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):
- 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
- 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.
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.
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.
