Geosynthetic

Geosynthetics is the term used to describe a range of generally synthetic 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. 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 in many civil and geotechnical engineering applications including roads, airfields, railroads, embankments, retaining structures, reservoirs, canals, dams, bank protection and coastal engineering.

History

Geosynthetics of different sorts have been used for thousands of years. They were used in roadway construction in the days of the Pharaohs to stabilize roadways and their edges. [Geotextiles, "The fabric of erosion control" [http://www.multigeo.com/index.html] Accessed 20th May 2006] These early geotextiles were made of natural fibres, fabrics or vegetation mixed with soil to improve road quality, particularly when roads were built on unstable soil. Although modern geosynthetics bear little resemblance to those used in Ancient Egypt, the general principles remain the same. The development of geosynthetics has been slow, mainly due to the limitations of the materials used. With the recent development of polymers, the modern development of geosynthetics has moved along swiftly. Their relatively recent development and acceptance by industry is highlighted by the fact that the International Geosynthetics Society (IGS) was founded in Paris in 1983. [IGS, "About the IGS" [http://www.geosyntheticssociety.org/about.htm] Accessed 27th May 2006]

Types

Geotextiles

Geotextiles 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 discernible warp and weft. [Joint Departments of the Army and Air Force (1995), "TM 5-818-8/AFJMAN 32-1030, Engineering Use of Geotextiles" Washington DC [http://www.army.mil/usapa/eng/DR_pubs/dr_a/pdf/tm5_818_8.pdf] Accessed 29th May 2006] 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 mainly polymeric yarns and fibres, consisting primarily of polypropylene, polyester, polyethylene and polyamide. There are a small group of Geotextiles that are still produced from fibrous materials, used mainly in erosion control. Their degradable characteristics are beneficial in some applications.

Geogrids and 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.

Geogrids are meshes typically made of a regular pattern of tensile elements usually made of a fairly rigid type of plastic. These are used to strengthen fill materials in geotechnical applications. They provide increased shear strength between soil strata interfaces. Their tensile strength can prevent or decrease the degree of differential settlement in some applications such as beneath structures or roads by transmitting the load over a broader area of soil, thereby diminishing the vertical stress — and subsequent compression — in the soil.

Geomembranes

Essentially impermeable sheets produced from polymeric materials. Geomembranes are manufactured several ways, excluding woven methods as they would leave unacceptably large voids in the material. Geomembranes can also be manufactured using fibreglass and bitumen. [Coletanche (online). [http://www.coletanche.com] ]

Geomembranes are used widely as cut-offs and liners. Until recent years, geomembranes were used mostly as canal and pond liners. One of the largest current applications is the containment of hazardous and municipal wastes and their leachates. In many of these applications geomembranes are employed with geotextile or mesh underliners which reinforce or protect the more flexible geomembrane whilst acting as an escape route for gases and leachates generated in some wastes.

Geomembranes are made of various materials including low density polyethylene (LDPE), high density polyethylene (HDPE), polyvinyl chloride (PVC) and polypropylene (PP). Another type of geomembrane is bituminous geomembrane, which is a layered product of glass and bitumen-impregnated non-woven geotextile.

Each type of geomembrane material has different characteristics which affect installation procedures, lifespan and performance. For example, PVC geomembranes are very flexible and as a result can conform to uneven surfaces without being punctured.

Geocomposites

Geocomposites are a combination of any of the above three. The materials and manufacturing methods vary with the composite geosynthetics used. For example, a geonet sandwiched between two non-woven geotextile layers can be used to provide a drainage layer with high transmissivity.

Applications

Among other uses, geosynthetics can be used for separation, filtration, reinforcement, drainage, protection and moisture barriers [Propex Fabrics (1994), "Technical Note No.1: Geosynthetic Functions" [http://www.geotextile.com/tech/WebVersions/TechnicalNote1web.pdf] Accessed 29th May 2006] . Different geosynthetics are suited for various applications and the diagram to the right illustrates their suitability.

Filtration can significantly enhance the performance of a geotechnical structure, and geosynthetics can be used to produce an effective filtration system [Kutay and Aydilek (2005), "Filtration Performance of Two-layer Geotextile Systems" Geotechnical Testing Journal, Volume 28 No.1] . 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 are 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 as "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 well suited to this application.

Drainage is 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 well 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 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 soils, thus maintaining design thickness and roadway integrity. Separators also help prevent fine-grained subgrade soils from being pumped into permeable granular road bases [IGS, "Geosynthetics Functions" [http://www.geosyntheticssociety.org/source_documents/Functions.pdf] Accessed 28th May 2006] ] . Geotextiles and geomembranes are most suited to this application.

Reinforcement: geosynthethics can be used to reinforce a soil mass, increasing the effective angle of shear and increasing the stability of an earth structure. In the reinforcement function, the geosynthetic is subjected to a sustained tensile force. 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

* 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 [Propex Fabrics (1993), "Technical Note No.2: Handling and Storage of Geosynthetics" [http://www.geotextile.com/tech/WebVersions/TechnicalNote2web.pdf] Accessed 28th May 2006] . Small damage to the material's matrix can seriously reduce the function of the material. A puncture in a leachate barrier is 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 [Button & Lyton (2003), "Geosynthetics in Flexible and Rigid Pavement Overlay Systems to Reduce Reflection Cracking" [http://www.utexas.edu/research/tppc/pubs/button_geosynthetics.pdf] Accessed 28th May 2006] .
* Some polymers are susceptible to chemical attack and are degraded by UV light and organic solvents [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] .

ee also

*Hard landscape materials

References

External links

* [http://www.iagi.org International Association of Geosynthetic Installers]
* [http://www.geosyntheticssociety.org International Geosynthetics Society Website]
* [http://www.army.mil/usapa/eng/DR_pubs/dr_a/pdf/tm5_818_8.pdf Engineering Use of Geotextiles]
* [http://www.gmanow.com Geosynthetic Materials Association]
* [http://www.geosynthetic-institute.org Geosynthetic Institute]