Geotextiles in civil engineering

A. Rawal, … S.C. Anand, in Handbook of Technical Textiles (Second Edition), 2016

4.4 Manufacturing of geotextiles

The majority of geotextiles are produced using classic or conventional fabric manufacturing techniques. The manufacturing processes for the production of geotextiles can be broadly classified in two categories, (1) classic and (2) special geotextiles.11 In classic geotextiles, the products of the textile industry, such as woven, knitted, nonwoven fabrics, etc., are considered, whereas special geotextiles, while having a similar appearance to classic geotextiles, are not the direct products of textile technology, i.e. webbing, mats, and nets. A typical classification of the production of geotextiles is depicted in Fig. 4.6.11

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Fig. 4.6. Production of geotextiles.11 Products are in lower case letters; processes are in capital letters. All special geotextiles are currently used. Notes: athe word ‘spinning’ has two meanings, extrusion through a spinneret to make a filament and fabrication of yarns from staple fibers; bstrips can be made using any appropriate process such as extrusion, calendering, weaving, yarn, fabric coating, etc.; csome classic geotextiles (those indicated) are used more than others; dcorrugated, waffled or alveolate structures are generally not used alone; they are used to make composite geotextiles.

The classic geotextiles are fabricated in two steps, i.e. the production of fibres, filaments, slit films (tapes), or yarns followed by converting these constituent materials into a fabric. The constituent materials required for the manufacture of geotextiles are produced using various techniques, as discussed below.11

Filaments. The filaments are produced by various extrusion techniques, i.e. wet, dry, and melt. Melt extrusion is used for polymers such as polyester and polypropylene, which are widely used for producing synthetic fibre-based geotextiles. Here, the molten polymers are extruded through spinnerets or dies and subsequently, they are drawn along the filament axis such that the molecular orientation along the filament is improved, resulting in higher tensile properties. When numerous filaments are extruded simultaneously through the spinneret, it is known as a multifilament yarn.

Short (staple) fibres. Filaments are chopped into short lengths ranging from 2 to 10 cm, which are known as staple fibres. These staple fibres are then twisted together to form a yarn.

Slit films. The films are produced through a melt extrusion process using slit dies which are subsequently slit with sharp blades. These films can be further fibrillated and broken into fibrous strands, known as a fibrillated yarn.

The above linear elements, namely filaments, fibres, slit films, or yarns, can be converted into several types of classic geotextiles, as briefly discussed below.

Woven geotextiles. A woven fabric consists of two sets of orthogonally interlaced filaments or staple-fibre yarns. The weave design or pattern is determined by the manner in which yarns or filaments are interlaced. Filaments or yarns placed in the longitudinal and transverse directions are known as warp and weft, respectively. Monofilament and slit woven geotextiles are anticipated to be thinner in comparison to multifilament, spun, and fibrillated woven geotextiles.

Nonwoven geotextiles. Nonwoven fabrics are defined as a sheet, web, or batt of directionally or randomly oriented fibres/filaments, bonded either by friction, and/or cohesion, and/or adhesion. In general, nonwoven fabric formation can be considered as a two-step process: web formation (aligning the fibres with certain orientation characteristics) and bonding these fibres by mechanical, thermal, or chemical means.16 This two-step process has formed the classification of nonwoven structures, i.e. carded, airlaid, spunlaid, meltblown, needlepunched, hydroentangled, adhesive bonded, thermal bonded, stitch bonded, etc. Some of the important processes that are used for the manufacture of nonwoven geotextiles are discussed below.

Spinlaying. This process includes various steps, i.e. filament extrusion, drawing, lay down, and bonding. The first two steps can be easily conceived from a typical melt extrusion process. The latter steps involve the deposition of filaments in a random manner on to the conveyor belt. It should be noted that the spunlaid nonwovens are generally self-bonded but additionally they can be bonded by means of thermal, chemical, or mechanical means in order to enhance their mechanical properties.

Chemical bonding. A binder such as glue, rubber, casein, latex, cellulose derivative, or a synthetic resin is used for bonding the filaments or short fibres together and these materials are known as chemically or adhesive bonded nonwoven geotextiles.

Mechanical bonding. This can be classified into two categories, needlepunching and hydroentanglement. Needlepunching is a physical method of mechanically interlocking fibrous webs by using barbed needles to reorientate some of the fibres from a horizontal to a vertical direction. With the hydroentanglement technique, high-pressure multiple rows of water jets are used in place of mechanical needles for reorienting and entangling a loose array of fibres into self-locking and coherent fabric structures. It should be noted that a significant proportion of needlepunched nonwovens are used for geotextile applications.

Thermal bonding. These fabrics are manufactured by applying the thermal or heat energy to the thermoplastic component present in the fibrous web and the polymer flows by means of surface tension and capillary action to form the required number of bonds at crossover regions of fibres.17 It is mainly classified into two categories: through-air bonding and calendaring. In through-air bonding, the fibrous web is passed in a heated air chamber (oven) for forming the bonds at the crossover positions of fibres. Calendaring involves the passage of the fibrous web through a heated pair of rollers that impart high pressure and temperature in order to melt the thermoplastic fibres.

Knitted geotextiles. These are produced by interlocking a series of loops of filaments or yarns to form a planar structure. The loops in the knitted structure are interlocked in different ways, similar to weave designs in woven fabrics.

Braided geotextiles. Braiding is generally used for producing narrow rope-like materials by interlacing diagonally three or more strands of filaments or yarns. The topology of strand interlacements in braided structures is similar to that of woven structures. Hence, the plain, 2/2 twill, and 3/3 twill are analogous to diamond, regular, and hercules braided structures, respectively. Furthermore, the braided structures can be categorised as biaxial and triaxial braids; both have two sets of braider strands, each strand aligned in the bias direction, but the latter also have an additional set of strands aligned parallel to the braid axis.18

The manufacturing of special geotextiles is also briefly discussed below.12

Webbings. These are produced from strips of moderate width and are similar to coarse woven slit film fabrics.

Mats. These are prepared from coarse and rigid filaments having tortuous shapes similar to those of open nonwoven fabrics.

Nets. These consist of two sets of extruded strands aligned in the bias direction and are bonded at the intersections, normally by partially melting one or both the strands. These net structures can also be produced by using a melt extrusion process consisting of rotating dies which have slots on their periphery through which the molten polymer is being extruded.19

Additionally, composite geotextiles can also be manufactured by combining several of the above listed products. For example, the combination of multiple layers of knitted/woven/nonwoven material can be produced by means of stitching, needlepunching, thermal bonding, etc. Similarly, mats/nets/plastic sheets can be sandwiched with different types of geotextiles for various geotechnical engineering applications.

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Methods for characterisation of nonwoven structure, property, and performance

N. Mao, in Advances in Technical Nonwovens, 2016

6.4.2 Nonwoven geotextiles

Nonwoven geotextiles are permeable geosynthetics161 made of nonwoven materials used with soil, rock, or other geotechnical-related material as an integral part of a civil engineering project, structure, or system. They are frequently made from synthetic polymers such as polypropylene, polyethylene, polyamide, and polyester, as well as natural fibres such as jute, sisal, and coir. The European standards (EN) for the specifications and performance requirements of geotextiles in 11 application areas (roads and other trafficked areas,162 railways,163 earthworks, foundation and retaining walls,164 drainage systems,165 erosion control,166 reservoirs and dams,167 canals,168 tunnels and underground structures,169 solid waste disposal,170 liquid waste containment,171 and asphalt reinforcement172) are defined. Their primary functions include filtration (to prevent soils from migrating into the adjacent material, such as drainage aggregate, while allowing water to flow through the system), drainage (to allow water to drain from or through low-permeability soils), separation (to separate the two dissimilar materials and prevent them from mixing), reinforcement (to increase shear strength of soils), and erosion control (to minimise the movement of soil particles due to flow of water). Therefore, there are a series of standard test methods defined in international and national standards to characterise its properties in relation to those five aspects of performance.

Some of the geotextile properties (H-properties), which are directly related to the functions and independent from the application, are imposed by the Mandates M/107 and M/386 of the European Commission for regulatory purposes, and some are of a voluntary nature to be used in all conditions of use (A-properties), and the rest of the properties are in some conditions of use (S-properties). The H-properties include tensile strength173 and elongation (at break),173 static puncture (California Bearing Ratio (CBR)) resistance,174,175 dynamic perforation resistance,176 water permeability (perpendicular to the plane),177,178 characteristic opening size,179,180 water flow capacity (in the plane),181,182 and different durability183–189 for various applications. The A- and S-properties (eg, tensile strength of seams and junctions,190 tensile and compressive creep191,192 abrasion,193 damage during installation,194–196 and friction197) may vary with applications and the actual conditions of use, which are specified in the individual standards. General provisions on dangerous substances and on fire behaviour (for tunnels and waste disposal) are also included.

There are also other test methods developed specifically designed for nonwoven geotextiles by some research institutions and industrial associations.198 EDANA and INDA had published a set of harmonised test standards for characterising the structure and properties of nonwoven geotextiles;199 however, these noncompulsory testing methods were removed from its latest NWSP test methods.200

The choosing of characterisation methods depends on the requirements of the geotextiles applications. The interpretation of the meaning of the results obtained for specific geotextiles performance and properties depends on the testing methods used and their testing conditions. For example, pore sizes in geotextiles can be characterised by using bubble point test method29 and pore size distribution by using capillary flow method,201 dry and wet sieving methods (AOS)179,180 and the meaning of the pore sizes obtained are significantly different in meaning. Another example is the tensile strength of geotextiles; there are three standard test methods for the tensile strength of geotextiles: grab tensile test (ASTM D4632), wide-width tensile test,173 and tension creep tests.191 While all of these three tensile strength tests provide an index of the ultimate strength of the specimen at failure, their test procedures vary in the speed required to perform each test, the fabric sample size and clamping feature, and the form of the test results. Both grab test and wide-width tests characterise the short-term tensile properties of geotextiles, while tension creep and rupture tests characterise the tensile properties of geotextiles under long-term sustained load and deformations. In grab tensile test, each specimen is clamped by 1-in jaws in the centre of the width and pulled in a greater speed, and the test results are expressed in units of total breakage force (Newton) rather than in terms of load per unit width. It is thus an excellent index strength test for verifying the quality and consistency of products in accordance with manufacturer’s specifications. In contrast, each specimen in wide-width tensile test is gripped across their full width and pulled in a smaller speed, it thus provides a better measure of true tensile strength in woven geotextiles. Unlike the grab tensile test, the wide-width strength results are expressed as a breakage force per unit width, and Young’s modulus can be obtained. However, as the specimen being tested is not confined as it would be in its end use, this test does not result in a true design value for nonwoven geotextiles. Tension creep tests are performed by sustaining a load on a test specimen across their full width for up to 10,000 h (417 days); the creep deformation or elongation (strain) of the sample is monitored over the test period, and the time to rupture at various load levels or the load level that will cause rupture at a given time is determined.

For commercial geotextile products, not only are the characterisation methods chosen crucial for their application performances, but also the reliability and reproducibility of the properties measured are critical for their quality assurance. Therefore, statistical values of the measured properties and performance of geotextiles of large batches such as minimum average roll values (MARVs) and typical values are usually required for the certain key properties of a batch of nonwoven geotextiles;202 both of the two terms are defined based an assumption that the data obtained are statistically in normal distribution.

MARV for geotextiles is defined as a manufacturing quality control tool used to allow manufacturers to establish published values such that the user/purchaser will have a 97.5% confidence that the property in question will meet published values.202 Typical value of a geotextiles product is the average of the test sample averages, and it is defined as a manufacturing quality control tool used to allow manufacturers to establish published values such that the user/purchaser will have a 50% confidence that the property in question will meet published values.202

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Developments in nonwoven as geotextiles

J.R. Ajmeri, C.J. Ajmeri, in Advances in Technical Nonwovens, 2016

12.2.3 Chemical bonding

Chemical bonding is the third technique used for bonding nonwoven geotextiles. A chemical binder, such as an acrylic resin, may be applied by total immersion or by spraying. After the binder is applied, the web is passed through an oven or hot rollers to cure the chemical bonding.

Another chemical bonding technique uses hydrogen chloride gas. In this process the gas is passed over web fibres, which are held in close contact by tension. The gas breaks the hydrogen bonds between the polymer chain and forms a complex with the amide group. By desorption the process is reversed and new hydrogen bonds are formed between polymer chains in different fibres. Chemical bonding often takes place after needle-punching or thermal bonding (Bhatia & Smith, 1996).

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Drainage for shallow foundations

Ruwan Rajapakse PE, CCM, CCE, AVS, in Geotechnical Engineering Calculations and Rules of Thumb (Second Edition), 2016

15.6.3 Geotextile wrapped granular drains (clayey surrounding soils)

Theory:

For cohesive soils, the direction of flow is not significant since the flow is not rapid as in sandy soils. Most Engineers prefer to use nonwoven geotextiles for cohesive soils.

Design Example 15.4

Design a Geotextile wrapped granular filter drain for a cohesive soil with D50 = 0.01 mm.

Solution

Step 1:

H50 (Geotextile) < (25 to 37) × D50 (soil) (one way and two way flow for clayey soils)

(Zitscher, 1975)

The equation in Step 1 is valid for cohesive surrounding soils for any type of flow.

H50 (Geotextile) < (25 to 37) × 0.01 mm

Since, D50 is equal to 0.01 mm, H50 should be between 0.25 mm and 0.37 mm.

Hence, select a geotextile with the H50 size equal to 0.30 mm.

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The use of geosynthetics as filters in civil engineering

FanninJ. , in Geosynthetics in Civil Engineering, 2007

Pore size opening

Geotextiles exhibit a distribution of pore size openings, with the variation in size being largely determined by attributes of the polymer strand and the manufacturing process. In contrast to a non-woven geotextile, see Fig. 6.4(a), which has a wide range of opening sizes, a woven geotextile tends to have narrow range of relatively larger openings, see Fig. 6.4(b). A characteristic opening size of the fabric is established through indirect means, typically by sieving a gradation of glass ballotini or sand through a specimen of the geotextile, and subsequent determination of the grain size distribution curve of the fraction that passes through the fabric under a prescribed disturbance. The disturbing action typically involves either dry shaking or hydrodynamic flushing. A characteristic opening size, e.g. O95 (μm), is taken to be the equivalent grain size of the fraction passing, in this case D95, with the implicit understanding that 95% of the pore openings are less than or equal to this value.

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6.4. Pore-size openings of (a) a needle-punched non-woven geotextile: (b) a woven geotextile.

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Multifunctional uses of geosynthetics in civil engineering

JonesC.J.F.P. , in Geosynthetics in Civil Engineering, 2007

5.2.2 Combined reinforcement and drainage

The design of reinforced slopes is concerned with the provision of an adequate factor of safety on strength and the control of settlements to acceptable limits. Conventional design and construction methods have used granular materials because of their high shear strength and good drainage properties. Research and long-term case histories have indicated that cohesive soils can be used in the construction of reinforced slopes if an adequate drainage system is provided.

When low-permeability fills are loaded, excess pore water pressures can be generated. This can result in a reduction in the available shear strength of the cohesive fill and also a reduction in the soil–reinforcement bond, requiring more reinforcement to provide an adequate bond length. The dissipation of excess pore water pressures results in consolidation and settlement of the reinforced structure, which can result in unacceptable face deflections.

The magnitude of excess pore water pressure present in a slope is a function both of the applied load and of the ability of the drainage system to dissipate the excess pore water pressure. At the base of a slope, with no drainage, large excess pore water pressures can develop. If drainage is provided and complete dissipation of excess pore water pressure occurs before construction of the next layer, the excess pore water pressure in the completed structure would be only a fraction of that otherwise present.

The ideal reinforcing material for cohesive soils requires the drainage characteristics of a non-woven geotextile and the strength of stiffer or stronger reinforcing geosynthetics. Alternatively, it is possible to combine existing materials (e.g. using a non-woven drainage geotextile together with geogrid reinforcement).

Heshmati (1993) studied the effects of combining a drainage material with grid reinforcement in clay soil. He concluded that the drainage and the reinforcement function were equally important in producing a stable and efficient structure. An important observation was that the method used to combine the drainage and reinforcing functions is critical. Simply placing a geotextile drain in conjunction with geogrid reinforcement can result in a reduction in strength as the presence of the drainage layer can lubricate the surface of the reinforcement. An essential requirement is that the combined functions of reinforcement and drainage have to be made integral.

An innovative geosynthetic material has been developed, which conforms with Heshmati’s findings relating to a reinforcing material, also providing drainage. The multifunctional geosynthetic material consists of high-tenacity polyester encased in a polyethylene sheath. The sheath both protects the load-carrying elements and maintains the shape of the product which is profiled to provide a drainage channel on one side. The profiled strap has a thermally bonded non-woven geotextile strip bonded on the shoulders of the drainage channel action as a filter. The geotextile allows excess pore water pressure to dissipate while retaining the cohesive soil (Fig. 5.1). Confirmation of the performance of the combined geogrid reinforcement and drainage material has been provided by Kempton et al. (2000) who identified the following.

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5.1. Integral drainage and reinforcement.

1

The effectiveness in dissipating excess pore water pressures under various confining stresses.

2

The pull-out resistance of the multifunctional material compared with a conventional geogrid of similar construction but with no drainage component.

3

The horizontal flow characteristics of the material under various hydraulic gradients and confining pressures.

4

Suitable parameters for use in design for constructing steep slopes using cohesive fills.

The test results show that the pore water pressure reduces to 20% of the applied pressure in a 36–42 h period at confining pressures of both 50 and 100 kPa. No noticeable difference was observed in pore water pressure values measured above and below the test specimen even though the drainage channel was only on one side of the combined reinforcement and drainage material.

Pull-out testing on the combined material and on conventional geogrid with no drainage component was conducted after partial and full dissipation of excess pore water pressure (Fig. 5.2). The improvement in pull-out resistance is explained by the rapid dissipation of the excess pore water pressure in the immediate vicinity of the composite material, thus allowing early development of bond between the reinforcement and the soil. Full dissipation of excess pore water pressure is assumed when the pore water pressure reaches 10% of the applied overburden pressure in the immediate vicinity of the composite material.

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5.2. Pull-out results for a new geosynthetic and conventional geogrid with no drainage component after dissipation of excess pore water pressure for 12 h (after Kempton et al., 2000).

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The use of geosynthetics as separators in civil engineering

W. Wilmers, in Geosynthetics in Civil Engineering, 2007

7.3.3 Requirements for separation layers

Calculation of damaging influences is not realistically possible; therefore, in different countries, individual approaches are developed, on the basis of experiences with particular applications. The following are outlined.

France (AFNOR G 38-063)

According to AFNOR G 38-063 (Association Française de Normalisation, 1993), the subsoil is separated into three classes: soil 1, cu > 60 kPa; soil 2, 20 kPa < cu < 60 kPa; soil 3, cu < 20 kPa.

The fill is defined by four properties.

1

Permeability in two classes: lower or higher kf = 1 × 10− 5 m/s or 100 times the permeability of subsoil.

2

Particle shape in two classes: sharp-edged or round.

3

Particle size in two classes: greatest particle size smaller or larger than 250 mm.

4

Thickness of first layer in two classes: middle, 0.30–0.50 m; thick, 0.50–1.00 m.

Geotextiles are characterized by the following properties.

1

Tensile strength and tensile strain.

2

Tear resistance.

3

Water permeability vertical to plane.

4

Water permeability in plane.

5

Opening size.

In a screen for different fill conditions, the required properties of geotextile separators are defined.

Germany

For separators in road construction, according to M Geok E-StB 05 (Forschungsgesellschaft für Strassen- und Verkehrswesen, 2005a) and TL Geok E-StB 05 (Forschungsgesellschaft für Strassen- und Verkehrswesen, 2005b) the fill is characterized by the following.

1

Angularity: sharp-edged or round

2

Particle size and grading in four classes

3

Stress during installation and use in five classes, defined by depth of ruts.

In a screen, these factors are combined to give five classes of requirements for the geotextiles. Corresponding to this the robustness of geotextiles is characterized in five classes depending on strength and mass per unit area. The strength of non-woven geotextiles is measured by the static puncture test CBR (EN ISO 12236), of woven geotextiles by the tensile test.

The hydraulic properties are as follows.

1

Permeability: kV > 1 × 10− 4 m/s and kV > kf soil.

2

Opening size O90; for non-woven geotextiles, 0.06 mm < O90 < 0.20 mm and, for woven geotextiles, 0.06 mm < O90 < 0.40 mm.

For separators between protection layers and fine-grained soils in railtracks, German Rail has developed special requirements (Eisenbahn-Bundesamt, 2003) (see Table 7.2).

Scandinavian countries (NorGeoSpec, 2002)

According to NorGeoSpec, 2002 (Stiftelsen for Industriell og Teknisk Forskning, 2002), five specification profiles are based on the following.

1

Subsoil conditions (two classes), defined by shear strength cu.

2

Construction conditions (two classes), based on construction traffic, angularity of fill material, particle or stone size and layer thickness.

3

Traffic in use (two classes), vehicles per day.

4

Maximum grain size and grading (four classes).

For five specification profiles, requirements for characteristics of geotextiles are given for the following.

1

Tensile strength and strain.

2

Cone drop diameter.

3

Energy index (product of maximum tensile strength multiplied by strain at maximum strength, divided by two).

4

Water flow velocity index.

5

Characteristic opening size O90 (0.15 mm or 0.20 mm).

6

Allowable tolerances for mass per unit area and static puncture strength (CBR).

Switzerland (SN 640 552a)

According to SN 640 552a (Vereinigung Schweizer Strassenfachleute, 1997), the fill is characterized by angularity and particle size, and particle grading in three classes. The bearing capacity of subsoil is separated into five classes by CBR value or plate-bearing test, and the traffic load into two classes by the addition of axel loads over the period of use. For three layer thicknesses of bearing layers, the requirements for geotextiles are defined according to tensile force and elongation. The hydraulic properties are as follows.

1

Permeability: four classes of kG > 1 × 10− 4 m/s to kG > 1 × 10− 6 m/s in relation to the soil.

2

Opening size Ow for non-woven geotextiles: four classes, 0.05 mm < Ow < 0.20 mm, up to 0.05 mm < Ow < 0.50 mm in relation to the soil.

UK

In road construction, the following requirements are given (Highways Agency, 2001a, 2001b). The tensile load is to be defined by the client. The water permeability at a right angle to its principle plane shall be not less than 101/m2 s under a constant water head of 100 mm, and the pore openings such that mean opening O90 is between 100 and 300 μm.

British Rail (1996) have the following requirements for separation layers. The minimum tensile breaking load shall be 10 kN/m and the CBR puncture resistance greater than 3000 N at a displacement of less than 60 mm. Water permeability shall be not less than 101/m2 s under a constant water head of 100 mm and the pore openings such that mean opening O90 is between 30 and 85 μm.

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The material properties of geosynthetics

S.W. Perkins, in Geosynthetics in Civil Engineering, 2007

2.2 Physical properties

Physical properties of geosynthetics are basic properties related to the composition of the materials used to fabricate the geosynthetic and include the type of structure, specific gravity, mass per unit area, thickness and stiffness. The type of structure of a geosynthetic describes the physical make-up of the geosynthetic resulting from the process used to manufacture the material. The structure of the geosynthetic often dictates the application area for which the material is appropriate. For example, a uniaxial geogrid is appropriate for applications where load is expected in one principal direction of the material, such as in a long slope or retaining wall. The geosynthetic structure is most often described for geogrids. The structure of geogrids of greatest importance is that associated with the manufacturing process used to form the junctions of the geogrid, with examples including woven, integral and welded junctions. Structure can also be described for geotextiles where the two main types of structure include woven and non-woven geotextiles.

The specific gravity of a geosynthetic is measured on the basic polymeric material or materials used to form the geosynthetic. The specific gravity is defined conventionally as the ratio of the material’s unit volume weight to that of distilled, de-aerated water at a standard temperature. Ranges of values for the specific gravity of commonly used geosynthetic polymers are listed in Table 2.1. The specific gravity of the geosynthetic polymer is important in applications where the geosynthetic will be placed underwater where polymers with values of specific gravity less than one will require weighting in order to sink the material into position.

Table 2.1. Specific gravities of common geosynthetic polymers

PolymerSpecific gravity
Polyamide1.05–1.14
Polyester1.22–1.38
Polyethylene0.90–0.96
Polypropylene0.91

Mass per unit area describes the mass (usually in units of grams) of a material per unit area (generally in square metres) and should be measured with no tension applied to the material. Typical values for geotextiles lie between 130 and 700 g/m2 while for geogrids the values range from 200 to 1000 g/m2.

The thickness of a geosynthetic is measured as the distance between the extreme upper and lower surfaces of the material. For geotextiles, this distance is measured while a specified pressure is applied to the material. Thicknesses of geotextiles range from 0.25 to 7.5 mm. The thickness of common geomembranes used today is 0.5 mm.

The physical property of stiffness refers to the flexibility of the material and is not a description of the mechanical property of stiffness which describes the material’s load-strain modulus. The flexibility of a geosynthetic is determined by allowing the material to bend under its own weight as it is being slid over the edge of a table. The properties of flexural stiffness or rigidity describe the material’s capability of providing a suitable working platform during installation and is an important property when installation is performed over soft soil sites.

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An Australian Perspective on Modernization of Rail Tracks Using Geosynthetics and Shockmats

Buddhima Indraratna, Sanjay Nimbalkar, in Ground Improvement Case Histories, 2015

20.2 Field study at Bulli

20.2.1 Site Geology and Track Construction

A site investigation was carried out to investigate the conditions of the subsurface profiles; the subgrade consisted of a stiff overconsolidated silty clay, and the bedrock was highly weathered sandstone with a low to medium strength (Choudhury, 2006).

A section of instrumented track was located between two turnouts at Bulli, part of Sydney Train’s South Coast Track, as shown in Fig. 20.1(a). The instrumented track section was 60 m long, and was divided into four equal sections (Fig. 20.1(b)). Fresh ballast, 300 mm thick, was used at Sections 1 and 2 and recycled ballast of the same thickness was used at Sections 3 and 4, respectively. A layer of geocomposite was placed over the 150-mm-thick subballast at Sections 2 and 3, respectively (Fig. 20.2).

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Figure 20.1. (a) Locations of experimental sections of Bulli track. (b) Details of instrumented track at Bulli.

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Figure 20.2. Placement of geocomposite over top of subballast.

20.2.2 Material specifications

The particle gradation of fresh ballast was in accordance with the Rail Infrastructure Corporation’s Technical Specification (RIC, 2001). Recycled ballast was obtained from spoil stockpiles of a recycled plant commissioned by Sydney Trains at Chullora Yard near Sydney. The subballast was a mixture of sand and gravel. The particle size distributions of fresh ballast, recycled ballast, and subballast materials are given in Fig. 20.3. Table 20.1 shows the grain size characteristics of these materials used at the site (Indraratna et al., 2010).

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Figure 20.3. Particle size distribution of the fresh ballast, recycled ballast, and subballast.

(Source: Data from Indraratna et al. (2010)).

Table 20.1. Grain size characteristics of ballast and subballast

Materialdmax (mm)dmin (mm)d50 (mm)CuCc
Fresh ballast7519351.51
Recycled ballast759.5381.81
Subballast (sandy gravel)190.050.265.01.2

Source: Data from Indraratna et al. (2010).

The layer of geocomposite consisted of biaxial geogrid placed over a layer of nonwoven polypropylene geotextile. Large-scale triaxial tests indicated that a geocomposite layer (a combination of geogrid and nonwoven geotextile) stabilized the ballast much better than the geogrids and geotextiles (Indraratna and Salim, 2003; Indraratna and Nimbalkar, 2013). Therefore, in this field trial, a geogrid–geotextile combination was used. In geocomposite, the biaxial geogrid provides a strong interlock with angular ballast particles, which improves the frictional characteristics, whereas the nonwoven geotextile keeps subgrade fines from fouling the ballast layer, and allows partial in-plane drainage. The technical specifications of the geosynthetic material used at this site are given in Table 20.2. The values are indicated as MD and TD; where MD is the machine direction (longitudinal to the roll) and TD is the transverse direction (across the roll).

Table 20.2. Mechanical properties of geocomposite used during the field trial

TypeGeocomposite
Biaxial geogridNonwoven geotextile
DirectionMDTDMDTD
Peak tensile strength (kN/m)3030
Strain at break (%)11101010
Aperture size (mm)4027
Thickness (mm)22
Mass per unit area (g/m2)420140

20.2.3 Track instrumentation

The performance of the experimental section was monitored using a series of sophisticated equipment. Ballast deformation was measured by settlement pegs and lateral displacement transducers. The use of displacement transducers is an established practice for measuring vertical displacement (Grabe and Clayton, 2003). In this field trial they were used to measure horizontal track movements. These transducers were placed inside two, 2.5-m-long tubes that can slide over each other and with 100-mm square end caps as anchors. The settlement pegs were 100-mm-square by 6-mm-thick end plates attached to 10-mm-diameter cylindrical rods. The settlement pegs and displacement transducers were installed above and below the ballast layer, as shown in Fig. 20.4.

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Figure 20.4. Installation of settlement pegs and displacement transducers in experimental sections of track at Bulli.

The vertical and horizontal stresses developed in the track substructure were measured by rapid response earth pressure cells containing semiconductor-type transducers. These earth pressure cells were placed in the track layer as shown in Fig. 20.5.

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Figure 20.5. Installation of pressure cells for measuring stresses in the track.

20.2.4 Track measurements

A total traffic tonnage of 90 million gross tons (MGT) resulted during the period of field measurements. The passenger and freight traffic was imparted from trains having four axles with loads of 20.5 and 25 tons, respectively. Data from the pressure cells and displacement transducers were obtained by operating a mobile data acquisition (DAQ) unit at a frequency of 40 Hz. Ballast deformation was measured in the field, against time, using simple survey techniques. A relationship between the annual rail traffic in MGT and axle load (At) was used to determine the number of load cycles (Selig and Waters, 1994):

(20.1) N t = 10 6 A t × N c

where

Nt = number of load cycles per MGT

At = the axle load in tons

Nc = number of axles per load cycle

The results were plotted against the time and number of load cycles, as discussed in the following.

Vertical deformation

Vertical deformation was determined from the mean of measurements at the sleeper-ballast and ballast–subballast interfaces. Vertical deformations were plotted against the number of load cycles (N) in Fig. 20.6.

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Figure 20.6. Vertical deformations of fresh and recycled ballast with and without geocomposite layer.

(Source: Data from Indraratna et al. (2010)).

As expected, a rapid vertical deformation of ballast occurred at the onset of the loading cycles, but the rate of deformation decreased to a controlled steady state after a certain number of load repetitions (defined as the stable zone). Rail-track settlement is usually related to the number of load cycles by a semilog relationship (Jeffs and Marich, 1987; Indraratna and Salim, 2005). Figure 20.7 shows the settlement of fresh and recycled ballast with and without geosynthetics, plotted in a semilogarithmic scale. Ballast settlement under cyclic loading may be represented by a simple semilogarithmic relationship, as proposed by Indraratna and Salim (2005):

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Figure 20.7. Vertical deformations of fresh and recycled ballast with and without geocomposite layer plotted in semilogarithmic scale.

(20.2) S V = a + b ln N

or by a more complex relationship as proposed by Indraratna and Nimbalkar (2013):

(20.3) S V = S V 1 1 + a ′ ln N + 0.5 b ′ ln N 2

where

SV = the vertical deformation of ballast

SV1 = the deformation of ballast after the first load cycle

N = the number of load cycles

a, a′, b, and b′ = empirical constants, depending on the type of ballast, type of geosynthetics used, initial density, and the degree of saturation

Recycled ballast experienced less vertical deformation than the very uniform fresh ballast because of its moderately graded particle-sized distribution. However, fresh ballast stabilized with the geocomposite experienced the least vertical deformation (i.e., a reduction of 33% compared to a 9% reduction for recycled ballast). This may be attributed to the fact that highly frictional, angular particles of fresh ballast develop a strong mechanical interlock with the geogrid layer, thus creating an enhanced confinement, whereas the performance of geotextile largely depends on the tension membrane effect.

Lateral deformation

Under repeated loading, the ballast layer undergoes vertical compression and expands in two lateral directions. The lateral deformation (SL) of ballast was determined by subtracting the displacement of the ballast–subballast interface from those at the sleeper–ballast interface. These deformations (SL) are plotted against the number of load cycles (N) in Fig. 20.8.

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Figure 20.8. Lateral deformations of fresh and recycled ballast plotted in normal scale.

(Source: Data from Indraratna et al. (2010)).

There was a significant lateral deformation in the ballast, with all sections showing almost similar trends in the variation of SL. The nonlinear variation of SL with increasing load cycles becomes linear in the semilogarithmic plot (Fig. 20.9), and may be expressed by a function similar to Eq. (20.2) (Indraratna and Salim, 2005):

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Figure 20.9. Lateral deformations of fresh and recycled ballast plotted in semilogarithmic scale.

(Source: Data from Indraratna et al. (2010)).

(20.4) S L = c + d ln N

or by a function similar to Eq. (20.3) (Indraratna and Nimbalkar (2013):

(20.5) S L = S L 1 1 + c ′ ln N + 0.5 d ′ ln N 2

where

SL1 = the lateral deformation of ballast after the first load cycle

c, c′, d, and d′ = empirical constants

Recycled ballast showed less lateral deformation because the corners of individual particles did not break as frequently due to their reduced angularity. The geocomposite reduced the lateral deformation of fresh ballast by 49% and recycled ballast by 11%. This was attributed to a better interlock of fresh ballast with the geogrid (aperture size of 40 mm × 27 mm). The field trial demonstrated the potential benefits of using a geocomposite at the base of the ballast layer in the track and the use of moderately graded recycled ballast.

Traffic-induced vertical stresses in ballast

Figure 20.10 shows the maximum vertical cyclic stresses (σV) recorded in Section 1, under the rail, from a passenger train traveling at 60 km/h, and indicates that σV decreased significantly with depth. The maximum cyclic stresses (σV) due to the passage of a coal train are also shown in Fig. 20.10. As expected, the maximum vertical cyclic stress (σV) measured in the layer of ballast and subballast was higher for a coal freight train than a passenger train. Thus, the greater axle load of the coal train imposed a higher σV, which resulted in more deformation and degradation of the ballast, implying the need for earlier track maintenance.

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Figure 20.10. Vertical cyclic stresses measured under the rails (σV) for passenger train (20.5-t axle load) and for a coal train (25-t axle load).

Traffic-induced lateral stresses in ballast

Figure 20.11 shows the maximum lateral cyclic stresses (σL), under the rail, from a passenger train (20.5-t axle load) and from a coal train (25-t axle load) traveling at 60 km/h. The large vertical stresses and relatively small lateral (confining) stresses caused large shear strains in the track. The corresponding ease of lateral spreading due to the absence of sufficient confinement increased the vertical compression of the ballast layer, as also confirmed by Selig and Waters (1994). Moreover, σL increased with an increase in the number of load cycles, which further degraded the track substructure including the ballast.

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Figure 20.11. Lateral cyclic stresses measured under the rails (σL) for passenger train (20.5-t axle load) and for a coal train (25-t axle load).

Shock mats can be used to mitigate damage induced by impact loads. The “in-field” performance of these artificial inclusions is described in the following section.

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URL: https://www.sciencedirect.com/science/article/pii/B9780081006986000209

Soil reinforcement

Anjan Patel, in Geotechnical Investigations and Improvement of Ground Conditions, 2019

7.2 Geosynthetic materials

As per the definition of American Society for Testing and Materials (ASTM), a geosynthetic material is a planar product manufactured from a polymeric material that is used with soil, rock, earth, or other geotechnical related material as an integral part of a civil engineering project, structure, or system.

A geosynthetic material can be categorized as geotextile, geogrid, geomembrane, or geocomposite (e.g. geotextile-geonets, geotextile-geogrids, geotextile-geomembranes, geomembrane-geonets, geotextile-polymeric cores, or a three-dimensional polymeric cell structures). Geotextiles are permeable synthetic materials made of textile materials such as polypropylene, polyethylene, or polyester. Further, based upon their preparation, a geotextile may be woven, nonwoven, or knitted, as shown in Fig. 7.2. Aside from these traditional forms of woven and nonwoven geotextiles, there are many forms of geotextiles that have come onto market over the last few years, as depicted in Fig. 7.3. Geotextiles are widely used in road construction and railway works (for separation, filtration, drainage, and soil reinforcement); for erosion control in river canals and coastal works; as a filtering media for drainage in earthen dam, behind retaining walls, and in deep drainage trenches; in sport field construction like Caselon playing fields and Astro turf; and for mud control in agriculture.

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Fig. 7.2. Woven, nonwoven, and knitted geotextiles.

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Fig. 7.3. Different forms of geotextile materials (Frank, 2004).

Geogrids are primarily used for reinforcement and they are heat-welded from strips of material (such as polyester, polyethylene, or polypropylene) or produced by punching a regular pattern of holes in sheets of material that are then stretched into a grid. As fill is placed on the geogrids, the mesh design locks soil in place providing differing levels of stability depending on the type of geogrid used. The geogrid may be uniaxial or biaxial, as shown in Fig. 7.4, depending upon their tensile strength in different directions. A uniaxial geogrid has high tensile strength in one direction and is useful for the reinforcement of retaining walls, steep slopes, and road embankments, and for repairing landslides. On the other hand, a biaxial geogrid has equal tensile strength in both directions and is useful for stabilizing roadways. It distributes loads over a larger area reducing pumping and shear failures while maximizing the load-bearing capacity of subgrades.

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Fig. 7.4. Uniaxial and biaxial geogrids.

Geomembranes are very low-permeable geosynthetic materials made from relatively thin continuous polymeric sheets (chlorosulphonated polythene (CSPE), high-density polythene (HDPE), very-low-density polythene (VLDPE), polyvinyl chloride (PVC), etc.), but they can also be made from the impregnation of geotextiles with asphalt, elastomer, or polymer. The geomembranes create an impermeable barrier that keeps contaminants and other dangerous chemicals contained so they cannot escape and damage the surrounding environment. Geomembranes are generally used as liner for sewage sludge, for the safe shutdown of nuclear facilities, for water and various waste conveyance canals, as secondary containment of underground storage tanks, as covers for solid waste landfills, as waterproof facing in various structures, to prevent infiltration of water and contaminants into sensitive areas, etc.

Geocomposites are made by combining different geosynthetics materials or by combining geosynthetic materials with nonsynthetic materials, such as bentonite clay, to address specific applications in the field in the optimal manner with minimum cost. Geocomposite materials include geotextile-geonets, geotextile-geogrids, geotextile-geomembranes, geomembrane-geonets, geosynthetics clay liner (GCL), geotextile-polymeric cores, or three-dimensional polymeric cell structures. Various forms of geocomposite materials are presented in Fig. 7.5.

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Fig. 7.5. Various forms of geocomposite materials.

So from this abovementioned information it can be concluded that the application of geosynthetic materials covers almost all types of civil engineering structures and its main functions are separation, filtration, drainage, reinforcement, protection, and waterproofing. In road construction, the application of geosynthetic materials is illustrated in Fig. 7.6.

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Fig. 7.6. Application of geotextiles in pavements (Koerner, 1998).

When reinforcement is required to be inserted into the soil mass, such as for retaining walls and embankments, it is very important to add the backfill material (for construction of manmade structures) or insert the reinforcing element into the soil mass (for stability and/or protection of natural backfill) in a systematic way by following the proper steps. Moreover, it is necessary to include a facing wall (either soft, such as growing vegetation, or hard, such as wraparound type, gabion, precast concrete panel, or modular concrete block) for the stability and longevity of the structure. Hence, the backfill materials and facing wall along with the reinforcing elements now become an integral part of the reinforced wall, as shown in Fig. 7.7, where a precast concrete panel has been used as the facing wall.

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Fig. 7.7. A typical picture of reinforced retaining wall (Holtz, 2001).

The use of geosynthetic materials in various civil-engineering structures and for ground treatment has tremendously increased in the last few decades. The advantages of using these materials are cost efficiency, convenience for transport and installation, lower repair and maintenance costs, predictability of the design, quick installation, applicability to a wide range of soils, space savings, improved performance and extended life, good-quality control due to homogeneity in nature, less environmentally sensitive, increased safety factor, and compatibility with field conditions. However, the geosynthetic materials need to meet certain requirements and to be checked and tested before using them in the field. The required properties are summarized in Table. 7.1.

Table 7.1. Requirement of geosynthetics material

PropertyParameters
General propertiesMaterial type and construction, polymer(s), mass, thickness, roll dimensions, specific gravity, absorption
Index propertiesStrip tensile strength, grab strength, creep resistance, flexural strength, cutting – trapezoidal tear strength, shear modulus, Poisson’s ratio, burst strength, puncture resistance, penetration, flexibility (flexural strength)
Endurance propertiesAbrasion resistance, UV stability, biological resistance, chemical resistance, wet/dry stability, temperature stability, long-term durability
Performance – soil/fabric propertiesStress–strain, creep, friction/adhesion, dynamic and cycling loading, soil retention, filtration
Hydraulic propertiesApparent opening size, percent open area, porosity, permeability/permittivity, soil retention ability, clogging resistance, in-plane flow capacity

The new generation of geosynthetic materials includes the nanofibre membrane with superhydrophobicity and superoleophilicity for selective separation of water and low viscous oil, nanocomposite electrospun nanofiber membranes (Nanocomposite ENM) for environmental remediation and water filtration, etc. The basic principles behind the concept of nanofibre is that by reducing fibre diameter down to the nanoscale, the specific surface area increases enormously to the order of about 1000 m2/g and this reduction in dimension and increase in surface area greatly affects the chemical/biological reactivity and electroactivity of polymeric fibres.