Geotextiles may be defined as fibre-based sheet products that hat are placed within or adjacent to soils to enhance the performance of ground-engineered structures; the prefix geo- means “relating to the earth”. The application of these textile materials in geo-engineering is aimed at making site soils more suitable for a desired end use than they would be naturally.
Historical records show that this concept is not new. The employment of reeds to reinforce compacted soils for foundations in construction of dwellings can be traced back to the first millennium BC where in biblical times ‘The Tower of Babel’ was built on a substrate reinforced by river bank reeds woven into sheet materials. In the third millennium BC, constructions of reed-reinforced clay were used for erosion control along the banks of the Tigris and Euphrates, and it is believed that around the fifth millennium BC, the Persians used compacted soil reinforced with reeds for the construction of dwellings. The early 19th century saw walls and slopes reinforced with brushwood, timber or canvas, and during the first quarter of the 20th century, cotton fabrics treated with asphalt were used in the USA to reinforce and protect sensitive soils. By the late 1950s geotextiles made from synthetic fibres were introduced and mainly used in drainage filtration. In the same decade woven nylon tapes were utilised by the Dutch as canal bed protection mattresses. However, in the late 1960s nonwoven geotextiles made of continuous spun Filament?, were gradually and successfully used in roads and railway track constructions. At about the same time as the introduction of these synthetic-fibre geotextiles, plastic sheet materials, referred to as geomembranes, became available as substitutes for waterproof clay layers, and were developed to become the principal waterproof liner materials for canals, riverbanks, water reservoirs, ponds, and lately, ground storage waste containment sites. Today there are various types of geosynthetics (i.e. geotextile and geomembranes). They have been classified by grouping those materials that are made to be porous and readily permeable to water and gas as ‘geotextiles’ and ‘geotextile related products’, and those made to be impermeable to fluids as geomembranes. Figure 1 gives a chart showing the various types of geosynthetics.
Fig.1 Geosynthetic Materials
The chart indicates that geotextile structures include continuous sheets of nonwoven, woven, and warp knitted fabrics, and stitch-bonded fibres or yarns. The fibre types used in making these geotextiles are mainly jute and coir for natural biodegrabale geotextiles and largely polyester and polypropylene for synthetic geotextiles for longevity.
These geosynthetic materials can be classified according to their basic physical structures as illustrated in Fig.2
Fig.2 Classification of Geosynthetics Structures
Nonwoven geotextiles are fibrous sheets in which the fibres can be almost randomly orientated. These materials may be manufactured from either staple fibers (synthetic or natural) or continuous filaments of PP or PET which are randomly distributed in layers onto a moving belt to form a "web" of assembled fibres; blends of PP and PET staple fibres are also used.
With staple fibres, the web is formed by a carding machine, usually a roller and clearer card [23 ], and is cross-laid onto the moving belt. Continuous Filament? webs are produced during the melt spinning process by extruding multifiament yarns to form a swirling pattern of fibres as they deposit onto the moving belt. In staple-fibre cross-laid webs, the fibre directions are semi-random within the two dimensional plane, whereas the swirling pattern of of the continuous fibre webs give, almost, a totally random fibre orientation. Increasing the number of layers and the number fibres in each layer forming the web, increases the thickness and thereby the bulk of the assembled mass which will also contains a high volume of open spaces.
To give cohesion and strength to the assembled fibre layers, the fibre lengths are either interlocked or thermally bonded. Interlocking of the fibre lengths is achieved through a process called “needle punching”. [See Section on Technologies]
Fig.3 shows a graphic illustration and photomicrographs of a typical nonwoven needle punched structure, and it can be seen that lengths of fibres that were caught by the barbed needles penetrate through the material thickness.
Fig. 3 Needle Punched Structure of Nonwoven Geotextile
Various methods may be used to thermally bond fibrous webs generally, but for thermally-bonded nonwoven geotextiles, bonding is performed by passing the web through heated rollers. The heated rollers compress the layers of loose fibres and cause partial melting of the polymer, leading to heat fusion at the fibre cross-over points. The webs processed in this way have their fibres in the form of filaments, deposited in fewer layers than for needle punching. Therefore, thicknesses and aerial densities are lower for thermally bonded geotextiles. The almost total random orientation of the filaments results in more isotropic strengths, where CD(strength) = MD(strength), when compared with needle punched materials. In general, both the needle-punched and thermally bonded nonwovens have a wider size distribution of open spaces than other geotextiles.
Woven geotextles are produced from synthetic fibre yarns, mainly PET or PP, and natural fibre yarns, largely jute or coir, using wide width looms such as the Sulzer projectile loom P7150, used to produce woven fabric widths of 190 to 540cm.
The type of yarns used to produce a woven geotextile may be monofilament, multifilament, a combination of each type, or slit film yarns. Two kinds of slit film yarn can be used, either flat tape yarns or fibrillated yarns. See Fig.4
Fig. 4 Woven Geotextile Structures
Monofilament and multifilament woven fabrics are generally made from PET, the use of monofilament gives the better permeability, whereas multifilament is used for higher strength reinforcement. Slit-film, flat tape fabrics are usually PP materials which are quite strong, but they form a fabric that has relatively poor permeability. Alternatively, fabrics made with fibrillated tape yarns have better permeability and more uniform interstice openings than flat tape products.
Woven constructions produce geotextiles with high strengths and moduli in the warp and weft directions with low elongations at rupture. The woven construction and the Filament? yarns used can be varied so that the finished geotextile has equal or different strengths in the warp and fill directions
Woven synthetic geotextiles usually have higher strengths and lower breaking extension than nonwovens geotextile of the same aerial weight and polymer type, as illustrated in stress-strain graphs of Fig.4. As shown, a woven fabric weighing 100gsm would have the same strength as a nonwoven weighing almost 300 gsm.
Fig. 5 Comparison of PP Woven and Nonwoven Geotextile Tensile Properties
Warp Knitted Geotextiles
Knitted geotextiles are a special type of warp knitted structure made with synthetic Filament? yarns and referred to as a directionally orientated structure (DOS) . Fig 5 illustrates the multiaxial warp knitted DOS structure. The term is somewhat self explanatory in that the load bearing Filament? yarns are kept straight and parallel to each other, and aligned with the fabric’s load bearing directions. These yarns introduced are placed in a fabric structure in four directions, warp, weft and diagonally, to give multiaxial strength. One set of the knitting machine operations lays down sheets of the multidirectional reinforcing yarns and then these are passed into the knitting zone, where they are held together by the knit loops, termed stitches, of a third Filament? yarn (knitting or stitching yarn) at their cross-over points to produce a coherent structure. DOS fabrics therefore have the advantage that the fabric modulus is effectively equal to the load bearing yarns, since yarn Twist? usually intended to be cut or stretch-broken for use in staple fibre or top form.">Tow?.">Crimp? is absent, and these reinforcing yarns enable the fabric to withstand loads from various directions.
Fig.6 Warp Knitted Multiaxial DOS Structure
DOS fabrics are mainly made for special reinforcement. The load bearing yarns used are high-strength polyester of 37 to 400dtex, and the DOS fabric strengths range from 35 – 110 kN/m in the warp and from 30 – 80kN/m in the weft and diagonal directions. The warp inlay yarns absorb the axial tensile forces, say for example down a slope, while the weft and diagonal inlay yarns give frictional resistance and transfer forces across the fabric to the cross-over points so that loads are uniformly induced into the warp yarns.
Geogrids are primarily made for reinforcing soil or aggregate. They can be manufactured by extruding polyolefin sheets (HDPE or PP) that are subsequently hole-punched, then heated, unidirectionally or bidirectionally stretched and cooled to give a grid structure, with large openings or apertures that enable the interlocking of the structure with the soil or aggregate to provide the reinforcing function. See Fig5. These grids have tensile strengths up to 86kN in the reinforcing direction, but they are usually stiff; the ribs of the grid being susceptible to facture during installation. Geogrids are also made by weaving (leno weave) or warp knitting (biaxial DOS fabrics) and are flexible and less susceptible to installation damage. These geotextile grid structures, See Fig 5, are comprised of either PP or high tenacity PET Filament? yarns and can be made to have directional strengths of 35kN/m to 110kN/m. After the structure is formed, the fabric is given a protective coating, which binds the filaments together in the structure. Application methods include spread coating with a knife or roll, dipping, and Spraying?.
Fig.7 Uniaxial and Biaxial Geogrids
Typical coatings may be bitumen or PVC, acrylic based or an ethylene/vinyl acetate (EVAc) copolymer coat, all containing carbon black for UV protection. The coating used must also protect against chemical and biological degradation of the underlying filaments, and in the case of PET hydrolytic degradation. Therefore, good adhesion to the Filament? yarns is essential to prevent wicking into internal voids. PET geogrids usually have PVC coatings that are typically in the form of plastisols, i.e. the resin dispersed in a plasticizer (dioctyl phthalate plasticizer). The purpose of plasticizers is to modified the resin, reducing its glass transition temperature, Tg , and making the polymer a more flexible and ductile coating. The formulations would also contain heat and light stabilizers, pigments, and biocides. The coating thickness is usually greater than 150 µm, as thinner coatings do not give adequate long term protection.
Geonets are stiff criss-cross, open grid-like sheet materials formed by two sets of coarse, parallel, extruded plastic strands intersecting at an acute angle. See Fig7. The network forms in-plane flow channels, making these sheet materials suitable for coupling with sheets of nonwoven geotextiles to produce drainage geocomposites. Nearly all geonets are made of polyethylene. The molten polymer is initially extruded through slits in counter-rotating dies to produce the intersecting plastic strands in the form of a tubular mesh where one layer of strands is overlaid at the acute angle by a second layer. The tubular mesh is then slit along its length to create the“bi-planar” grid-like sheet. A third layer can also be extruded to give a “tri-planar” sheet, having increased thickness and, thus, increased flow capacity. A further development of a plastic sheet material with high in-plane flow capacity is the cuspated drainage sheet, also shown in Fig7. This is essentially a PP sheet, hot pressed to give parallel rows of truncated cones protruding from the plane of the sheet. The rows define the flow channels which are usually more widely spaced than the strands of the geogrid, enabling a greater in-plane flow capacity. If made from a plastic sheet of adequate thickness, the rows of cone-like protrusions can give a cuspated drainage material greater compressive strength than a geogrid.
Fig.8 Geonet and Cuspated Sheet
Fig.9 Geocells used for soil containment
Geocells are relatively thick, three-dimensional networks constructed from strips of plastic sheets. See Fig 8. The strips are joined together to form interconnected cells that are infilled with soil and sometimes concrete. In some cases 0.5 m to 1 m wide strips of polyolefin geogrids have been linked together with vertical polymeric rods used to form deep geocell layers called geomattresses.
Geopipes are perforated l polymeric pipes used for drainage of liquids or gas. They are generally used in combination with a nonwoven or woven geotextile.
Geomembranes are continuous flexible sheets manufactured from synthetic materials such as high density or low density polyethylene, which must incorporate a thermal and UV stabiliser usually carbon black. They are relatively impermeable and are used as liners for liquid or gas containment barriers.
The structures yet to describe are those of geocomposites. However, before doing so consideration must be given to the various functions of the geotextile structures thus far described. This is because geocomposites are combination of geotextile and geotextile-related structures and their structures are directly related to their multifunctionality.