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.
Geotextiles are used to provide one or more of the following functions in geotechnical applications:
· moisture barrier
The Filtration Function
The use of geotextiles in filter applications is probably the oldest, the most widely known, and the most used function. A geotextile can be structured to be a filter so that liquids can pass through its thickness (i.e across the plane of the material) whilst preventing the passage of soil particles from the upstream side, as illustrated in Fig.9. This means that the geosynthetic filter must be made to meet two conflicting requirements. It must have a suitable level of permeability to enable the required liquid flow, as well as an average pore size and pore-size distribution sufficiently small to prevent all but the finest of particles migrating through its thickness. Generally, textile structures are the only material form that can be readily manufactured to achieve these conflicting requirements.
The performance of geotextiles as filter media depends largely on two properties, the materials cross-plane permeability and pore size characteristics
The commercial testing of geotextiles for filtration involves the use of one of two standard methods, namely BSENISO 11058:”Determination of water permeability characteristics normal to the plane without load” or ASTM D4491:”Water permeability of geotextiles by permittivity”. The former is the international standard, the latter the USA standard.
Fig.9 Geotextile Filtration Function
Pore Size Characteristics
Several terms may be used to describe the pore size of geotextile materials, namely: apparent opening size (AOS), equivalent opening size (EOS), and filtration opening size (FOS); the most commonly used is the AOS. This term is linked with the symbol Ox which refers to the apparent pore size diameter which is greater than x% of the pore diameters contained in the material. Thus, O95 would be the ‘near-largest’ pore diameter in the material. AOS or O95 must be considered in relation to the soil particle distribution as this will indicate susceptibility of the geotextile to clogging.
Various methods have been reported for determining the pore size characteristics of geotextile materials, but the two standard methods are: ASTM D4571 “Determining apparent pore size of a geotextile”, and BSENISO 12956: “Determination of the characteristic opening size.” Both methods use the progressive sieving of glass beads of known diameters through the pores of the geotextile until a value is reached where 5% or less (by mass) of the beads pass through the material, thereby giving O95. The ASTM method, also termed the dry sieving method, is performed under strictly controlled humidity with the beads and geotextile in their dry state, but this is susceptible to static electricity causing particles to be held by the material. The BSENISO method attempts to circumvent the problem by employing a water spray and the test is therefore referred to as the wet sieving method.
The Drainage Function
Structures of geotextiles and geonets ) which will be described later.
Fig.10 Geotextile Drainage Function
Similar to the filtration function the requirements for drainage are soil retention, adequate flow capacity and long-term soil-to-geotextile flow equilibrium, in respect of minimum clogging of the pore spaces. A part from the flow capacity within the plane of the geosynthetic material, the other aspects have been dealt with above. We will therefore now consider the in plane flow characteristics.
One of two standard test methods may be used to measure the in-plane permeability of materials; BSEN ISO 12958: “Determination of in-plane water flow capacity”, and ASTM D4716: “Determining the in-plane flow rate per unit width and the hydraulic transmissivity of a geosynthetic using a constant head”.
The Separation Function
Fig.11 illustrates that for the separation function, the geotextile is placed between a fine soil or subsoil and a coarser aggregate material to prevent the two from mixing, even when subjected to the action of repeated applied normal loads. If the stone aggregate is placed on the subsoils without a geotextile separator then, when loads are applied to the aggregate, the stone at the interface will be pushed into the subsoil while concurrently the displaced volume of soil moves up into voids of the aggregate stone. With repeated loading, overtime the two materials become effectively a homogenous mix.
The soil retention function of the geotextile separator can be seen as similar to the geotextile filter, in that for a suitable separator the AOS of the geotextile must be related to the particle size distribution.
When the aggregate layer is subjected to loads significant local stresses are created in the separator. The separator must therefore be of sufficient strength to withstand these stresses. The bust strength, localized tensile strength properties and puncture resistance are of specific importance to the separation function.
Fig. 11 Separation Function of Geotextiles
The Reinforcement Function
A compacted soils generally have good compressive strength but very poor tensile resistance, therefore to give resistance to a compacted soil, i.e. preventing it from breaking up under tensile stresses, a geotextile can be used to reinforce the soil. See Fig 12. Evidently, there are three important mechanical properties of a geotextile used for reinforcement: tensile modulus, tensile strength and surface friction.
What is termed the “wide-width” tensile properties are generally measured according to the standards: ASTM D4595 Tensile Properties of Geotextiles by the Wide-Width Strip Method’ or ISO10319 ‘Geosynthetics – Wide-Width Tensile Test’. This is essentially the load-elongation test carried out on tensile testing machines. However, whereas in general textile testing, a 50mm width is used (e.g. BSENISO 13934-1 Textiles – Tensile Properties of Fabrics or ISO 9073 Nonwovens – Determination of Tensile Strength and Elongation), for wide width tests, specimens are cut to 200mm width. This gives a more representative measure of the geotextile structure and circumvents the narrowing of the sample width underload (Poison’s ratio effect) having a major influence on measured values. Similar to general textiles, the Tensile stress? is calculated as load per unit width, the unit width in this case is the metre and the unit of stress kNm-1. The usual plot of stress v’s strain enables the modulus and work of rupture to be estimated.
Surface Friction (Reinforcement by Shear Strength)
It can be reasoned from Fig.12 that if the interfacial contact does not fully transfer the shear force to the geotextile as a tensile load, then slippage of the top layer of aggregate can occur. This is particularly important if the reinforcing arrangement is on an incline, e.g. a slope. A measure of the interfacial behaviour is therefore useful and this is usually determined by the ASTM D5321 direct shear test
Pullout Strength (Reinforcement by Anchorage)
In certain applications of the reinforcing function, the geotextile has to provide anchorage. This obtained by sandwiching the geotextile between two soils layers. In this case, the important property is the resistance of the geotextile to pullout from between the soil layers. A test can be devised to obtain the pull-out strength in relation to applied load on the top layer of soil.
Creep is the term applied to the time dependent, continued strain or extension of a material when subjected to a constant load over a prolonged period of time. For the reinforcing function it is an important requirement that the geotextile has low or preferably negligible creep over tens to hundreds of years of constant loads. ASTM D5262 and ISO13431 describe standard test procedures for creep tests. Essentially, a stress is applied to the geotextile, equal to a specified percentage (commonly 20%, 40% or 60% ) of the material’s tensile strength. Since results are required as quickly as possible, the time period is important for the results to be meaningful and usually a period of 1000hrs to 10,000hrs is specified. Measurements are taken of the materials elongation under the set percentage stress at increasing time intervals, typically 1,2,5,10 and 30mins, initially, then 1,2,5,10,30,100, 200, 500, 750 and 1000hrs; for longer required periods, every 500hrs after the 1000hrs. Environmental conditions such as temperature rise and/or increased moisture may influence the creep of geotextile materials susceptible and these factors should be accounted for in testing.
Moisture Barrier (liquid Containment)
The specific objective, here, is to prevent the movement of liquids, through a geosynthetic and into neighbouring soil. For this function the most commonly used geosynthetic is an impermeable plastic membrane, i.e. a geomembrane, usually high density polyethylene .However, PVC coated woven textiles and bitumen impregnated nonwoven geotextiles are may also be used for some applications. A geocomposite referred to GCL is also much in evidence and will be described later.
The Protective Function
Fig.13 illustrates the compressibility of a geotextile being utilized for the protective function and as such, largely thick nonwoven geotextiles are used. The main objective is to prevent penetration by stones in the soil base layer causing failure of the geomembrane fluid barrier. Failure can be by an immediate puncture of the membrane or by longer term localized stress cracking of the membrane. As depicted in Fig.13(B), at a localized projection, the weight of water contained within membrane lined pond can induce localized tensile stresses, Tm , in the membrane, stretching the material as its deforms for the liquid to occupy the void. Over time this can lead to a type of creep rupture or stress cracking as it is termed. Placing a nonwoven geotextile between the membrane and the compacted soil would reduce the induced localized stress to prevent creep rupture.
Fig.13 Illustrating Protective Function of Geotextiles
Geocomposite are basically combinations of two or more different types of geosynthetic. As most of the individual components are thermoplastic they can be thermally laminated, but adhesive bonding and needle punching are also used. Examples include: geotextile-geonet; geotextile-geogrid; geonet-geomembrane; or what is termed a geosynthetic clay liner (GCL). There is almost no limit to the variety of geocomposites that are possible and the development of these materials results from the anticipated usefulness of their multifunctionalities and the opportunity for more rapid installation than by using the individual components. The three main geocomposite material types are: drainage geocomposites, reinforcement geocomposites and fluid barrier geocomposites.
Control of water is critical to the stability of most geotechnical constructions and drainage geocomposites have become important materials for such a requirement. Common configurations of drainage geocomposites are of a geonet sandwich between two nonwoven geotextile filters (termed a blanket drain – see Fig 14), or a sandwiched thick or thin preformed core (panel drain, edge drain or wick drain). Blanket drains are commonly used as liquid collection - removal layers.
Fig.14 Drainage Geocomposites
Reinforcement geocomposites are structures in which a spun-bonded or melt blown nonwoven web is incorporated into a knitted geogrid by the stich-knit action of holding yarns or bonded by needle punching to one or both sides of a woven or knitted geogrid. The nonwoven adds separation and filtration functions to the geogrid reinforcement to give the multifunctionality of the geocomposite.
Both woven and nonwoven geotextiles can serve as moisture barriers when impregnated with bituminous, rubber-bitumen, or polymeric mixtures. Such impregnation reduces both the cross-plane and in-plane flow capacity of the geotextiles to a minimum. However, for liquid containment applications what is referred to as a geosynthetic clay liner will be more effective.
Geosynthetic clay liners (GCLs) are geocomposites that are typically prefabricated with a sodium bentonite clay layer sandwiched between two geotextile layers; two needle-punched nonwovens layers or one needle-punched nonwoven and one woven layer. The process commonly involves attaching the top and bottom layers by stitching or needle-punching through the bentonite core which also gives the structure its internal shear resistance. When hydrated the bentonite core swells and becomes an effective barrier to liquid or gas.
Index & Performance Testing
The tests referred to in the above descriptions of the functionalities are generally used in two different ways.
They can be employed to generate data that are utilized for product comparisons, specifications, quality control purposes, and as indicators of how a product might survive during the installation process and of its longterm effectiveness. For such uses the tests are called ‘index tests’ and they include mass per unit area, uniaxial mechanical strength (grab tensile; load-strain; creep, tear, and seam strength); multiaxial rupture strength (puncture, burst, and cutting resistance; flexibility); and hydraulic tests (apparent opening size, percent open area; pore size distribution; porosity; cross-plane permeability and in-plane permeability). For indication of longterm endurance or durability the materials are tested after being subjected to specific mechanical and/or simulated environmental conditions in order to determine their abrasion resistance; UV stability; chemical and biological resistance; wet-dry and temperature stability.
A common practice is to subject a geotextile product to a required test while it is in contact with the particular soil or granular material in which it is to be installed. Such tests are termed ‘performance tests’ and are used to obtain a direct assessment of the geotextile’s properties under simulated geotechnical conditions of a site specific installation. Performance tests are not normally used in specifications; rather, a geotextile should be preselected for performance testing based on index values, or performance test results should be correlated to index values for use in specifications. Examples of performance tests include in-soil stress-strain, creep, friction/adhesion, and dynamic tests: puncture, cylinder, chemical resistance, and filtration/clogging resistance tests.
Within the European Union (EU), the Construction Products Directive - 89/106/EEC has resulted in minimum standard test requirements of any geotextile that is to be utilised for the possible functions described above. Table 1 lists the required properties to be measured and standard test methods to be used. These are referred to as the CE marking requirements. All manufacturers of building and construction products in the EU, are required to comply with the CE marking requirements and provide the appropriate test certificates. This became mandatory for geotextiles in October 2002. The CE mark is therefore seen as a guarantee that, for application of a particular functionality, the properties of a product will match those claimed by its manufacturer.
Table-1 CE Marking Requirements for Geotextiles & Related Products
H: required by the Mandate (CE Marking )
A: relevant to all conditions of use
S: relevant to specific conditions of use
Required characteristics for Filtration
EN ISO 10319
Elongation at maximum load
EN ISO 10319
Tensile strength of seams and joints
EN ISO 10321
Static puncture (CBR test)
EN ISO 12236
Dynamic perforation resistance (cone drop test)
prEN ISO 12957-1 and -2
Damage during installation
ENV ISO 10722-1
Characteristic opening size
EN ISO 12956
Water permeability normal to the plane
EN ISO 11058
Resistance to weathering
Resistance to chemical ageing
Resistance to microbiological degradation
The applications of geotextiles cover almost all areas of geotechnical engineering. See Fig. 15. Examples fall within transportation, such as road, airport runways, railway, embankment and bridge constructions; liquid and gas containment structures; and erosion control, including inland, river bankings and coastal defences. Generally in these applications the geotextiles used to fulfil several functions.
Fig.15 Geotextile Applications by Industrial Sector
With road construction the following functions are incorporated into the engineered ground structure:
- Separation and filtration
- Moisture Barrier
Separation, Filtration and Drainage
The base of a road is likely to consist of a compacted subgrade soil on which a thick layer of coarse aggregate is placed for primarily bearing traffic loads. Based on the earlier description of the separation and filtration functions, a geotextile would be needed to prevent repetitive, cyclic traffic loads push the coarse stone aggregate into the subsoil; this termed pumping failure. See Fig 16(A)
Fig16(A) Separation Filtration and Drainage Functions of Geotextiles in Road Construction
Without the geotextile present, the cyclic loads will cause the bottom stones of the aggregate to spread, while simultaneously moving the soil fines up into the void spaces of the aggregate, i.e. the aggregate layer becomes contaminated with fines from the subgrade.
[Insert Video No.1]
Rising ground water table and/or rain, in the case of untarmac (unpaved) roads, accelerates the fines migration leading to signs of rutting on the road surface and ultimately full deterioration of the surface. An aggregate base layer which is saturated 10% of its time, will have its service life reduced by as much as 50%. Thus, eliminating saturation of the base course is an important requirement.
The geotextile placed at the interface of the subgrade and aggregate has to provide, in addition to separation and filtration, the drainage function in order to retain the strength characteristics of the aggregate material. This is achieved by making part of the bottom layer of aggregate a drainage blanket, encapsulated within the wrap of the geotextile. The in-plane permeability of the geotextile-aggregate system must be effective in removing rising ground water, and where necessary surface water ingress, to a subsurface geopipe drainage run at the road side; with tarmac/pavement roads, this is referred to as ‘a pavement edge drain’ illustrated in Fig.35(B).
[Insert Video No.2]
The type of geotextile which provides the best combination of the three functions is a nonwoven fabric with suitable through-plane and in-plane permeability, as well as adequate puncture resistance, and bust, grab and tear strengths to resist the aggregate penetration under cyclic loads. Where ground water is substantial, geocomposite blanket drains have been used to enhance road base drainage.
The reinforcement function is required for aggregate restraint, particularly where high loads will occur, such as motorways (highways) and especially runways at airports. In such cases geotextiles made from para-aramid filaments may be used for aggregate reinforcement.
Under loading, the total road structure behaves like a beam in bending. The upper region is under compressive stress while the bottom region experiences tension. Since the aggregate layer has no useful tensile resistance, the bottom region of an unreinforced base course will spread allowing the upper region to progressively collapse downwards. Although the nonwoven geotextile used for the separation, filtration and drainage functions will impart some degree of reinforcement to resist the spread, it is more effective to use a geogrid, a woven or a warp knitted geotextile within the aggregate layer when high loads are anticipated. The fabric would be placed at a depth where it is likely to coincide with what would be the failure plane in the absence of the geotextile. See Fig17. The encapsulation effect of the aggregate by the nonwoven geotextile gives a supporting role by enhancing the CBR of the drainage blanket.
The use of a base course reinforcement is also seen as a requirement for road construction on soft cohesive soils, weak collapsible soil and in areas having underground cavities, whether of natural (karstic phenomena) or anthropic origin (e.g. closed mines).
On weak substrates the geotextile is likely to extend the service life of a road by a factor of 2.5 to 3.0 times compared to a non-stabilized construction. Roads built in closed mining areas can be susceptible to sink holes suddenly occurring owing to the collapse of old mine shafts resulting from suberosion processes. Road constructions in mining areas are, therefore, reinforced with uni-axial or bi-axial geogrids.
Besides the above technical advantages, there is a significant economic advantage for using geotextile reinforcments. Local soils can be used for the construction, dispensing with the need to transport stronger mineral materials to replace weaker local soils, thereby eliminating the added material cost and the cost of transportation. In addition, because the geotextile reinforcement greatly improves the load bearing resistance of the road, a reduced thickness of aggregate layer may be used compared to that for an unreinforced structure, thus further reducing cost.
Fig.17 Geotextile Reinforcement and Moisture Barrier
The vast majority of roads and airfields in the built environment are constructed with an asphalt / Tarmacadam? surface or pavement and the modern method is to incorporate a geotextile fabric interlayer, called a pavment geotextiles. See Fig 17.
Traditionally laid surfaces have a shorter service life than those with a pavement fabric. Besides flexural fatigue by the dynamic loading from traffic, natural ageing through changes in weather conditions of temperature and moisture will cause deterioration of road surfaces. These factors cause differential expansion and contraction between the various material layers of the constructed road, resulting in small horizontal or vertical movements generating cracks which eventually propagate to the road surface. Water entering surface cracks can penetrate deep into the pavement layer and severe temperature changes can cause water within the cracks to freeze and damage the bitmac layer leaving pot holes and ruts. Repairing by just placing a new surface overlay often results in the cracks from the existing pavement to propagate up through the new overlay surface (termed: reflective cracking). The inclusion of a paving geotextile interlayer in both new road constructions and rehabilitation work, significantly retards the potential for such deterioration of the road surface, and notably increases the pavement service life.
Commonly, a PET nonwoven geotextile of 135 gm m-2 to 200 gm m-2 is used. When laid it becomes impregnated with bitumen to form a moisture-barrier flexible membrane, relatively impermeable to cross-plane and in-plane liquid flow, minimizing ingress of water into the pavement structure. Although the primary function of the paving geotextile is to prevent penetration of surface water, it may also improve the bitamc layer performance by acting as a stress- relieving /absorbing membrane interlayer. PET is preferable to PP because of its higher thermal resistance, since bitmac is normally heated to temperatures of 150°-180°C for onsite 'workability'. However 'doping' the bitumen with a light oil as a solvent (often kerosene, or creosote), known as "Cutback", can render the bitmac workable at lower temperatures.
RAIL TRACK CONSTRUCTION
Geotextiles were introduced into railway construction in order to improve track support, for both the laying of new lines and railtrack rehabilitation
[Insert Video No.3]
Subgrade soils make up the main foundation to a railtrack on which layers of granular materials are placed, and subsequently the sleepers and rail lines. The role of the aggregate layers is essentially that of a load supporting intermediary between the rail lines and the subgrade; the aggregate layers are subjected to a repeated cyclical stress as the wheels on each axel of a rolling stock transverses the line above a sleeper. In heavily trafficked areas crushed stone or gravel is normally placed on top of low cost uncrushed aggregate.
Though economical, the use of an uncrushed stone layer can cause a decrease in the stability and the holding capacity of crushed granular cover. To circumvent this, a geogrid reinforcement is incorporated within the construction. However, further problems can still arise with the longer-term stability of railway track.
Similar to roadway construction these problems are associated with ‘erosion pumping failure’. Over time, the track stability becomes greatly weakened by the ingress of water and the movement of ‘fines’ from the sub-grade into the ballast layers. Water ingress along with train effluent (waste water and diesel /oil / grease) occurs from the environment above the track; water ingress can also result from a rising water table and run-off from the side embankments of, say, a railway cutting.
The movement of the fines is much amplified by the presence of water. In addition to the fines from the sub-grade, stone damage due to the physical pounding of the ballast by the concrete sleeper and the frictional wear of the stones can cause wet contaminants in the aggregate. The movement of the fines and muddying of the initially separate layers in the rail bed leads to uneven settlement of the line. The consequences would be the speed reduction of the line (i.e. the speed at which it is safe for rail traffic to proceed along it) and eventually the removal of the line from service whilst the track undergoes repair. Drainage is therefore an important factor in maintaining track stability and various geotextile materials can be used for this purpose.
For example, the ingress of a rising water table can be restricted by a cross-slope drainage geocomposite with a GCL layer or geomembrane placed above to act as a capillary break in the water flow. A second drainage geocomposite above impermeable layer would then remove surface water penetrating the aggregate. The geotextiles used is such drainage composites would also provide the functions of separation and filtration, similar to the situation for road construction.
Modern tunnel design and construction involves engineered lining systems that provide effective drainage and a long-term moisture barrier for the tunnel walls. See Fig 18 (A). After excavation of the tunnel, the rock is sprayed with concrete (shotcrete) to smooth out the unevenness of the rock face. See Fig 18(B) Either a geosomposite [Fig.18(C)] or a needle punched nonwoven geotextile, is then placed on the sprayed concrete and anchored using steel bolts coupled with polymer fastening discs. An HDPE membraneliner (AGRUFLEX) is then welded to the discs. A second geocomposite or nonwoven geotextile lining is then positioned and a final concrete layer (ring concrete) is subsequently put in place.
The geotextile performs two primary functions:
- protection of the liner from abrasion and puncture by protrusions from the base surface of the tunnel wall.
- drainage of water seeping through the rock and sprayed concrete layer. The in-plane permability of the geotextile enables the water to flow to a subsurface drainage collection system, where it is directed away from the tunnel.
REINFORCED EMBANKMENTS/SLOPES/RETAINING WALLS/ABUTMENTS
The three primary applications of soil reinforcement using geotextiles are:
- reinforcing the base of embankments constructed on very soft foundations,
- increasing the stability and steepness of slopes, and
- reducing the earth pressures behind retaining walls and abutments.
In the first two applications, geotextiles permit construction that otherwise would be cost prohibitive or technically not feasible. With the case for retaining walls, significant cost savings are possible in comparison with conventional retaining wall construction.
Fig 19 shows the three ways in which embankments constructed on soft foundations can fail. The application of high strength woven or knitted geotextiles, or geogrids can enhance the bearing capacity. The geotextile can be made sufficiently strong to prevent rotational failures, and lateral spreading failures can be prevented by the development of adequate shearing resistance between the base of the embankment and the reinforcement. For some very soft grounds, the use of geogrids may require a lightweight geotextile separator to provide filtration and prevent contamination of the material used for the embankment, i.e. the embankment fill. Also a drainage layer may be needed to ensure ingress of water doe not adversely affect the direct shear properties.
Fig.18 Tunnel Lining System
Fig.19 Embankment Failure Modes and Reinforcement
Slope reinforcement is also employed in the construction of embankments, and as an alternative to retaining walls. To provide slope stability, multiple layers of geogrids, or woven / knitted geotextiles may be placed during construction within the earth used to make the slope.
Reinforcement in a slope contributes to stability in two ways. Firstly, the reinforcement directly improves the shear resistance of the soil to resist the shear loading caused by the steep face. Secondly, the reinforced zone holds the soil mass in equilibrium without overstressing the underlying foundation soils.
A major cost advantage of geotextile reinforcements is that a wider range of slope angles can be used with any type of soil. Similar to other situations of reinforcement, filtration, sepration and drainage functions may be needed and a suitable nonwoven geotextile with in-plane drainage capabilities would be incorporated.
Reinforced Retaining Walls and Abutments
Retaining walls are generally required where a soil slope is uneconomical or not technically feasible. When compared with conventional retaining structures, walls with geotextile reinforced backfills offer significant advantages. They are very cost effective, especially for high walls. They are also more flexible than conventional earth retaining walls, such as reinforced concrete cantilever or gravity walls. They are particularly suitable for sites with poor foundations, and for seismically active areas. A further significant benefit of geotextile reinforced retaining walls is that they permit a wider variety of wall facings to be used than traditional systems, resulting in greater aesthetic and economic options.
A geotextile reinforcing supporting (GRS) bridge abutment is essentially a GRS mass with a wall facing (See Fig.20), and it is therefore similar to a geotextile reinforced retaining wall. The facings used are precast concrete blocks , a cast-in-place structure, or an assembly of natural rocks or gabions.
Fig.20 Geotextile Reinforcing Supporting Bridge Abutment
Fluid containment in the present context means fresh water, liquid waste including that leached liquids from solid waste, and biodegradation gases such as methane.
Water Containment Systems
Geosynthetic lining systems for water reservoirs, dams, cannals, ponds and irrigation channels can conserve millions of gallons of water by preventing seepage losses. These lining systems can eliminate 70% of the water losses that occur between the storage and usage points. Additionally, for small scale storage units like ponds, lining covers can prevent contamination, control evaporation and prevent chlorine loss. Generally, therefore, facilities capable of storing 25000m³of water and above have some form of man-made lining system as an alternative water barrier to conventional clay liner systems.
When geomembrane liners are employed, it is necessary to ustilise the protective function of a needele-punched nonwoven geotextile, above and below the plastic sheet, with a layer of gravel or other erosion resistant mineral material on the upper geotextile. A GCL may be used as a secondary barrier liner beneath the geomembrane in addition to the nonwoven geotextile protector. For non-critical water storrage systems, such as architectural ponds, and recreational ponds, GCLs are used as alternatives to geomembranes.
Waste Containment System
One of the most important applications of lining systems is in landfill.
[See Video No.5]
A lined landfill is a carefully engineered excavation in the ground into which waste is contained. At the bottom of the excavation would be a geomembrane liner, called a basal liner, to minimise ground and surface water contamination by the waste. There are several categories of waste: municipal solid waste (MSW) comprised of consumer and household items; hazardous waste (e.g. toxic chemicals and flammable, radioactive, or biological substances);, industrial waste (i.e. waste chemicals, paints, building demolition residues,); agricultural wastes, such as animal farm manure and crop residue; and then mining wastes, such as slag heaps and coal refuse piles. At mine sites, basal liners are used for liquid containment (drainage waters, process solutions, treatment ponds), as well as tailings impoundments, heap leach facilities and other solid waste facilities. The principal difference between the landfill constructions for these categories of waste is that MSW landfills generally have one basal liner whereas the others a multiple of basal liners. The upper (primary) basal liner (or liners) performs the principal barrier function against contamination of the neighbouring ground environment and surface water, while the lower liner acts as both a secondary barrier and leak-detection system. If local conditions require it (e.g. high water table) a double liner system may be used in MSW landfills.
The contamination risk is usually caused by leachate which is the liquid that is drained from the landfill basal layer. Leachate forms during the operation of the landfill site mainly by rain water percolating through the decaying solid waste. With the body of the waste of an active landfill average temperatures can be between 40 ºC and 50 ºC, reaching up to 70 ºC in the basal area. Therefore as the water moves through the waste, the thermal energy can be sufficient for it to slowly dissolve metals, organic compounds and other contaminants, collecting to form toxic liquid
Fig.21 shows a cross-sectional diagram of a typical MSW landfill construction. The structure is made up of an HDPE membrane placed on a compacted soil layer (compacted clay); optionally a nonwoven geotextile protector, a drainage geocomposite (for leakage detection) or a GCL may be present beneath the plastic membrane. This type of basal layer is referred to as a single composite (compacted clay plus geomembrane) liner.
A needle-punched nonwoven geotextile protector (typically 1200 -2000gsm) is placed on top of the membrane and on top of this is laid a 300 mm thick stone drainage layer (usually a deposit of 20 mm semi-angular lime stones), through which the leachate can be tapped off from the waste. The nonwoven protects the membrane from installation damage and from stress cracking due to long term compression by stone projections at the drainage interface.
A second, but thinner, nonwoven geotextile is placed above the drainage layer as a separator/filter to prevent waste particles, particularly fine waste, caught in the downward flow of the leachate, entering the granular drainage layer. Typical geotextiles used above the drainage system are spun-bonded nonwoven PET, 271 to 542 gsm.
On the slopes of the landfill a geonet may be used for reinforcement of the basal liner against the angled load of the waste. Analyses of past slope failures have shown that liner induced failures occurred at the geomembrane interface with under- or overlying materials. The geonet is therefore placed above the nonwoven geotextile to avoid slippage of the geonet on the membrane.
Fig. 21 also illustrates the capping structure of the landfill. The primary aim of landfill capping is to control and minimise leachate generation by restricting water ingress into the landfill. A low permeability domed cap is therefore constructed over whole site to achieve the highest possible surface water run-off. The cap is also constructed to facilitate landfill gas control and collection. Landfill gas can be equally polluting as leachate and may also be dangerous, if not controlled, causing risk of fires and explosions. Geopipes are usually installed to collect and remove the generated gases from the waste.
The capping of a MSW landfill is usually done by incorporating a nonwoven geotextile separator/filter above the waste, and then geopipes enveloped by gravel with a second nonwoven geotextile filter above the gravel. This combination is covered with a layer of compacted clay, on which is placed a geomembrane, followed by a needle-punched nonwoven, a granular drainage system or a substitute drainage geocomposite, and then a geonet and finally soil for vegetation. The cap must be built to withstand agricultural machinery, drying and cracking, plant root penetration, burrowing by animals and erosion.
Erosion can result in major damage to coastal areas, river banks and ground slopes, particular where the vegetation hold is poor or there is an absence of vegetation. The basic principle of erosion control is to prevent or limit soil movement by the erosive forces such as moving water and driving wind. The control techniques typically involve the use of armored protection with geotextile support for tidal situations (i.e. coastal and rivers) that occur either naturally or through water movement induced by marine transport. Geotextiles are also employed for the containment of silt and the retention of soil cover and revegetation on steep slopes. With the former only synthetic materials are effective as long term solutions, whereas with the latter both natural and synthetic materials are used for erosion control.
[See Video N.6]
River and Costal Areas
Protection from tidal and wave erosion commonly involves the use of revetments constructed from rock armour or pre-cast concrete units on the bank or shoreline (See Fig.22), the size of which is determined by the anticipated wave action. A filter layer is required beneath these constructions in order to prevent the progressive removal of the underlying soil by the infusion of water, which would result in the collapse of the revetment into the increasing void
Fig. 21Cross Section of Basal Liner and Capping in MSW Landfill Systems
and the loss of its effectiveness against the tidal action. Traditionally, granular mineral filter layers were used, one metre thick and graduated so that progressively larger grain sizes are nearer the rock or concrete material, as illustrated in Fig22. Installation of such filter layers is difficult and time consuming, particularly if this involved inter-tidal working. The filter layers must allow the free movement of water in both directions, i.e. repeatedly into the shore or banking and then out again, in accordance with high and low tide, at the same time preventing the leaching of the underlying soil. Without the ability to facilitate this action over the entire life of the revetment, there is the potential for the armour to be undermined as beach material is progressively eroded, which then leads to the failure of the revetment. Besides the cost and difficulty in construction, traditional filters do not provide good long term performance. The use of geotextile filters as replacements for traditional filters, see Fig. 21, enables better restriction of movement of the underlying soil and thereby its retainment. The repeated reversed action of the water flow reduces the chances of the filter blocking and so gives the geotextile a long-time performance.
Fabric formed revetments (FFR) can be used as alternative rock rip rap structures, as shown in Fig22. They are constructed by pumping concrete into a woven fabric envelope. FFRs can be engineered to perform as impermeable or permeable wave barriers.
Fig.22 Application of Geotextiles in Costal Erosion Protection
Although geotextile are used for river banking and coastal defences, there is also a need to use such materials to protect near-shore waters, streams, rivers, lakes and other aquatic resources from contamination by silt and sediment forming debris. Silt fences are a common solution to such problems and are placed along the perimeter of the areas of the polluting source, typically they are 1 m (3 ft) high and 30 m (100 ft) long. A PP slit film or flat-tape woven geotextile, which is permeable, is usually used by attaching it to supporting posts that are firmly inserted into the ground. The fabric is usually wider than the wood post length to allow burial at the toe or base of the silt fence. The erected fence functions by initially holding back sediments (silt and sand particles) carried by precipitation runoff, to allow a slow flow through of the water and create a shallow a pond behind the fence. The pond serves as a sedimentation basin to collect suspended soils from the runoff water. Geotextile filter fabrics are not suitable for silt fences because their pore sizes are too small. Their use would result in a backup of silt in the water flow, followed by water overtopping the silt fence or causing a 'blow-out' of the structure itself.
To deal with silt in water runoff near deeper water shorelines, turbidity curtains are used. See Fig. 23. These are reusable floating geosynthetic tubular panels that block movement of sediment. The top edge of each curtain contains floats and a cable or chain. Weights are attached to the lower edge of the curtain to keep it vertical in the water. Posts, piling, or anchors hold the curtains in place. Generally, they are intended for use against currents no greater than about 1.5 m/s and depths of no more than 5 to 8 m.
Fig.23 Geotextile Turbidity Curtain
(To print off any of Geotextiles fig 1 to 25, click here)
ong>Slopes and Poor Vegitation Ground Cover
Inland soil erosion follows a process of detachment and transportation of soil particles by wind and rain water under the influence of gravity. Wind erosion is largely a problem in open areas (lack of shelter); exposed slopes of non-cohesive (loose and dry) soils or exposed smooth bare surfaces with little, if any, protective vegetation. Wind erosion occurs when air turbulence imparts sufficient energy to initiate movement and impact of smaller size gains on heavier size soil particles. In geology this action is called saltation.
Saltation grains are 0.05 to 0.5mm dia. in size, but can move heavier size particles of up to 200 times their own weight by force of impact, giving what is referred to as surface creep – i.e. slow movement of larger soil surface particles, 0.05 to 1mm dia. Some of the salting particles may become suspended in the airflow to be eventually deposited miles away from their original ground.
Water erosion is generally more destructive than wind erosion. The movement of falling or running water begins the erosion process, through conversion of kinetic energy to impact energy, loosening soil particles in the surface layer of the ground, enabling them to travel with the rainwater flow. Fig.24 illustrates the process of rain erosion of a slope. For a certain length at the top of the slope, there is no erosion. Beyond this initial zone, water starts picking up soil particles along the slope and the erosion begins. The concentration of soil material in the water increases as the flow travels down the slope to the lowest area where most of the eroded mass will be deposited. This area may be a stream, a river or part of a shoreline.
If the flow is concentrated, then small rivulets initially form, and as the water becomes greater in volume and prolonged the rivulets will get larger, and with repeat of significant amounts of rainfall over a period of time, gullies will form and ultimately the eroded surface may be turned into a ditch, which widens and deepens in the process. If not arrested in the initial stages, the sequential chain of erosion can impair ground slope stability, leading to catastrophic failure.
Fig. 24 The Process of Rain Erosion of a Slope
The basic solution to soil erosion is to create a cover for the soil that keeps the wind and water from dislodging the soil particles. A range of geotextile products has been developed to meet the various erosion control demands. Geotextile materials are available for situations such as: re-vegetation of bare soils; the support of vegetation in erodible soil; temporary Biodegradable? support for new seedings; and, long-term permanent non-biodegrabale support. These products are classed as erosion-control nets (ECN), erosion-control meshes (ECM), erosion-control blankets (ECB) and turf reinforced mats (TRM).
Where biodegrability is a requirement, jute and coir woven or nonwoven fabrics are the most used. See Fig.25. These materials are most useful because as the vegetation growth becomes established, the degraded by-products from these fibres are beneficial to the plants. The ability of jute and coir fibres to absorb water aids in attenuating soil movement by water runoff and ameliorates any high variation of soil Moisture content?. This is in addition to the regulating effect of soil temperature variation that erosion control geotextiles in general have.
For longer term permanency (i.e. retaining 75% of its original strength after a 10-year life) PP, PET and PA are employed, the fibres being made with additives for UV resistance.
Fig.25 Examples of Geotextile Erosion Control Fabrics
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Geotextiles and Geomembranes, Volume 23, Issue 2, April 2005, Pages 117-143
F. Cartaud, N. Touze-Foltz, Y. Duval
Geotextile reinforcement of embankments on peat
Geotextiles and Geomembranes, Volume 2, Issue 4, 1985, Pages 277-298
R.K. Rowe, K.L. Soderman
Geotextile filtration principles, practices and problems
Geotextiles and Geomembranes, Volume 11, Issues 4–6, 1992, Pages 337-353
B.R. Christopher, G.R. Fischer
A case study of geotextile-reinforced embankment on soft ground
Geotextiles and Geomembranes, Volume 20, Issue 6, December 2002, Pages 343-365
Dennes T Bergado, Pham V Long, B.R Srinivasa Murthy
Material properties for the design of geotextile reinforced slopes Geotextiles and Geomembranes, Volume 2, Issue 2, 1985, Pages 83-109
Geotextiles and Geomembranes (Special Issue), Volume 29, Issue 4 (2011)
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