Geosynthetics

 

Geosynthetics is the term coined to describe a class of synthetic materials that has been developed for geotechnical applications. Essentially this refers to applications relating to geological materials, earth structures and foundations. Geotextiles made from natural fibers have been used for thousands of years. For example, they were used to stabilize roadways in ancient Egypt, where the dryness of the climate offset natural fibers' tendency to deteriorate when submerged in the soil.

 

What Are Geosynthetics?

Geosynthetics are man-made materials used to improve soil conditions. The word is derived from:

Geo = earth or soil + Synthetics = man-made

Geosynthetics are typically made from petrochemical-based polymers (“plastics”)

that are biologically inert and will not decompose from bacterial or fungal action.

While most are essentially chemical inert, some may be damaged by petrochemicals

and most have some degree of susceptibility to ultraviolet light (sunlight).

Geosynthetic materials are placed on or in soil to do one of four things (some may

perform more than one of these functions simultaneously):

• separation/confinement/distribute loads

- improve level-grade soil situations such as roads, alleys, lane ways

- improve sloped-grade situations such as banks, hillsides, stream access points

• reinforce soil

- soil walls, bridge abutments, box culverts/bridges, and soil arches

• prevent soil movement (piping) while letting water move through the material

- such as in drainage systems and back fill around water intakes

• controlling water pressure allowing flow (drainage) in the plane of the material

-          such as on foundation walls to allow water to move down to perimeter drains

 

  Types of geosynthetics

1. geotextiles, used for drainage, separation and reinforcement, are in two forms

- woven - cloth-like materials with fibers woven perpendicular to each other

- non-woven - felt-like materials with randomly-oriented fibers

2.geogrids are open mesh-like materials used for stabilization and reinforcement

3.geonets are cavity-like materials in a web used for stabilization

4.geomembranes are very low permeability liner or fluid containment materials

5.geosynthetic clay liner

6.geofoam

 

 

 

2.Geotextiles

           Geotextiles are defined as “any permeable textile used with foundation soil, rock, earth, or any other geotechnical engineering-related material as an integral part of a human-made project, structure, or system”.

           Geotextiles form one of the two largest categories of geosynthetic materials. Their rise in growth during the past 40-years has been nothing short of outstanding. They are indeed textiles in the traditional sense, but consist of synthetic fibers (all are polymer-based) rather than natural ones such as cotton, wool, or silk. Thus, biodegradation and subsequent short lifetime is not a problem. These synthetic fibers are made into flexible, porous fabrics by standard weaving machinery or they are matted together in a random nonwoven manner. Some are also knitted. The major point is that geotextiles are porous to liquid flow across their manufactured plane and also (to a limited extent) within their thickness. There are hundreds of specific application areas for geotextiles that have been developed; however, the fabric always performs at least one of four discrete functions; separation, reinforcement, filtration and/or drainage. as they are known and used today, were first used in filtration applications and were intended to be an alternative to granular soil (sand and gravel) filters. Thus the original, and still sometimes used, term for geotextiles is filter fabrics. Barrett tells of work originating in the late 1950s using geotextiles behind precast concrete seawalls, under precast concrete erosion control blocks, beneath large stone riprap, and in other erosion control situations. He used different styles of woven monofilament fabrics, all characterized by a relatively high percentage open area (varying from 6 to 30%), since sand was the soil being retained. He discussed the need for both adequate permeability and soil retention, along with adequate fabric strength and minimal elongation and set the tone for geotextile use in granular soil filtration situations.

            In the late 1960s Rhone-Poulenc Textiles in France began working with nonwoven  needle-punched fabrics for somewhat different applications. Here emphasis was on unpaved roads, beneath railroad ballast, within embankments and earth dams, and the like. The primary function in many of these applications was that of separation and reinforcement. Additionally, a quite different use of this particular style of fabric was also recognized, that is, that thick nonwoven fabrics can also transmit water within the plane of their structure (i.e., they can act as drains). Such uses as dissipation of pore-water pressures and liquid flow interceptors, grew out of this particular drainage function. Today's use of the word geotextiles recognizes these many possible functions of fabrics when used within a soil mass.

 

Geotextiles can be produced as a non-woven, a knitted, or a woven fabric We will focus on the nonwoven and woven fabrics since knitted fabric is rarely used Whether the fabric is woven or non-woven is an important characteristic in choosing a geotextile for a particular use.

 

 

 

2.1.Woven.

These cloth-like fabrics are formed by the uniform and regular interweaving

of threads or yarns in two directions as shown in Figure 1, below. These products have a regular visible construction pattern, and where present, have distinct and measurable openings. Woven geotextiles are typically used for soil separation, reinforcement, load distribution, filtration, and drainage. They can have high tensile strength and relative low strain or limited elongation under load (typically up to 15%).

Figure 1 A Typical Woven Geotextile (Enlarged View)

 

2.2.Non-Woven.

These felt-like fabrics are formed by a random placement of threads in

a mat and bonded by heat-bonding, resin-bonding or needle punching, as shown in Figure 2, below. These products do not have any visible thread pattern. Non-woven geotextiles are typically used for soil separation, stabilization, load distribution, and drainage but not for soil reinforcement such as in retaining walls. They have a relatively high strain and stretch considerably under load (about 50%).

Figure 2 A Typical Non-Woven Geotextile (Enlarged View)

In the road industry there are four primary uses for geotextiles: separation, drainage, filtration and reinforcement

In separation, inserting a properly designed geotextile will keep layers of different sized particles separated from one another. In drainage, water is allowed to pass either downward through the geotextile into the subsoil, or laterally within the geotextile which functions as a drain How it is used depends on the drainage requirements of the application. In filtration, the fabric allows water to move through the soil while restricting the movement of soil particles. In reinforcement, the geotextile can actually strengthen the earth or it can increase apparent soil support For example, whenplaced on sand it distributes the load evenly to reduce rutting.

Geotextiles now are most widely used for stabilizing roads through separation and drainage When the native soil beneath a road is very silty, or constantly wet and mucky, for example, its natural strength may be too low to support common traffic loads, and it has a tendency to shift under those loads Although the subgrade may be reinforced with a base course of gravel, water moving upward carries soil fines or silt particles into the gravel, reducing its strength Geotextiles keep the layers of subgrade and base materials separate and manage water movement through or off the roadbed However, a layer of geotextile cannot be used as a substitute for using an adequate thickness of free draining

soil (like clean sand) to reduce frost heaving.

 

2.3.Functions

2.3.1Geotextiles for separation

 

In separation functions geotextiles keep fines in the subgrade from migrating into the base course Tests show that it takes only about 20% by weight of subgrade soil mixed into the base course to reduce its bearing capacity to that of the subgrade.

This problem usually is due to the movement of large amounts of water, when large loads cross the surface of the roadway they set up a pumping action which accelerates this water movement and soil particle migration, and speeds up the failure of the road.

Two important criteria for selecting a geotextile for separation are permeability and strength The geotextile used for separation must allow water to move through it while retaining the soil fines or sand particles It should let water pass through it at the same rate or slightly faster than the adjacent soil It must also retain the smallest soil particle size without clogging or plugging. To select a geotextile, you will need to know the grain size distribution of the subgrade and the subbase as well as the permeability of the geotextile.

In selecting a specific geotextile for separation you must consider its basic strength properties Be sure to take into account how its physical properties will survive the construction process as well as how it will survive the pressures of traffic on the gravel cover and enhance the life of the road These strength properties are described in manufacturers’ literature and design manuals in a variety of terms including burst and abrasion resistance, and puncture, grab, and tearing strength.

 

2.3.2.Geotextiles in runoff and sediment control

 

Most units of government are responsible for erosion, runoff and sediment control, both during construction and afterwards until vegetation is established A variety of statutes, ordinances and other regulations establish this responsibility You can use geotextile fabrics as silt fences to hold back sediments carried in snow melt or precipitation runoff, and in seeding and mulching operations.

Erecting a silt fence can be relatively simple, but should follow certain standards:

1. Select a geotextile fabric permeable enough that runoff will flow through the fence and not overtop or bypass it.

2. The perforations or permeability should be small enough to retain the smallest soil particle, but not so small as to plug immediately Monitor silt fences periodically and remove silt so the fences will remain effective.

3. Erect silt fences with adequate support to withstand the hydraulic pressures it will bear from one side during peak runoff periods Space supporting posts two to ten feet apart and use wire to reinforce the downstream side of the geotextile

Geotextiles have also been very successful in seeding and mulching operations when properly applied There are no design standards or comparative records for this use which recommend one specific type of geotextile over another Where you anticipate intense precipitation, it might be worthwhile to consider using

geotextile-related materials –mats and grids, or meshes -instead of just a geotextile fabric because they are less likely to wash down the hill.

Where you anticipate rapid vegetation growth, consider using geotextiles made of natural materials which will degrade rapidly In other situations, the synthetic fabrics will become entwined with the plants’ root systems providing permanent erosion protection In either case the mats or fabrics will permit seedlings to root and grow through the opening without any negative consequences.

It is very important that the fabric will be in close contact with the soil Staples or pins can be used to secure the fabric. It is also important to keep runoff water from undercutting the fabric ends at the top and bottom of the hill Thoroughly secure fabric ends by imbedding them in small trenches, or use staples, pins or any method which will keep them in place.

 

2.3.3.Geotextiles for erosion control

 

Geotextiles can be used many ways for erosion control, One of these is with rip-rap along stream banks, lake shores, and other bodies of water to keep finer soils beneath the rip-rap from eroding Geotextiles recommended for erosion control should have permeability, resistance to abrasion, and high resistance to

ultraviolet rays as primary considerations.

Erosion control covers a variety of conditions from high velocity stream flow to heavy wave action, to less severe conditions All conditions should be considered before selecting a fabric.

The following instructions describe how to install geotextiles on stream banks and similar steep

slopes These may be modified for applying geotextiles in less severe conditions such as rip-rapping in ditches.

Geotextile/rip-rap installations may also be used in specifically designed systems to protect against scouring around bridge piers and abutments, and in other water installations.

To install geotextiles for any riprap system:

1.Before starting, review such design considerations as wave action, bank steepness, etc.

2.Identify soils by particle size and permeability as these will determine certain geotextile specifications.

3. Identify the size of rip-rap planned for this application.

4. Review past weather and climate conditions for such information as levels of ice, wave action, and amount of sunlight for their effect on riprap/ geotextile installations Ultraviolet rays in sunlight deteriorate most synthetic materials If exposure to ultraviolet rays is anticipated, select a geotextile with high resistance to ultraviolet rays.

5. Depending on the type of installation and the care it will need, you may have to consider abrasion to ensure that the geotextile will survive installation.

            The protected soil surface should be as smooth as possible Remove large stones, roots and other materials that might project and puncture or tear the fabric during construction and installation Then place the fabric loosely and overlap it as required Sewing the seams is preferable Pin or weight down the fabric so that you can place the rip-rap without the fabric bubbling, shifting or slipping.

            Always being placing rip-rap at the base of the slope and move upward, and from the center of the textile strip to its side edges Do not allow stones weighing over 100 pounds to roll Specify a minimal drop height of one foot for stones up to 250 pounds and no freefall for stones exceeding 250 pounds If fabric is on a cushion layer, height drops can be up to three feet for stones less than 250 pounds, with no freefall for stones greater than 250 pounds Avoid machine grading or any method of shifting rip-rap after it is placed unless the fabric is covered sufficiently to avoid damage.

 

2.4.Summary

 

With experience, geotextiles are being used more often in road construction and maintenance, Certain fundamental considerations are necessary for success in any application, You must know the soils to select the proper geotextile Study the application thoroughly to determine the severity of conditions facing the geotextile.

In many installations, permeability may override concern for durability and resistance to bursting, puncturing and tearing. In other installations, such as a separator in a road where the geotextile will be subjected to severe loads, durability is of concern permeability should also always be considered in separation uses to allow moisture to move freely through the system. This avoids excessive hydrostatic

pressures which cause soil failure.

Most geotextile system failures result from improper installation, improper selection of fabrics, a change of conditions from the original design, or a combination of these factors.

Many states have successfully used geotextiles for stabilization Here, too, you should carefully determine the type and frequency of usage for these roads since heavy, high speed traffic could

cause premature failure of the system.

Manufacturers’ technical manuals will help guide you in installation techniques and fabric selection for that manufacturer’s products.

 

 

 

3.Geogrids

These are open grid-like materials of integrally connected polymers, as shown in

Figure below. They are used primarily for soil reinforcement. Their strength can

be greater than the more common geotextiles. Geogrids have a low strain and stretch

only about 2 to 5% under load. Where practicable they would likely be used in heavy load or high demand agricultural situations.            

A Typical Geogrid

Geogrids represent a rapidly growing category within geosynthetics. Rather than being a woven, nonwoven or knitted textile fabric, geogrids are polymeric materials formed into a very open, grid like configuration, i.e., they have large apertures between individual ribs in the machine and cross machine directions. Geogrids are                      (a) homogeneously stretched from perforated polymer sheets in one or two directions for improved physical properties,

(b) made from yarns on weaving or knitting machinery by standard textile manufacturing methods and then coated, or

(c) by bonding polymeric rods or straps together. There are many specific application areas, however, they function almost exclusively as reinforcement materials.

The development of methods of preparing relatively rigid polymeric materials by tensile drawing, in a sense "cold working," raised the possibility that such materials could be used in the reinforcement of soils for walls, steep slopes, roadway bases and foundation soils. Used as such, the major function of the resulting geogrids is in the area of reinforcement. This area, as with many other geosynthetics, is very active, with a number of different products, materials, configurations, etc., making up today's geogrid market. The key feature of all geogrids is that the openings between the adjacent sets of longitudinal and transverse ribs, called “apertures,” are large enough to allow for soil strike-through from one side of the geogrid to the other. The ribs of some geogrids are often quite stiff compared to the fibers of geotextiles. As will be discussed later, not only is rib strength important, but junction strength is also important. The reason for this is that in anchorage situations the soil strike-through within the apertures bears against the transverse ribs, which transmits the load to the longitudinal ribs via the junctions. The junctions are, of course, where the longitudinal and transverse ribs meet and are connected. They are sometimes called “nodes”.

Currently there are three categories of geogrids. The first, and original, geogrids (called unitized or homogeneous types) were made in the United Kingdom by Netlon, Ltd., and were brought in 1982 to North America by the Tensar Corporation. A conference in 1984 was helpful in bringing geogrids to the engineering design community. A similar type of drawn geogrid which originated in Italy by Tenax is also available, as are products by new manufacturers in Asia. The second category of geogrids are more flexible, textile-like geogrids using bundles of polypropylene coated polyester fibers as the reinforcing component. They were developed first by ICI in the United Kingdom around 1980. This led to the development of polyester yarn geogrids made on textile weaving machinery. In this process hundreds of continuous fibers are gathered together to form yarns which are woven into longitudinal and transverse ribs with large open spaces between. The cross-overs are joined by knitting or intertwining before the entire unit is protected by a subsequent coating. Bitumen, latex or PVC are the usual coating materials. Geosynthetics within this group are manufactured by many companies having various trademarked products. There are possibly as many as 25 companies manufacturing coated yarn-type polyester geogrids on a worldwide basis. The third category of geogrids are made by laser or ultrasonically bonding together polyester or polypropylene rods or straps in a gridlike pattern. Two manufacturers currently make such geogrids.

           The geogrid area is extremely active not only in manufacturing new products, but also in providing significant technical information to aid the design engineer.

 

 

 

 

 

 

 

4.Geonets

Geonets, called geospacers by some, constitute another specialized category within the geosynthetics area. They are formed by continuous extrusion of parallel sets of polymeric ribs at preset angles to one another. When the ribs are opened, relatively large apertures are formed into a netlike configuration. They are usually factory fabricated with one or two geotextiles on their surfaces. Their design function is completely within the in-plane drainage area where they are used to convey all types of liquids.

Geonets were originally developed by Sir Bryan Mercer, of Netlon, Ltd. in the United Kingdom. Mercer patented the machinery and processing methods for the lightweight plastic nets commonly seen in supermarkets for carrying produce, fruits and vegetables. Experimentation with gradually thicker ribs in various configurations led to robust drainage nets of the type used in geosynthetic engineering. The first known use of geonets was in 1984 for the environmental application of leak detection in a double lined hazardous liquid waste impoundment in Hopewell, Virginia. Geonets are indeed grid-like materials but their use dictates a separate identity. The reason for their separate treatment from geogrids lies not in the material or its configuration, but in its function. Geonets are used for their in-plane drainage capability, while geogrids are used for reinforcement. It should be stated at the outset, however, that geonets are not weak, flimsy materials. They have reasonable tensile strength, but are used exclusively in drainage applications. Note that geonets are generally used with one or two geotextiles on their upper and/or lower surfaces to prevent soil intrusion into the apertures which would tend to block the in-plane drainage function of the material. Hence, they are often manufactured as a composite and are then referred to as a geocomposite but in so doing are best referred to as a drainage composite. They certainly deserve mention in their own right. They can also be used by themselves—for example, when placed between two geomembranes.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

5.Geomembranes

Whereas geotextiles, geogrids and geocells are usually porous to allow water to filter through them, geomembranes are polymer sheets used to control fluid movement.

These materials have very low permeability and would be used for lining ponds, pits etc to control leachate. They may be used over top of a geotextile.

           Geomembranes represent the other largest category of geosynthetics and in dollar volume their sales are even greater than that of geotextiles. Case histories of reservoir liners date from the 1950's, but the major growth in the USA and Germany was stimulated by governmental regulations originally enacted in the early 1980’s . The materials themselves are relatively thin impervious sheets of polymeric materials used primarily for linings and covers of liquid- or solid-storage facilities. This includes all types of landfills, reservoirs, canals, tunnels and other containment facilities. Thus the primary function is always containment thereby functioning as a liquid and/or vapor barrier. The range of applications is very great, and in addition to the geoenvironmental area, applications are rapidly growing in geotechnical, transportation, hydraulic, and private development engineering.

In 1839, Charles Goodyear cured (via vulcanization) natural rubber with sulfur, resulting in a synthetic rubber which is the current classification of thermoset polymers. The impetus was the inherent instability of natural (gum) rubber which was brittle in cold weather and sticky in hot weather. Today, the production of synthetic rubber materials is a major industry. The original geomembrane for use in civil engineering applications was a rubber product and was used as a waste water pond liner. It was made from butyl rubber, which is a copolymer of isobutylene with about 2% isoprene. Butyl rubber is quite impermeable and presently has its major use as inner tubes and as the liners of tubeless tires. Many other combinations and variants of rubber materials are possible, e.g., nitrile and EPDM. Since the 1980’s, however, the geosynthetics industry has shifted from thermoset polymers to thermoplastic polymers; the exception being EPDM geomembranes. Thus, almost all of the geomembrane materials used in civil engineering fall into the category of polymers classified as thermoplastic materials. By definition, thermoplastic materials become soft and pliable when heated without any substantial change in inherent properties and when cooled revert back to their original properties. They are readily seamed by heat, extrusion or chemical methods.

Some of the resins used to manufacture today’s polymeric geomembranes are described as follows. Polyethylene is formed by the polymerization of compounds containing an unsaturated bond between two carbon atoms. Production in quantity began in 1943. Its main original uses were (and continue to be) in the packaging and molding industries. Polyethylene, in its various densities, is the most widely used polymer in the manufacturing of geomembranes. A related polyolefin is polypropylene. The development of crystallizing polypropylene is an outgrowth of low-pressure polymerization of ethylene and is the basic material from which many geosynthetics are made. Polyvinyl chloride is yet another common resin used to manufacture plastic pipe and, when plasticized, geomembranes. This resin was developed in 1939 and has extensive uses. It ranks second in use to the various density polyethylenes. It is interesting to note that polyethylene geomembranes were first used in Europe and South Africa and moved to North America, while polyvinyl chloride used for geomembranes had its roots in the U.S. and moved to Europe and elsewhere. Other types of geomembranes, were being developed in the 1960’s and used by the U.S. Bureau of Reclamation. These geomembranes served primarily as canal liners, and their use spread to Canada, Hawaii, Russia, Taiwan, and Europe. Another early geomembrane, chlorosulfonated polyethylene (CSPE), resulting from the reaction of chlorine and sulfur chloride on polyethylene, was introduced for reservoir and landfill liners in the late 1960's and this geomembrane type was used in Europe shortly thereafter. Today’s polymeric geomembranes are made from the above different thermoplastic resins and are manufactured and distributed the world over, making all types of products readily available. The area of geomembranes is probably the largest of the geosynthetic material categories insofar as sales volume is concerned.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

6.Geosynthetic Clay Liners

           Geosynthetic clay liners, or GCLs, are an interesting juxtaposition of polymeric materials and natural soils. They are rolls of factory fabricated thin layers of bentonite clay sandwiched between two geotextiles or bonded to a geomembrane. Structural integrity of the subsequent composite is obtained by needle-punching, stitching or physical bonding. GCLs are used as a composite component beneath a geomembrane or by themselves in geoenvironmental and containment applications as well as in transportation, geotechnical, hydraulic, and many private development applications

Geosynthetic clay liners (or GCLs) are factory manufactured hydraulic barriers consisting of a thin layer of bentonite (or other very low permeability material) supported by geotextiles and/or geomembranes, being mechanically held together by needling, stitching, or chemical adhesives. Sodium bentonite is the usual type, but calcium bentonite can be modified to give a similar product.

The use of GCLs as a separate category of geosynthetics appears to have been in the U.S. in 1988 in solid waste containment as a backup to a geomembrane. The product was Claymax® which is bentonite mixed with an adhesive so as to bond the clay between two geotextiles; one below (the carrier textile) and the other above (the cover textile) the bentonite in the center. About the same time a different product in Germany, Bentofix®, was manufactured by placing bentonite powder between two geotextiles and then needle punching the three-component system together.

Other names used for GCLs since their initiation are “clay blankets”, “bentonite blankets”, “bentonite mats”, “prefabricated bentonite clay blankets” and “clay geosynthetic barriers”, the latter currently favored by the International Organization for Standardization (ISO). The engineering function of a GCL is containment as a hydraulic barrier to water, leachate or other liquids and sometimes gases. As such, they are used as replacements to either compacted clay liners or geomembranes, or they are used in a composite manner to augment the more traditional liner materials. The ultimate in liner security is probably a three component composite geomembrane/geosynthetic clay liner/compacted clay liner which has seen use as a landfill liner in many occasions.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

7.Geofoam

           Geofoam is a bulky product created by a polymeric expansion process resulting in a “foam” consisting of many closed, but gas-filled, cells. The skeletal nature of the cell walls is the unexpanded polymeric material. The resulting product is generally in the form of large, but extremely light, blocks which are stacked side-by-side providing lightweight fill in numerous applications. The primary function is dictated by the application; however separation is always a consideration and geofoam is included in this category rather than creating a separate one for each new type of material.

Geofoam is expanded polystyrene (EPS) or extruded polystyrene (XPS) manufactured into large lightweight blocks. The blocks vary in size but are often 2 m x 0.75 m x 0.75 m. The primary function of geofoam is that of separation typically between foundation soils and an overlying highway or parking lot. Geofoam is also used in much broader applications, the major ones being as lightweight fill, compressible inclusions, thermal insulation, and (when appropriately formed) drainage.

It should be noted that the area of geofoam can nicely seque into geocombs, previously called ultralight cellular structures which Horvath defines as “any manufactured material created by an extrusion process that results in a final product that consists of numerous open-ended tubes that are glued, bonded, fused or otherwise bundled together.” The cross-sectional geometry of an individual tube typically has a simple geometric shape (circle, ellipse, hexagon, octagon, etc.) and is of the order of 25 mm across. The overall cross-section of the assemblage of bundled tubes resembles a honeycomb that gives rise to its name. Presently, only rigid polymers (polypropylene and PVC) have also been used as geocomb material.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

8.Geocomposites

           A geocomposite consists of a combination of geotextiles, geogrids, geonets and/or geomembranes in a factory fabricated unit. Also, any one of these four materials can be combined with another synthetic material (e.g., deformed plastic sheets or steel cables) or even with soil. As examples, a geonet with geotextiles on both surfaces and a GCL consisting of a geotextile/bentonite/geotextile sandwich are both geocomposites. Soil filled honeycombed cells made from geomembranes or geotextiles are also considered as geocomposites. The geocomposite category brings out the best creative efforts of the engineer and manufacturer. The application areas are numerous and constantly growing. The major functions encompass the entire range of functions listed for geosynthetics discussed previously; separation, reinforcement, filtration, drainage, and containment.

The basic philosophy behind geocomposite materials is to combine the best features of different materials in such a way that specific applications are addressed in the optimal manner and at minimum cost. Thus, the benefit/cost ratio is maximized. Such geocomposites will generally be geosynthetic materials, but not always. In some cases it may be more advantageous to use a nonsynthetic material with a geosynthetic one for optimum performance and/or least cost. As seen in the following, the number of possibilities is huge — the only limits being one's ingenuity and imagination.

In considering the following geocomposites, keep in mind that there are five basic functions that can be provided: separation, reinforcement, filtration, drainage, and containment.

8.1.Geotextile-Geonet Composites

When a geotextile is used on one or both sides of a geonet, the separation and filtration functions are always satisfied, but the drainage function is vastly improved in comparison to geotextiles by themselves. Such geocomposites are regularly used in intercepting and conveying leachate in landfill liner and cover systems and for conducting vapor or water beneath pond liners of various types. These drainage geocomposites also make excellent drains to intercept water in a capillary zone where frost heave or salt migration is a problem. In all cases, the liquid enters through the geotextile and then travels horizontally within the geonet to a suitable exit.

8.2.Geotextile-Geomembrane Composites

Geotextiles can be laminated on one or both sides of a geomembrane for a number of purposes. The geotextiles provide increased resistance to puncture, tear propagation, and friction related to sliding, as well as providing tensile strength in and of themselves. Quite often, however, the geotextiles are of the nonwoven, needle-punched variety and are of relatively heavy weight. In such cases the geotextile component acts as a drainage media, since its in-plane transmissivity feature can conduct water, leachate or gases away from direct contact with the geomembrane.

 

8.3.Geomembrane-Geogrid Composites

Since some types of geomembranes and geogrids can be made from the same material (e.g., high-density polyethylene), they can be bonded together to form an impervious membrane barrier with enhanced strength and friction capabilities.

8.4.Geotextile-Geogrid Composites

A needle punched nonwoven geotextile bonded to a geogrid provides in-plane drainage while the geogrid provides tensile reinforcement. Such geotextile-geogrid composites are used for internal drainage of low-permeability backfill soils for reinforced walls and slopes. The synergistic properties of each component enhances the behavior of the final product.

8.5.Geotextile-Polymer Core Composites

A core in the form of a quasi-rigid plastic sheet, it can be extruded or deformed in such a way as to allow very large quantities of liquid to flow within its structure; it thus acts as a drainage core. The core must be protected by a geotextile, acting as a filter and separator, on one or both sides. Various systems are available, each focused on a particular application. The first is known as wick drains in the U.S. and prefabricated vertical drains, PVDs, in Europe. The 100 mm wide by 5 mm thick polymer cores are often fluted for ease of conducting water. A geotextile acting as a filter and separator is socked around the core. The emergence of such wick drains, or PVDs, has all but eliminated traditional sand drains as a rapid means of consolidating fine-grained saturated soils.

The second type is in the form of drainage panels, the rigid polymer core being nubbed, columned, dimpled or a three-dimensional net. With a geotextile on one side it makes an excellent drain on the backfilled side of retaining walls, basement walls and plaza decks. The cores are sometimes vacuum formed dimples or stiff 3-D meshes. As with wick drains, the geotextile is the filter/separator and the thick polymer core is the drain. Many systems of this type are available, the latest addition having a thin pliable geomembrane on the side facing the wall and functioning as a vapor barrier.

The third type within this area of drainage geocomposites is the category of prefabricated edge drains. These materials, typically 500 mm high by 20 to 30 mm wide are placed adjacent to a highway pavement, airfield pavement, or railroad right-of-way, for lateral drainage out of and away from the pavement section. The systems are very rapid in their installation and extremely cost effective.