Reinforced Retaining Wall

Concept

Retaining walls are required where a soil slope is uneconomical or not technically
feasible. When compared with conventional retaining structures, walls with reinforced
backfills offer significant advantages. They are very cost effective, especially for higher walls. Furthermore, these systems are more flexible than conventional earth retaining walls such as reinforced concrete cantilever or gravity walls. Therefore, they are very suitable for sites with poor foundations and for seismically active areas.

Modern reinforced soil technology was developed in France by H. Vidal in the mid
1960s. His system is called Reinforced Earth and is shown in Fig. 4. Steel strips are used to reduce the earth pressure against the wall face. The design and construction of Vidaltype reinforced earth walls are now well established, and many thousands have been successfully built throughout the world in the last 25 years. Other similar proprietary reinforcing systems have also been deve loped using steel bar mats, grids, and gabions.
The use of geotextiles as reinforcing elements started in the early 1970’s because of
concern over possible corrosion of metallic reinforcement. Systems using sheets of
geosynthetics rather than steel strips are shown in Figure

The maximum heights of geosynthetic reinforced walls constructed to date are less
than 20 m, whereas steel reinforced walls over 40 m high have been built. A significant benefit of using geosynthetics is the wide variety of wall facings available, resulting in greater aesthetic and economic options. Metallic reinforcement is typically used with articulated precast concrete panels or gabion-type facing systems.


Design Considerations



Reinforced wall design is very similar to conventional retaining wall design, but
with the added consideration of internal stability of the reinforced section. External stability is calculated in the conventional way--the bearing capacity must be adequate, the reinforced section may not slide or overturn, and overall slope stability must be adequate. Surcharges (live and dead loads; distributed and point loads) are considered in the conventional manner. Settlement of the reinforced section also should be checked if the foundation is compressible.

A number of different approaches to internal design of geotextile reinforced
retaining walls have been proposed (Christopher et al., 1990; Allen and Holtz; 1991; Holtz,1995), but the oldest and most common--and most conservative--method is the tieback wedge analysis. It utilizes classical earth pressure theory combined with tensile resisting “tiebacks” that extend back of the assumed failure plane (Fig. 6). The KA (or Ko) is assumed, depending on the stiffness of the fa cing and the amount of yielding likely to occur during construction, and the earth pressure at each vertical section of the wall is calculated. This earth pressure must be resisted by the geosynthetic reinforcement at that section.

To design against failure of the reinforcement, there are two possible limiting or failure cond itions: rupture of the geosynthetic and pullout of the geosynthetic. The corresponding reinforcement properties are the tensile strength of the geosynthetic and its pullout resistance. In the latter case, the geosynthetic reinforcement must extend some distance behind the assumed failure wedge so that it will not pull out of the backfill. Typically, sliding of the entire reinforced mass controls the length of the reinforcing elements. For a detailed description of the tieback wedge method, see Christopher and Holtz (1985), Bonaparte et al. (1987), Allen and Holtz (1991), and Holtz et al. (1997). Recent research (e.g., Lee et al., 1999; Lee, 2000; Bathurst et al.,2000) has indicated that the tieback wedge approach is overly conservative and uneconomical, and modifications and deformation-based designs are rapidly being deve loped. Other important design considerations include drainage and potential seismic loading.


Material Properties

Geosynthetic properties required for reinforced walls are similar to those listed in Table 1, Section 8.3 and discussed in Section 9.3 for reinforced slopes. Properties are required for design (stability), constructability, and durability. Allowable tensile strength and soil- geosynthetic friction are required for stability design, and similar to reinforced slopes, a partial factor or reduction factor approach is common. The ultimate wide width strength is reduced to account for uncertainties in creep strength, chemical and biological degradation effects, installation damage, and joints and connections. Berg (1993), Holtz et al.(1997), and Koerner and Hsuan (2001) give details about the determination of the allowable geosynthetic tensile strength. They also describe how soil- geosynthetic friction is measured or estimated.

Backfill for geosynthetic reinforced walls should be free draining if at all possible. If not, then adequate drainage of infiltrating surface or groundwater must be provided. This is important for stability considerations because drainage outward through the wall face may not be adequate. Soil properties required include gradation, percent fines,chemical composition, compaction, unit weight, and shear strength. To insure stability,appropriate consideration of the foundation and overall slope stability at the site is also important (Holtz et al., 2001b).


Wall Facing Considerations

A significant advantage of geosynthetic reinforced walls over conventional retaining structures is the variety of facings that can be used and the resulting aesthetic options that can be provided. Aesthetic requirements often determine the type of facing systems. Anticipated deflection of the wall face, both laterally and downward, may place further limitations on the type of facing system selected. Tight construction specifications and quality inspection are necessary to insure that the wall face is constructed properly; otherwise an unattractive wall face, or a wall face failure, could result.

Facing systems can be installed (1) as the wall is constructed or (2) after the wall is built. Facings installed as the wall is constructed include segmental and full height precast concrete panels, interlocking precast concrete blocks, welded wire panels, gabion baskets, treated timber facings, and geosynthetic face wraps. In these cases, the geosynthetic reinforcement is attached directly to the facing element. Systems installed after construction include shotcrete, cast-in-place concrete facia, and precast concrete or timber panels; the panels are attached to brackets placed between the layers of the geosynthetic wrapped wall face at the end of wall construction or after wall movements are complete. Facings constructed as the wall is constructed must either allow the geosynthetic to deform freely during construction without any buildup of stress on the face, or the facing connection must be designed to take the stress. Although most wall design methods assume that the stress at the face is equal to the maximum horizontal stress in the reinforced backfill, measurements show that considerable stress reduction occurs near the face, depending on the flexibility of the face. See Allen and Holtz (1991) and Holtz et al. (1997) for a detailed discussion of wall facing systems.


Constuction

Construction procedures for geosynthetic reinforced walls and abutments are given
by Christopher and Holtz (1985) and Holtz et al. (1997). Procedures are relatively simple and straightforward, but failures are surprisingly common, especially with proprietary precast segmental concrete block-faced wall systems. It appears that most of these failures are due to (1) inadequate design, particularly of the foundation and back slope of the wall, and/or (2) problems in construction. The latter include poor inspection and quality control, poor compaction, use of inappropriate backfill materials, lack of attention to facing connections, and lack of clear lines of responsibility between designers, material suppliers, and contractors.

Reinforced Steep Slope (with geosynthetics)

Concept

The first use of geosynthetics for the stabilization of steep slopes was for the reinstatement of failed slopes. Cost savings resulted because the slide debris could be reused in the repaired slope (together
with geosynthetic reinforcement), rather than importing select materials to reconstruct the slope. Even if foundation conditions are satisfactory, costs of fill and right-of-way plus other considerations may require a steeper slope than is stable in compacted embankment soils without reinforcement. As shown in Fig.3, multiple layers of geogrids or geotextiles may be placed in a fill slope during construction or reconstruction to reinforce the soil and provide increased slope stability. Most steep slope reinforcement projects are for the construction of new embankments, alternatives to retaining walls, widening of existing embankments, and repair of failed slopes. Another use of geosynthetics in slopes is for compaction aids (Fig. 3). In this application, narrow geosynthetic strips, 1 to 2 m wide, are placed at the edge of the fill slope to provide increased lateral confinement at the slope face, and therefore increased compacted density over that normally achieved. Even modest amounts of reinforcement in compacted slopes have been found to prevent sloughing and reduce slope erosion. In some cases, thick nonwoven geotextiles with in-plane drainage capabilities allow for rapid pore pressure dissipation in compacted cohesive fill soils.


Design Considerations

The overall design requirements for reinforced slopes are similar to those for unreinforced slopes--the factor of safety must be adequate for both the short- and long-term conditions and for all possible modes of failure. These include: (1) internal--where the failure plane passes through the reinforcing elements; (2) external--where the failure surface passes behind and underneath the reinforced mass; and (3) compound--where the failure surface passes behind and through the reinforced soil mass. Reinforced slopes are analyzed using modified versions of classical limit equilibrium slope stability methods (e.g., Terzaghi et al., 1996). Potential circular or wedge-type failure surfaces are assumed, and the relationship between driving and resisting forces or moments determines the factor of safety. Based on their tensile capacity and orientation, reinforcement layers intersecting the potential failure surface increase the resisting moment or force. The tensile capacity of a reinforcement layer is the minimum of its allowable pullout resistance behind, or in front of, the potential failure surface and/or its long-term design tensile strength, whichever is smaller. A variety of potential failure surfaces must be considered, including deep-seated surfaces through or behind the reinforced zone, and the critical surface requiring the maximum amount reinforcement determines the slope factor of safety. The reinforcement layout and spacing may be varied to achieve an optimum design. Computer programs are available for reinforced slope design which include searching routines to help locate critical surfaces and appropriate consideration of reinforcement strength and pullout capacity. Additional information on reinforced slope design is available in Christopher et al. (1990), Christopher and Leshchinsky (1991), Berg (1993), Holtz et al.(1997), and Bathurst and Jones (2001).
For slide repair applications, it is very important that the cause of original failure is addressed in order to insure that the new reinforced soil slope will not have the same problems. Particular attention must be paid to drainage. In natural soil slopes, it is also necessary to identify any weak seams that could affect stability.


Material Properties

Geosynthetic properties required for reinforced slopes are similar to those listed in Table 1. in the previous post Properties are required for design (stability), constructability, and durability. Allowable tensile strength and soil-geosynthetic friction are most important for stability design. Because of uncertainties in creep strength, chemical and biological degradation effects, installation damage, and joints and connections, a partial factor or reduction factor concept is recommended. The ultimate wide width strength is reduced for these various factors, and the reduction depends on how much information is available about the geosynthetics at the time of design and selection. Berg (1993), Holtz et al. (1997), and Koerner and Hsuan (2001) give details about the determination of the allowable geosynthetic tensile strength. They also describe how soil-geosynthetic friction is measured or estimated. An inherent advantage of geosynthetic reinforcement is their longevity, especially in normal soil environments. Recent studies have indicated that the anticipated half-life of reinforfcement geosynthetics in between 500 and 5000 years, although strength characteristics may have to be adjusted to account for potential degradation in the specific environmental conditions. Any soil suitable for embankment construction can be used in a reinforced slope system. From a reinforcement point of view alone, even lower-quality soil than conventionally used in unreinforced slope construction may be used. However, higher-quality materials offer less durability concerns, are easier to place and compact, which tends to speed up construction, and they have fewer problems with drainage. See Berg (1993) and Holtz et al. (1997) for discussion of soil gradation, compaction, unit weight, shear strength, and chemical composition.


Construction

Similarly to reinforced embankments, proper construction is very important to insure adequate performance of a reinforced slope. Considerations of site preparation, reinforcement and fill placement, compaction control, face construction, and field inspection are given by Berg (1993) and Holtz et al. (1997).


Reference :
R.D. Holtz, Ph.D., P.E., Geosynthetics Soil Reinforcement, Department of Civil & Environmental Engineering, University of Washington

Reinforced Embankment on Soft Foundation (Using Geosynthetics)

Concept

The design and construction of embankments on soft foundation soils is a very challenging geotechnical problem. As noted by Leroueil and Rowe (2001), successful projects require a thorough subsurface investigation, properties determination, and settlement and stability analyses. If the settlements are too large or instability is likely, then some type of foundation soil improvement is warranted. Traditional soil improvement methods include preloading/surcharging with drains; lightweight fill; excavation and replacement; deep soil mixing, embankment piles, etc., as discussed by Holtz (1989) and Holtz et al. (2001a). Today, geosynthetic reinforcement must also be considered as a feasible treatment alternative. In some situations, the most economical final design may be some combination of a traditional foundation treatment alternative together with geosynthetic reinforcement. Figure 2a shows the basic concept for using geosynthetic reinforcement. Note that the reinforcement will not reduce the magnitude of long-term consolidation or secondary settlement of the embankment.


Design Considerations

As with ordinary embankments on soft soils, the basic design approach for reinforced embankments is to design against failure. The ways in which embankments constructed on soft foundations can fail have been described by Terzaghi et al. (1996), among others.



In figure above shows unsatisfactory behavior that can occur in reinforced embankments. The three possible modes of failure indicate the types of stability analyses that are required for design. Overall bearing capacity of the embankment must be adequate, and the reinforcement should be strong enough to prevent rotational failures at the edge of the embankment. Lateral spreading failures can be prevented by the development of adequate shearing resistance between the base of the embankment and the reinforcement. In addition, an analysis to limit geosynthetic deformations must be performed. Finally, the geosynthetic strength requirements in the longitudinal direction, typically the transverse seam strength, must be determined. Discussion of these design concepts as well as detailed design procedures are given by Christopher and Holtz (1985), Bonaparte et al. (1987), Holtz (1989 and 1990), Humphrey and Rowe (1991), Holtz et al. (1997), and Leroueil and Rowe (2001). The calculations required for stability and settlement utilize conventional geotechnical design procedures modified only for the presence of the reinforcement. Because the most critical condition for embankment stability is at the end of construction, the total stress method of analysis is usually performed, which is conservative since the analysis generally assumes that no strength gain occurs in the foundation soil. It is always possible of course to calculate stability in terms of effective stresses provided that effective stress shear strength parameters are available and an accurate estimate of the field pore pressures can be made during the project design phase. Because the prediction of in situ pore pressures in advance of construction is not easy, it is essential that the foundation be instrumented with high quality piezometers during construction to control the rate of embankment filling. Preloading and staged embankment construction are discussed in detail by Ladd (1991) and summarized by Leroueil and Rowe (2001).


Material Properties

Based on the stability calculations, the minimum geosynthetic strengths required for stability at an appropriate factor of safety can be determined. In addition to its tensile and frictional properties, drainage requirements, construction conditions, and environmental factors must also be considered. Geosynthetic properties required for reinforcement applications are given in Table 1.

Table 1. Geosynthetic properties required for reinforcement applications



When properly designed and selected, high-strength geotextiles or geogrids can provide adequate embankment reinforcement. Both materials can be used equally well, provided they have the requisite design properties. There are some differences in how they are installed, especially with respect to seaming and field workability. Also, at some very soft sites, especially where there is no root mat or vegetative layer, geogrids may require a lightweight geotextile separator to provide filtration and prevent contamination of the embankment fill. However, a geotextile separator is not required if the fill can adequately filter the foundation soil. A detailed discussion of geosynthetic properties and specifications is given by Holtz et al. (1997) and Koerner and Hsuan (2001), so only a few additional comments are given below. The selection of appropriate fill materials is also an important aspect of the design. When possible, granular fill is preferred, especially for the first few lifts above the geosynthetic.

Environmental Considerations
For most embankment reinforcement situations, geosynthetics have a high resistance to chemical and biological attack; therefore, chemical and biological compatibility is usually not a concern. However, in unusual situations such as very low (i.e., < 3) or very high (i.e., > 9) pH soils, or other unusual chemical environments (for example, in industrial areas or near mine or other waste dumps), chemical compatibility with the polymer(s) in the geosynthetic should be checked. It is important to assure it will retain the design strength at least until the underlying subsoil is strong enough to support the structure without reinforcement.

Constructability (Survivability) Requirements
In addition to the design strength requirements, the geotextile or geogrid must also have sufficient strength to survive construction. If the geosynthetic is ripped, punctured, torn or otherwise damaged during construction, its strength will be reduced and failure could result. Constructability property requirements are listed in Table 1. (These are also called survivability requirements.) See Christopher and Holtz (1985) and Holtz et al. (1997) for specific property requirements for reinforced embankment construction with varying subgrade conditions, construction equipment, and lift thicknesses. For all critical applications, high to very high survivability geotextiles and geogrids are recommended

Stiffness and Workability
For extremely soft soil conditions, geosynthetic stiffness or workability may be an important consideration. The workability of a geosynthetic is its ability to support workpersons during initial placement and seaming operations and to support construction equipment during the first lift placement. Workability is generally related to geosynthetic stiffness; however, stiffness evaluation techniques and correlations with field workability are very poor (Tan, 1990). See Holtz et al. (1997) for recommendations on stiffness.

Construction
The importance of proper construction procedures for geosynthetic reinforced embankments cannot be overemphasized. A specific construction sequence is usually required in order to avoid failures during construction. Appropriate site preparation, low ground pressure equipment, small initial lift thicknesses, and partially loaded hauling vehicles may be required. Clean granular fill is recommended especially for the first few construction lifts, and proper fill placement, spreading, and compaction procedures are very important. A detailed discussion of construction procedures for reinforced embankments on very soft foundations is given by Christopher and Holtz (1985) and Holtz et al. (1997). It should be noted that all geosynthetic seams must be positively joined. For geotextiles, this means sewing; for geogrids, some type of positive clamping arrangement must be used. Careful inspection is essential, as the seams are the “weak link” in the system, and seam failures are common in improperly constructed embankments. Finally, soft ground construction projects usually require geotechnical instrumentation for proper control of construction and fill placement; see Holtz (1989) and Holtz et al. (2001a) for recommendations.



Reference :
R.D. Holtz, Ph.D., P.E., Geosynthetics Soil Reinforcement, Department of Civil & Environmental Engineering, University of Washington

Using Geogrid in Flexible Pavement

Geogrid technology has developed steadily since the products werefirst introduced in the early 1980’s. The initial geogrid products rapidlygained popularity within the civil engineering industry, principallydue to their ability to provide simple, cost-effective solutions in variousroadway and grade separation applications.They have gained widespread acceptance over the last 25 yearsas a solution to problems associated with roads constructed on softor problematic subgrades, but their use with roads on competentsubgrades has been less common. Clear, well-established, design methodology from the AmericanAssociation of State Highway and Transportation Officials (AASHTO)is now available that allows the design engineer to quantify the benefitsof using geogrids to extend pavement design life. This approach canbe applied for the design of major highways or light duty pavementsassociated with local housing or retail store developments.


Geogrid Composition



A geogrid is a regular grid structure of polymeric material used toreinforce soil or other geotechnical engineering related materials.Products are generally classified as either Uniaxial Geogrids orBiaxial Geogrids, depending on whether their strength is predomi-nantly in one or two directions. Uniaxial Geogrids are principally used in grade separation appli-cations for retaining walls and steep slopes. Biaxial Geogrids areused mainly in roadway applications to either stabilize a soft subgrade, or to provide reinforcement to the unbound base coursematerials (referred to as base reinforcement). Benefits of usingBiaxial Geogrids for base reinforcement are typically a reduction ofrequired base course material thickness and/or a significant extension in service life of the pavement structure. In base reinforcement applications, the existing subgrade is of afirm nature or has been rendered such through the use of a subgradeimprovement technique. One of the principal failure mechanisms ofa pavement under these firm subsoil conditions is rutting–resultingfrom progressive lateral movement of the aggregate base courseduring traffic loading. The amount of lateral movement can be reduced greatly byincluding a Biaxial Geogrid within or at the bottom of the basecourse layer. Partial penetration of coarse aggregate particlesthrough the geogrid apertures and subsequent compaction, resultsin “mechanical interlock” or “confinement” of the aggregate particles.


Application Benefits



The principal benefit of using a geogrid within the unbound aggregatecomponent of a flexible pavement is less rutting at the surface. Thisis due to reduced lateral spreading of the unbound aggregate. However, an additional feature of the reinforcement is that thegeogrid confined aggregate results in a much stiffer base courselayer and a lower dynamic deflection of the pavement structureduring traffic loading. Fatigue cracking of the asphalt is thereforealso reduced due to the presence of the geogrid reinforcement.In order for geogrids to work successfully in base reinforcementapplications, they must have the capacity to facilitate efficient loadtransfer between the aggregate and the geogrid. Webster (1992) reported a large-scale research program undertakenby the U.S. Army Corps of Engineers to investigate and determinethe key physical properties of a geogrid required to create optimalinteraction and load transfer. A summary of the key material propertiesdetermined in the study are presented in the table below.Key geogrid properties as determined by the U.S. Army Corps of Engineers.


Expanded Use



As the population of our towns and cities continues to expand rapidly,new or recently constructed housing in the form of sub-division devel-opments are becoming increasingly commonplace. One of the morefrequent problems associated with the roads in these developmentsis adirect result of their method of construction.Phased construction has become an extremely common practice,particularly in residential developments. In order to build a roadwayto gain site access, contractors will initially placethe aggregate component of the pavement and,usually, a thin asphalt layer on top. This techniqueis particularlyuseful when local trenches arerequired for installation of utilitypipes and cables. Pavement distress in the form of asphalt crackingat the surface is common on phased roads withinsub-divisions. In many cases, these “alligatorcracks” start to appear within a very short periodof time following construction – perhaps as little asone or two years. The simple solution to this problem is a layer ofBiaxial Geogrid installed at the bottom or withinthe base course during initial construction. Forrelatively little additional expense at the start of construction, thelifetime of their road is extended enormously, while expensive anddisruptive rehabilitation or reconstruction activities are avoided.Another use of geogrid technology can be found in the developmentof pavements around retail stores. Typically, thicker heavy-duty pavements are adopted in the loading areas around suchstores, while thinner lighter duty pavements are used for the carparking areas.
One of the main problems associated with this approach is thepotential for a “bath tub” effect – this is where the subgrade is at alower level in the areas of the heavy duty pavements. These areas areprone to water ingress and build up resulting in a reduction in thelong-term strength of the pavement. In colder regions, these areas are also more susceptible to theeffects of freeze-thaw activity. Both of these situations result in areduction in the design life of the pavement but there are additionalpractical problems for the contractor associated with this more complicated method of construction.In addition to offering protection against the “bath tub” problemsdescribed above, the reinforced sections offer significant material cost savings. Additional benefits result from increased speed of construction – fewer stake out procedures, less undercut/disposal offill, simpler construction, etc.


A Glimpse Into The Future

Currently, AASHTO provides guidelines for the design of flexible pave-ments in their current design guide (AASHTO,1993) and in InterimStandard PP46-01 (AASHTO, 2001). New pavement design approaches,based on advanced mechanistic-empirical (M-E) principles, are beingdeveloped and refined by AASHTO and other entities. A few DOT’s have already made the leap to improved M-E designmethods, but most are still awaiting official publication of AASHTO’snew design guide which will advocate adoption of this approach topavement design. Official publication of the new AASHTO designguide may still be several years out, but the availability of a M-E baseddesign method incorporatinggeogrids within the pavement structureis currently being finalized by The University of Illinois atChampaign-Urbana.

Geosynthetics Terminology

Bituminous geomembrane : see Geomembrane, bituminous.

Bonded geogrid : see Geogrid, bonded.

Drainage composite : see Geocomposite drain.

Elastomeric geomembrane : see Geomembrane, elastomeric.

Electrokinetic geosynthetic : A composite material which may provide filtration, drainage, reinforcement in addition to electrical conduction.

Extruded geogrid : see Geogrid, extruded.

Geoarmour : A permeable geosynthetic material placed over the surface of the soil, in conjunction with pattern-placed block armour units, to prevent erosion.

Geobar : A polymeric material in the form of a bar, used in contact with soil/rock and/or any other geotechnical material in civil engineering applications.

Geoblanket : A permeable, biodegradable (synthetic or natural) structure placed over the soil for temporary erosion control applications, usually while vegetation is being established.

Geocell : A three-dimensional, permeable, polymeric (synthetic or natural) honeycomb or web structure, made of strips of geotextiles, geogrids or geomembranes linked alternatingly and used in contact with soil/rock and/or any other geotechnical material in civil engineering applications.

Geocomposite : A manufactured or assembled material using at least one geosynthetic product among the components, used in contact with soil/rock and/or any other geotechnical material in civil engineering applications.

Geocomposite clay liner : An assembled structure of geosynthetic materials and low hydraulic conductivity earth materials (clay or bentonite), in the form of a manufactured sheet, used in contact with soil/rock and/or any other geotechnical material in civil engineering applications.

Geocomposite drain : A prefabricated subsurface drainage product which consists of a geotextile filter skin supported by a geonet or a geospacer.

Geocomposite reinforcement : An assembled structure of dissimilar geosynthetic materials used for soil reinforcement.

Geofoam : A polymeric material which has been formed by the application of the polymer in semi-liquid form, through the use of a foaming agent, and results in a lightweight material with high void content, used in contact with soil/rock and/or any other geotechnical material in civil engineering applications.

Geoform : A three-dimensional, permeable geosynthetic structure, filled with soil or sediment waste such that the fill takes the shape of the inflated geoform.

Geogrid : A planar, polymeric structure consisting of a regular open network of integrally connected tensile elements, which may be linked by extrusion, bonding or interlacing, whose openings are larger than the constituents, used in contact with soil/rock and/or any other geotechnical material in civil engineering applications.

Geogrid, bonded : A geogrid manufactured by bonding, usually at right angles, two or more sets of strands or elements.

Geogrid, extruded : A geogrid manufactured by extruding polymers and drawing in a sheet form.

Geogrid, knitted : A geogrid manufactured by knitting together yarns or elements, usually at right angles to each other.

Geogrid, woven : A geogrid manufactured by weaving yarns or elements, usually at right angles to each other.

Geomat : A three-dimensional, permeable, polymeric structure, made of bonded filaments, used to reinforce roots of grass and small plants and extend the erosion-control limits of vegetation for permanent erosion control applications.

Geomattress : A three-dimensional, permeable geosynthetic structure, placed over the surface of a soil, and then filled with concrete mortar or soil, to prevent erosion.

Geomembrane : A planar, relatively impermeable, polymeric (synthetic or natural) sheet used in contact with soil/rock and/or any other geotechnical material in civil engineering applications.

Geomembrane, bituminous : A planar, relatively impermeable sheet manufactured from natural bituminous materials.

Geomembrane, elastomeric : A planar, relatively impermeable sheet manufactured from elastomeric polymers. Geomembrane, plastomeric: A planar, relatively impermeable sheet manufactured from plastomeric polymers.

Geonet : A planar, polymeric structure consisting of a regular dense network, whose constituent elements are linked by knots or extrusions and whose openings are much larger than the constituents, used in contact with soil/rock and/or any other geotechnical material in civil engineering applications.

Geospacer : A three-dimensional polymeric structure with large void spaces, used in contact with soil/rock and/or any other geotechnical material in civil engineering applications.

Geostrip : A polymeric material in the form of a strip, used in contact with soil/rock and/or any other geotechnical material in civil engineering applications.

Geosynthetic : A planar, polymeric (synthetic or natural) material used in contact with soil/rock and/or any other geotechnical material in civil engineering applications.

Geotextile : A planar, permeable, polymeric (synthetic or natural) textile material, which may be nonwoven, knitted or woven, used in contact with soil/rock and/or any other geotechnical material in civil engineering applications.

Geotextile, knitted : A geotextile produced by interlooping one or more yarns, fibres, filaments or other elements.

Geotextile, nonwoven : A geotextile in the form of a manufactured sheet, web or batt of directionally or randomly orientated fibres, filaments or other elements, mechanically and/or thermally and/or chemically bonded.

Geotextile, woven : A geotextile produced by interlacing, usually at right angles, two or more sets of yarns, fibres, filaments, tapes or other elements.

Knitted geogrid : see Geogrid, knitted.

Knitted geotextile : see Geotextile, knitted.

Nonwoven geotextile : see Geotextile, nonwoven.

Plastomeric geomembrane : see Geomembrane, plastomeric.

Woven geogrid : see Geogrid, woven.

Woven geotextile : see Geotextile, woven.

Fundamental of Geosynthetics

Definition

Geosynthetics is the catch all term used to describe a range of generally synthetic products used to solve geotechnical problems. The term is generally regarded to encompass four main products: geotextiles, geonets/geogrids, geomembranes and geocomposites. The synthetic nature of the products make them suitable for use in the ground where high levels of durability are required. Needless to say, those materials may be destructible. Geosynthetics are available in a wide range of forms and materials, each to suit a slightly different end use. These products have a wide range of applications and are currently used in many civil and geotechnical engineering applications including roads, airfields, railroads, embankments, retaining structures, reservoirs, canals, dams, bank protection and coastal engineering.

Types and Material

Geotextiles These are usually produced as either woven or non-woven textiles. Woven geotextiles are produced by the interlacing of yarns to leave a finished material that has a discernible warp and weft^[3]. Non-woven geotextiles are produced by various methods other than weaving, mainly heat bonded, needle punched and chemically bonded. Woven and non-woven geotextiles are manufactured from mainly polymeric yarns and fibres, consisting primarily of polypropylene, polyester, polyethylene and polyamide. There are a small group of Geotextiles that are still produced from fibrous materials, used mainly in erosion control. Their degradable characteristics are beneficial in some applications.

Geogrids/Geonets These are discernibly stiffer than geotextiles and have relatively large voids within the material. Methods of production vary but include extrusion, bonding or interlacing. They can be produced from nearly all polymeric materials. These are used to strengthen fill materials in geotechnical applications. They provide increased shear strength between soil strata interfaces. Their tensile strength can prevent or decrease the degree of differential settlement in some applications by transmitting the load over a broader area of soil thereby diminishing the vertical stress in the soil.

Geomembranes Essentially impermeable sheets produced from polymeric materials. Geomembranes are manufactured several ways, excluding woven methods as they would leave unacceptably large voids in the material. Suitable materials include PVC, Polypropylene, Polyethylene and HDPE.

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

Applications

Among other uses, geosynthetics can be used for Separation, Filtration, Reinforcement, Drainage, Protection and Moisture Barriers^[4]. Different geosynthetics are suited for various applications and the diagram to the right illustrates their suitability.

Filtration can significantly enhance the performance of a geotechnical structure, and geosynthetics can be used to produce an effective filtration system^[5]. The job of a filter is to allow water to pass through the plane of the filter, whilst retaining particles of the filtered soil. Filtration can improve the performance of a geotechnical structure by controlling the erosion of the structure and reducing the amount of fines that are washed out of the soil matrix. When fines get washed out of a soil it can reduce the cohesion of the matrix and thus the strength of the soil, referred to piping. Mitigating these two problems also improves the durability of a structure. Geosynthetic filters can improve the reliability and performance of traditional graded soil filters and require less work to construct. Geotextiles are well suited to this application.

Drainage required in nearly all geotechnical structures. Whether used to remove surface water from a sports field, or to reduce lateral pressure on a retaining wall, the need for effective drainage cannot be underestimated. Drains of various designs have been used in the past, most based on the use of a high permeability layer built into the ground using aggregates, single layers of Geosynthetics can produce the same results. Drains can be distinguished from filters as such; water travels across the plane of filters and travels with the plane of drains. Geotextiles and geocomposites are well suited to this application.

Protection/Barrier In some geotechnical applications it is necessary to separate or protect one section of the works from another. This could be for a multitude of reasons, including stopping leachate seepage, protecting a structure from moisture and protecting a geotechnical structure from erosion. Geotextiles and Geomembranes are suited to this application.

Separation The geosynthetic acts to separate two layers of soil that have different particle size distributions. For example, geotextiles are used to prevent road base materials from penetrating into soft underlying soils, thus maintaining design thickness and roadway integrity. Separators also help prevent fine-grained subgrade soils from being pumped into permeable granular road bases^[6]. Geotextiles and geomembranes are most suited to this application.

Reinforcement Geosynthethics can be used to reinforce a soil mass in, increasing the effective angle of shear and increasing the stability of an earth structure. In the reinforcement function, the geosynthetic is subjected to a sustained tensile force. Soil and rock materials are noted for their ability to withstand compressive forces and their relative low capacity for sustained tensile forces. In much the same way that tensile forces are taken up by steel in a reinforced concrete beam, the geosynthetic supports tensile forces that cannot be carried by the soil in a soil/Geosynthetic system. Geogrid/geonets and geotextiles are best suited to this function.