geosynthetics is the “Geosynthetics Bibliography” prepared for the IGS by Giroud and Beech (1993).

It is both timely and significant that the next International Conference of the IGS (the 5th International Conference on Geotextiles, Geomembranes and Related Products) will be held in Singapore 5-9 September 1994. The level of interest in geosynthetics is evident from the fact that more than 530 abstracts were submitted to this conference. The 5th International Conference will provide an excellent and relatively close opportunity for Australian engineers and others interested in the applications of geosynthetics to be updated on the latest advances of the discipline.

Education

One of the greatest challenges facing the discipline, and the IGS, is the dissemination of information and the education of designers, salespersons and contractors in the appropriate use of geosynthetics. This is where local chapters play an important role in providing a forum for education and discussion. The majority of individuals involved in the design and use of geosynthetics graduated long before geosynthetics were introduced into the under-graduate program at universities; indeed there are still many universities where geosynthetics are not mentioned at all. Thus there is a need firstly to provide generic, unbiased, information to designers and contractors regarding appropriate methods of design, the differences between different types of geosynthetic products and the appropriate methods of construction. The IGS, through its local chapters, is addressing this issue by means of short courses and seminars for practising engineers and technologists. The North American chapter of the IGS is leading the way in addressing the problem of educating the educators and is in the process of establishing a week-long training program for faculty members interested in geosynthetics. This program is directed at providing generic teaching materials for use in undergraduate courses. These materials are being developed in a modular form so that they can be incorporated as individual lectures relevant to specific topics in existing undergraduate courses.

Geosynthetics in reinforcing applications

The use of geosynthetic reinforcement to allow cost effective construction of steep slopes, reinforced soil walls and reinforced embankments in soft soils is now a proven and widely accepted technology. The use of geosynthetics in these types of applications are now incorporated in design codes (e.g. the U.K) and in national documents which provide guidance to design engineers (e.g. Canadian Foundation Engineering Manual, 1993 and in the U.S. the Federal Highway Administration Guidelines). An excellent review of the state-of-the-art is contained in a paper by Jewell (1990) and in the keynote papers presented at the International Symposium on Soil Reinforcement (IS Kyushu-92).

The main challenge facing engineers designing geosynthetic reinforced soil structures is to recognize that there are many different geosynthetic products available. These include:

• woven geotextiles which cover a range in strength from the order of 10 kN/m up to 1000 kN/m;
• nonwoven geotextiles which typically have a strength (in isolation) ofless than about 30 kN/m;
• and different types of geogrids which typically have ultimate tensile strengths in the range of20- 200 kN/m.

These different materials each have advantages and disadvantages; it is important to recognize that the objective of a design is to find a cost effective solution to a given need and that often, alternate designs can be obtained using different types of geosynthetic materials.

The ultimate choice of the most desirable geosynthetic is entirely dependent on the specific application and conditions and may be as much related to the cost of other materials (e.g. fill material) as it is to the cost of the geosynthetic. However, the design of reinforced soil structures will depend both on the characteristics of the class of the geosynthetic being considered as well as the characteristics of the soil. Thus, for example one needs to be cautious in developing a product specific design (i.e. brand X or equivalent) since frequently this design will incorporate inherent characteristics of that specific geosynthetic and this may unnecessarily limit the range of alternative geosynthetics and correspondingly increase costs. When a design is being developed it is important that consideration be given to the various alternative types of geosynthetic and the implications with respect to the fill material that is used. Once the design has been completed, it is important to only specify those geosynthetic (and soil) properties essential to the performance of the reinforced soil structure being considered.

When tenders are called, it is also important to allow consideration of alternative designs. For example, one might be able to achieve a more economic design by substituting a geosynthetic with a lower interaction coefficient between soil and reinforcement but requiring a longer embedment length. Alternatively, one might also be able to achieve an equally effective but cheaper design using a shorter length of reinforcement which has a higher interaction factor. One should not prejudge what will necessarily be the most cost effective solution. On the other hand, substitutions should only· be made on a sound technical basis and the specifications should never be arbitrarily “relaxed” to allow the use of a “lower quality” geosynthetic or soil to be used simply to reduce cost.

There has been much discussion of the relative advantages and disadvantages of different types of geosynthetic reinforcement. This discussion sometimes focuses on the physical nature of the product (e.g. the type of geogrid or woven geotextile) or on the type of polymer (e.g. polyester, polypropylene, and high density polyethylene). The reality is that for the majority of applications, providing the differences in material properties is considered in the design, many different geosynthetic products with different structures and polymers can be satisfactorily used to provide adequate reinforcement of a wide range of reinforced soil structures. The key issue then becomes one of economics. Thus, there is not one form of geosynthetic reinforcement that is intrinsically better than another, they are simply different and these differences need to be understood and considered in the design.

Geosynthetics in filtration and drainage

One of the earliest and still major applications for geosynthetics is in the area of filtration and drainage. An excellent review of this application has heen provided by Gourc and Faure (1990). Geosynthetics often serve to provide filtration and drainage along highways, in dams, for river and coastal bank protection and in landfills. Often, geosynthetics will serve filtration and/or drainage functions in projects where other geosynthetics will serve alternative functions. For example in the construction of retaining walls with a full face concrete panel, geosynthetics may be used to provide drainage between the retaining wall and the reinforced soil mass. Similarly in the construction of reinforced embankments over soft soils, geosynthetic drains may be used to enhance pore pressure dissipation and to increase overall embankment stability. An IGS supported conference dealing with filtration and drainage was Geofad ’92 held in Cambridge, U.K.

Landfills

A major growth area in the use of geosynthetics is in landfills. In this context, people most commonly think of geomembranes and indeed geomembranes play a very important role with respect to creating low permeability landfill liners or covers. However, for each square metre of geomembrane used in a landfill there are usually many square metres of geotextiles and geonets used to provide for separation, filtration, drainage and protection. In some cases, geosynthetics may also be used to provide reinforcement of the soil mass. An excellent review of the use of geosynthetics in landfill applications has been provided by Koerner, (1990) and the Proceedings of the 4th International Conference on Geotextiles, Geomembranes and Related Products (1990), contains a number of important papers dealing with the subject as does the Proceedings of the 4th International Landfill Symposium (1993).

One of the key issues to be considered in the use of geomembranes to provide barriers and more generally, in the use of geosynthetics in landfill applications, is to ensure there is an adequate level of quality assurance associated with the project. This is becoming increasingly more important as landfill designs become more sophisticated and as larger and larger landfills are being constructed with the expectation that the barrier system will provide long term protection to the environment. There are two key factors influencing long term performance of a barrier system. The first is the design. The second is the construction. The performance of the overall system can never be better than the design allows and consequently it is essential that the designers of waste disposal systems (either with or without geomembranes) have a clear understanding of the factors affecting the performance of barrier systems and the interaction between the landfill and the local hydrogeology (e.g. see Rowe, 1992). As landfill designs become more and more complex, it is becoming increasingly important to ensure that there is a peer review of designs in order to provide a check on the quality of the design itself.

Assuming that an adequate design has been developed and checked, the next important step is to ensure that the construction of the facility is in accordance with the designer’s intention. This is where construction quality assurance becomes essential. The first step is the development of the construction quality assurance plan. This should be developed in parallel with the design itself and not as an afterthought. The construction quality assurance plan should then be implemented on the site in a manner which will ensure quality of the entire system. This will require the use of a quality assurance team which has sufficient specialized knowledge to deal with all of the components of the engineered system including such features as compacted clay liners, the installation of geotextiles, geonets, geomembranes, granular drainage systems, pipes and manholes. For example, it serves little purpose to ensure the construction of a high quality compacted clay liner as part of a composite liner system if one then allows the liner to desiccate and crack during the seaming of the geomembrane. Similarly, the great deal of effort devoted to ensuring good quality geomembrane seams is wasted if one doesn’t then pay an equal amount of care and attention to ensuring that placing of the overlying layers of the drainage system does not cause damage or perforation to the underlying geomembrane. Thus the barrier system is designed and must be constructed as a system, paying careful attention to each component while never losing sight of the need to ensure the integrity of the entire system.

Conclusion

This article has attempted to summarize and highlight some of the important issues relating to the use of geosynthetics in civil and geotechnical engineering projects. Geosynthetics now have a proven track record of providing cost effective alternatives to conventional methods of construction. In some cases, geosynthetics make it possible to undertake construction where otherwise it would not be practicable. However, like all engineering materials, geosynthetics should be treated with respect. If they represent an important part of the civil engineering structure then the same consideration should be given to them in the design as is given to the other components. Similarly, in construction, geosynthetics should be treated with the same level of respect as is accorded more conventional engineering materials. The IGS is actively promoting research and the dissemination of information concerning the appropriate design and use of geosynthetics. We are delighted to have been invited to have this opportunity to make an overview contribution to a special issue of Australian Geomechanics dealing with geosynthetics.

References

Canadian Foundation Engineering Manual (1993) – 3rd Edition; The Canadian Geotechnical Society, Richmond, B.C., Canada, 512 p.

Fourth International Conference on Geotextiles, Geomembranes and Related Products. (1990), Ed. G. den Hoedt, Balkema, Rotterdam, 1250 p.

Geofad ’92 – Proceedings of the Conference on Geotextiles in Filtration and Drainage, Cambridge, U.K, Ed. S. Corbet & J. King, Thomas Telford, London.

Giroud, J-P., with cooperation of Beech, J.F. and Khatami, A. (1993). “Geosynthetics Bibliography”, Volume 1, IGS, IFAI Publishers, St. Paul, MN, USA, 781 p.

Gourc, J-P and Faure, Y-H (1990), “Soil particles, water and fibres – A fruitful interaction now controlled”, Proceedings Fourth International Conference on Geotextiles, Geomembranes and Related Products, Ed. G. Den Hoedt, Balkema, Rotterdam, pp. 949-973.

IS Kyushu ’92 – Proceedings of the International Symposium on Soil Reinforcement (1992), Ed. H.Ochiai, S. Hayashi, and J. Otari, Balkema, Rotterdam, 725 p.

Jewell, R.A., (1990), “Strength and deformation in reinforced soil design”, Proceedings Fourth International Conference on Geotextiles, Geomembranes and Related Products, Ed. G. den Hoedt, Balkema, Rotterdam, pp. 913-946.

Koerner, R.M. (1990), “Preservation of the environment via geosynthetic containment systems”, Proceedings Fourth International Conference on Geotextiles, Geomembranes and Related Products, Ed. G. Den Hoedt, Balkema, Rotterdam, pp. 975-989.

Rowe, R. K (1992), “Integration of hydrogeology and engineering in the design of waste management sites”, Proceedings of the International Association of Hydrogeologists Conference on Modern Trends in Hydrogeology, Hamilton, Canada pp. 7-21.

Sardinia 93 – Proceedings of the Fourth International Landfill Symposium, Cagliari, Italy.

U.S. Department of Transportation (1989), Reinforced Soil Structures – Design and Construction Guidelines, 287 p.