Fundamental of Geosynthetics

Definition

Geosynthetics is the catch all 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. Needless to say, those materials may be destructible. 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.

Types and Material

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 discernible 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 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/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. 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 by transmitting the load over a broader area of soil thereby diminishing the vertical stress 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. Suitable materials include PVC, Polypropylene, Polyethylene and HDPE.

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

Applications

Among other uses, geosynthetics can be used for Separation, Filtration, Reinforcement, Drainage, Protection and Moisture Barriers^[4]. 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^[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 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 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 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^[6]. Geotextiles and geomembranes are most suited to this application.

Reinforcement Geosynthethics can be used to reinforce a soil mass in, 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.