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Geosynthetics in use

Geosynthetics is the umbrella term used to describe a range of synthethic products used to solve geotechnical problems. The term is generally regarded to encompass four main products; Geotextiles, Geonets/Geogrids, Geomembranes and Geocomposites. The synthetic nature of the products make them suitable for use in the ground where high levels of durability are required, this is not to say that they are indestructible. Geosynthetics are available in a wide range of forms and materials, each to suit a slightly different end use. These products have a wide range of applications and are currently used to advantage in many civil and geotechnical engineering applications including roads, airfields, railroads, embankments, retaining structures, reservoirs, canals, dams, bank protection and coastal engineering.

History

File:Geotextileroad.jpg

Geotextile used in road construction

Geosynthetics of different sorts have been used for thousands of years; they were used in roadway construction in the days of the Pharaohs to stabilise roadways and their edges ^[1]. These early geotextiles were made of natural fibres, fabrics or vegetation mixed with soil to improve road quality, particularly when roads were made on unstable soil. Although the modern Geosynthetics bare little resemblance to those used in Egyptian times, the general principles remain the same. The development of Geosynthetics has been slow, mainly due to the limitations of the materials used. However, with the relatively recent development of Polymers and their respective characteristics the devlopment of Geosynthetics has also moved along swiftly. Their relatively recent development and accptance by industy is highlighted by the fact that the International Geosynthetics Society was founded in Paris 1983. ^[2].

Types, Materials & Manufacture

Geotextiles These are usually produced as either woven or non-woven textiles. Woven geotextiles are produced by the interlacing of yarns to leave a finished material that has a discernable warp and weft^[3]. Non-woven geotextiles are produced by various methods other than weaving, mainly heat bonded, needle punched and chemically bonded. Woven and non-woven geotextiles are manufactured from polymeric yarns and fibres respectively, the constituent materials consist primarily of polypropylene, polyester, polyethylene and polyamide.

Geogrids/Geonets These are discernibly stiffer than geotextiles and have relatively large voids within the material. Methods of production vary but include extrusion, bonding or interlacing. They can be produced from nearly all polymeric materials.

Geomembranes Essentially impermeable sheets produced from polymeric materials. Geomembranes are manufactured in a range of ways, excluding woven methods which would leave unacceptably large voids in the material matrix. Suitable constituent materials include PVC, Polypropylene, Polyethylene and HDPE.

Geocomposites Term used to describe Geosynthetics which are a combination of any of the above three. The materials and manufacturing methods vary with the composite Geosynthetics used.

Applications

Collection of illustrations showing the basic functions of Geosynthetics

Among other uses, Geosynthetics can be applied to serve the purpose of; Separation, Filtration, Reinforcement, Drainage, Protection and Moisture Barriers^[4]. Different Geosynthetics are suited to the various applications and the diagram to the right illustrates the suitability of the different materials.

Filtration can significantly enhance the performance of a geotechnical structure, and Geosynthetics can be used to produce an effective filtration system^[5]. The job of a filter is to allow water to pass through the plane of the filter, whilst retaining particles of the filtered soil. Filtration can improve the performance of a geotechnical structure by controlling the erosion of the structure and reducing the amount of fines that get washed out of the soil matrix. When fines get washed out of a soil it can reduce the cohesion of the matrix and thus the strength of the soil, referred to Piping. Mitigating these two problems also improves the durability of a structure. Geosynthetic filters can improve the reliability and performance of traditional graded soil filters and require less work to construct. Geotextiles are most suited to this application.

Drainage required in nearly all geotechnical structures. Whether used to remove surface water from a sports field, or to reduce lateral pressure on a retaining wall, the need for effective drainage cannot be underestimated. Drains of various designs have been used in the past, most based on the use of a high permeability layer built into the ground using aggregates, single layers of Geosynthetics can produce the same results. Drains can be distinguished from filters as such; water travels across the plane of filters and travels with the plane of drains. Geotextiles and Geocomposites are most suited to this application.

Protection/Barrier In some geotechnical applications it is necessary to separate or protect one section of the works from another. This could be for a multitude of reasons, including stopping leachate seepage, protecting a structure from moisture and protecting a geotechnical structure from erosion. Geotextiles and Geomembranes are most suited to this application.

Separation The geosynthetic acts to separate two layers of soil that have different particle size distributions. For example, geotextiles are used to prevent road base materials from penetrating into soft underlying soft subgrade soils, thus maintaining design thickness and roadway integrity. Separators also help to prevent fine-grained subgrade soils from being pumped into permeable granular road bases^[6]. Geotextiles and Geomembranes are most suited to this application.

Reinforcement Geosynthethics can be used to reinforce a soil mass in the hope of increasing the effective angle of shear and reducing the instability of an earth structure. In the reinforcement function, the Geosynthetic is subjected to a sustained tensile force or load. Soil and rock materials are noted for their ability to withstand compressive forces and their relative low capacity for sustained tensile forces. In much the same way that tensile forces are taken up by steel in a reinforced concrete beam, the Geosynthetic supports tensile forces that cannot be carried by the soil in a soil/Geosynthetic system. Geogrid/Geonets and Geotextiles are best suited to this function.

Disadvantages

File:Geostorage.JPG

Illustration of how to store Geosynthetics on site

As the materials are relatively new to the Geotechnical industry there is still concern about their long term performance. They are still regarded by some sections of the industry as not being “time-served.”

Unlike soil and rock, Geosynthetics require careful handling and storage^[7]. Small damage to the materials matrix can seriously reduce the functionality of the material. A puncture in a leachate barrier would be a good example of this.

The performance of Geosynthetics under dynamic flow conditions is relatively unknown and most design codes assume a constant seepage.

Moisture barriers may stop water ingress to a structure but they also stop water vapour leaving the structure^[8].

Some Polymers are susceptible to chemical attack and are degraded by UV light and organic solvents^[9].

References

^ Geotextiles, "The fabric of erosion control" [1]Accessed 20th May 2006
^ IGS, "About the IGS" [2] Accessed 27th May 2006
^ Joint Departments of the Army and Air Force (1995), "TM 5-818-8/AFJMAN 32-1030, Engineering Use of Geotextiles" Washington DC [3] Accessed 29th May 2006
^ Propex Fabrics (1994), "Technical Note No.1: Geosynthetic Functions" [4] Accessed 29th May 2006
^ Kutay and Aydilek (2005), "Filtration Performance of Two-layer Geotextile Systems" Geotechnical Testing Journal, Volume 28 No.1
^ IGS, "Geosynthetics Functions" [5] Accessed 28th May 2006]
^ Propex Fabrics (1993), "Technical Note No.2: Handling and Storage of Geosynthetics" [6] Accessed 28th May 2006
^ Button & Lyton (2003), "Geosynthetics in Flexible and Rigid Pavement Overlay Systems to Reduce Reflection Cracking" [7] Accessed 28th May 2006
^ ASTM (1992), "D 4873 Standard Guide for Identification, Storage and Handling of Geotextiles", Annual Book of ASTM Standards, Vol. 4.08. American Society for Testing and Measurements, Philadelphia, p1056-1057

External Links

[1] Geotextiles, "The fabric of erosion control" [1]Accessed 20th May 2006

[2] IGS, "About the IGS" [2] Accessed 27th May 2006

[3] Joint Departments of the Army and Air Force (1995), "TM 5-818-8/AFJMAN 32-1030, Engineering Use of Geotextiles" Washington DC [3] Accessed 29th May 2006

[4] Propex Fabrics (1994), "Technical Note No.1: Geosynthetic Functions" [4] Accessed 29th May 2006

[5] Kutay and Aydilek (2005), "Filtration Performance of Two-layer Geotextile Systems" Geotechnical Testing Journal, Volume 28 No.1

[6] IGS, "Geosynthetics Functions" [5] Accessed 28th May 2006]

[7] Propex Fabrics (1993), "Technical Note No.2: Handling and Storage of Geosynthetics" [6] Accessed 28th May 2006

[8] Button & Lyton (2003), "Geosynthetics in Flexible and Rigid Pavement Overlay Systems to Reduce Reflection Cracking" [7] Accessed 28th May 2006

[9] ASTM (1992), "D 4873 Standard Guide for Identification, Storage and Handling of Geotextiles", Annual Book of ASTM Standards, Vol. 4.08. American Society for Testing and Measurements, Philadelphia, p1056-1057

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