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GeoTextiles

Geotextiles are textiles in the traditional sense. Fibers are made into flexible porous fabrics by standard weaving or knitting machinery, or matted together in a random, non-woven manner. Geotextile are the major category of geosynthetics. They are used in nearly every highway project, and can be found in almost all civil construction works.

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Geotextiles are the major category of geosynthetics. They are used in nearly every highway project, and can be found in almost all civil construction works.

Geotextiles are the largest product category within geosynthetics.

They play a key role in many civil construction projects such as roads, railways and landfills. They can reduce cost and time and enable more innovative construction practices.

The most common applications for geotextiles are in roads, railways, landfills and coastal works. In all these areas they are used in conjunction with soil to :

  • Provide additional strength 
  • Prevent contamination of good quality fill material 
  • Allow the passage of water whilst retaining the drained soil particles 
  • Prevent damage to plastic liners
  • Form flexible containers for coastal protection 

Road and Railways

In roads and railways, a solid formation is always required. Uneven settlement of the carriageway will lead to undulations or a breakdown of the surface.

The traditional method of providing a uniform base is excavation and importation of aggregate. This can be expensive in materials and time.

Geotextiles are often used at the base of roads and railways. they can significantly reduce the required depth of excavation particularly when traversing poor quality soils.

Geotextiles provides strength. They prevent the underlying soil from contamination from the imported aggregate. This increases durability of the structure.

It is important to keep moisture content in the formation low. This extends the pavement life. Geotextiles are used in subsurface drains to allow moisture to drain out of the formation.

Geotextiles are also used to seal and reinforce the surface of aging pavements.

the classic use of a geotextile is at the base of a road formation as well as separating the weak soil from the introduced road aggregate.

The geotextile allows water to drain from the road formation.

Geotextiles can be found in combination with geogrids or other Geosynthetics on extremely weak or waterlogged soils. They are also used in other civil infrastructure:

in this case, a bridge abutment, the geotextile is placed underneath the geocell to protect the abutment during flood conditions.
A geotextile separation layer is often used in railway formations. They are even rolled out under existing railways
Geotextile provides a separate function. Geogrid provides reinforcement by interacting with the rail ballast

Landfills

Modern landfill structures are designed with sophisticated containment systems. They ensure the waist remains separated from the surrounding environment.

Engineers must ensure this separation, both during the work life of the dump and long after it has been decommissioned.

Low permeability liners (that is geomembranes and geosynthetic clay liners) are often used. If these products are punctured, hazardous leachate will escape into the surrounding ground water.
Geotextile layers above and below the non-porous liner mitigates the risk of puncture. They provide protection from sharp-edged waste material as well as the base underneath

Geotextiles are also used in the leachate collection system of landfills. During its working life, the landfill will be open to rainfall. As the water filters through the waste, it creates leachate that collects at the bottom of the landfill.

Leachate collection systems are designed to control the movement of this hazardous liquid within the landfill

Geotextiles are used as a filter layer. They allow the leachate to pass through, while preventing fine materials from entering and clogging the leachate collection system.

geotextile is rolled out at the base of the landfill lining system. It protects the geomem brane from being punctured from the rock below.

Where the base soil has been compacted, and leveled:

a lower strength Geotextile can be used.
The geomembrane is then laid out on top of the geotextile.

Finally, a second layer of geotextile is placed at the top of the lining system.

In this case, on top of the drainage aggregate, the geotextile acts as a filter.

Costal works

In coastal environments engineers must contend with extremely strong forces from water moving in multiple directions.
The challenge that coastal areas face from storm surges is quite different to that faced by road formations or landfills dealing only with rainfall or water seepage.
Traditionally, rock structures have been used to resist coastal forces and protect land, beaches and infrastructure.
Geotextiles offer a modern, cost-effective and more flexible option for coastal protection works.
  They are used as a filter layer under rock walls, to replace the thick and expensive graded filter layer.
  They are also used as a building material in their own right when stitched into bags to contain sand.

Geotextiles are used under traditional Roxie wall structures.
They separate the sand from the wall and help prevent it from being washed away.
Sand-filled containers are made from geotextiles.
They offer engineers an alternative building material

they are also used beneath the sea in artificial reefs which are designed to dissipate wave energy before it reaches the shore.

Some geotextiles provide a host for marine growth.

Geotextiles are no different to other textiles they are made from fibers or yarns. Yarns come in continuous, long threads rolled onto spools. they can be made up of many individual fiber.

Fiber are discrete pieces. They are often extruded from polymers, and can come in bales.

Yarns are woven or knitted together, and fibers are needled together. In both cases, a mat or blanket is created and is transported in rolls.

Geotextiles can also be made from natural fiber such as jute or coconut, which are used in erosion applications.

Here is a clump of fiber in the form used in manufacturing.

Different fiber types are distinguished by their thickness length and crimp as well as strength UV resistance and color.

Fibers are transported in bales. The bales are opened and spread to commence the manufacturing process.

In the manufacture of geotextiles, engineers consider :

  • The function and performance requirements. 
  • The type of fiber used to make up the fabric
  • The polymer from which the fiber is derived 

The required function and performance requirements usually determines the manufacturing method – woven, knitted or nonwoven.

Woven

Woven geotextiles are created using traditional textile weaving techniques. Many different weave variations are used.

The different variations influence the physical, mechanical and hydraulic properties of the finished textile.

When engineers are seeking the right product for practical problems, they often segment woven geotextiles into:

  • Medium and high strength - for reinforcement functions.
  • High flow - for filtration functions.
Here is an example of a woven geotextile
you can see the individual yarns interweaving. This example has a tighter weave. It is a stronger product and has a lower flow rate. It is less porous.

Knitted:

Knitted geotextile have emerged in the last decade due to manufacturing innovations.

Instead of yarns interlocking together in a weave, they are set down in two directions and knitted together with binding yarn.

Knitted manufacture is used exclusively for reinforcement geotextiles. Polyester yarns with high strength at low stretch are used.

Here is a geotextile knitting line:

on the right side, you can see many spools of yarn that are being knitted together. A reinforcing yarn is knitted to a non-woven geotextile to provide greater strength.

Non-woven

Nonwoven geotextiles are made by laying down fibers in a random manner and needle punching them together to form a mat of fabric. Needle punching entangles the fibers to mechanically hold the mass together.

Fibers may be created as a part of the manufacturing process, or introduced.

  • In spun bond manufacture, fibers are extruded directly from the polymer.
  • In carding manufacture, the fibers are pre-fabricated. They are often supplied in bales.

The two different non-woven processes determine the performance of the finished product. For example:

  • A staple fiber geotextile can be made using various raw material inputs and combinations (that is various fibers), whereas a spun bond process uses one raw material input only.
  •  Spun bond geotextiles are stronger per unit of mass than staple fiber products.
  • A staple fiber geotextile may elongate more than a spun bond geotextile.
The random pattern of fibers can be seen clearly in this spun bond geotextile.
In this cross section of a two-layered staple fiber geotextile

you can clearly see the individual fibers that have been needled through both layers. They hold the mat together both within and across the layers.

This schematic shows the spun bond process

Raw materials are sourced as pellets and stored in silos. the pellets are melted and then extruded into fibers. The fibers are laid down to form a loose mat which is then needled together and compressed to create the finished geotextile. The geotextile is then rolled and wrapped ready for dispatch.

let's take a moment to look at each of these steps:

Pellets of raw material are supplied in bags.
Here are the individual pallets
In this example, polyester chips are used as the raw material.
The chips are stored in silos and fed into the extrusion process.
The polyester chips are melted and extruded into thin continuous fibers.
The fibers are laid down in a random manner to form a loose mat.
boards of needles are punched into the fibers to entangle them.

the needle punching compresses the mat and increases its strength.

the finished product is rolled and wrapped.
in UV resistant material for outdoor storage

Engineers need to be aware that the strength of a geotextile is not necessarily the same in both directions.

this is a function of the manufacturing process. often the machine direction is stronger than the cross machine direction

product data sheets should specify the strength in both directions.

in applications where the load is multi-directional (such as a highway or landfill base) the engineer must design to the weaker direction

the machine direction is along the rolled out product. the cross machine direct is across the roll

geotextiles are generally made from either polyester or polypropylene.

it is therefore important to understand how the polymer will perform in the chosen application. for example, an embankment uses a high-strength geotextile as a reinforcement layer in its foundation. the high strength geotextile will be woven or knitted from polyester yarns. these yarns have the required stress-strain relationship to support the reinforcing function.

Engineers should also consider the polymers chemical properties. for example geotextiles used to contain landfill or mining waste may need to contend with extreme pH or temperature.

geotextiles in coastal structures need to resist the effects of intense sunlight over many years. geotextiles used in road pavements need to withstand extreme temperatures as the bitumen is being laid.

Consider:

  • polyester is more suitable for low pH say less than four.
  • polypropylene is more suitable for high pH say greater than 10 
  • polyester is better suited for high temperatures whereas polypropylene starts to degrade above 70 degrees Celsius 
       
  • polyester has a natural resistance to sunlight additives are needed to increase polypropylene resistance.
       

The selection of the correct geotextile for the application is critical to the long-term performance of the structure into which it is incorporated. each function has its key performance requirements.

For Example:

  • 1. reinforcement -requires consideration of high strength low elongation and long-term creep properties.
  • 2. drainage and filtration- require consideration of flow rate and pore size. 
  • 3. separation - requires consideration of strength and elongation.
when selecting this nonwoven geotextile as a separation layer in a highway project.

the CBR of the sub base was considered. this determines how strong the geotextile needs to be. the lower the CBR the greatest strength required. geotextile selection also needed to consider the combination with imported aggregate greater strength is required for larger more angular aggregate.

in this drainage application

(the geotextile is selected on the basis of flow rate and pore size. these properties need to be considered in relation of the surrounding soil and filtration aggregate. they must be selected and designed in a system not individually.)

this non woven geotextile is being used to protect the geomembrane beneath from puncture.

you can see highly angular drainage aggregate being spread in a thick layer. therefore this geotextile is also selected on its strength properties. it needs to be stronger than in our highway example because it supports a large single grade aggregate.

this high-strength woven geotextile is being used as a reinforcing layer under an embankment

selection is determined by the bearing capacity of the subgrade, height of the embankment to be built and the load it is to carry.

in this case, the geotextile was selected on its filtration properties

such as pore size and flow rate. the strength also needs to be sufficient to tolerate the roughness of the subgrade. UV resistance was also an important factor for selection as this geotextile will be exposed to sunlight for several years until the tailings dam fills.

Specification

once a suitable geo textile has been selected the engineer must specify the key attributes and assess the suitability of products submitted for approval.

different suppliers will present performance data in different ways. the engineer must interpret this data and compare options.

geotextile datasheets most often describe their performance in terms of Minimum Average Roll Values (MARVs) or Typical values. these utilize different statistical methods.

Minimum Average Role Value or MARV

  • is the mean minus two standard deviations of a representative batch of samples. 
    provides the engineer with a 97.5 percent confidence level that the goods purchased perform at or above the nominated value on the data sheet.
    typical values
  • are usually defined as the average or mean of a representative batch of samples.
  • they provide the engineer with a 50% confidence level that the goods purchased perform at or above the nominated value on the data sheet.
  •  the specification nominate the minimum acceptable strength of the geotextile. 
    maRV results ensure that ninety seven point five percent of test results will meet or exceed specification.

when engineers specify typical values they need to accept that 50% of test results will not meet specification. for the same specification, the typical value will be higher than the MA RV. engineers need to understand that it does not represent a stronger product just less certainty that the required strength will be met.

as you can see, these alternative methods of specifying geotextiles can have quite different outcomes.

the specification is the means of communicating design requirements to the construction phase.

the design engineer must be very careful that the specification accurately reflects the design requirements and clearly states the designers minimum expectations.

this specification will determine the geotextile purchased by the construction engineer, and will ultimately be reflected in the performance of the completed project.

engineers need to be guided in their specification approached by the relevant statutory body.

for example:

  • new south wales Road Transport Authority specification R63
  • Queensland main roads specification MRTS27 
  • transit New Zealand specification tNZ f/7

these specifications guide the engineer on how to select the geotextile. they provide a framework to calculate the strength grade required.

for example, the are 63 specification considers the CBR of the subgrade, thickness of the road formation and aggregate size within the formation to identify the required strength grade of the geotextile. R63 then specifies the minimum performance required of the Geotextile within that strength class.

the specifications are limited to separation and drainage functions in highway applications.

There are two forms of quality assurance available to the engineer:

  • manufacturing quality/assurance -
        is undertaken in the factory. Results are reflected in the geotextile datasheets. Reputable manufacturers will stand behind these results.
  • Construction quality/assurance -
        is undertaken on site to ensure the geotextile delivered meets project specifications.

Both forms of quality assurance are important to manage the risk of errors or misunderstanding in the project chain - from design to procurement, through delivery of product, to construction.

Manufacturing Quality Assurance

geotextile manufacturers should operate within a quality assurance program. they must ensure the product is consistent, performs to specification and is traceable from manufacturing batch to delivery on site.

manufacturing QA data should form the basis for published data.

construction quality assurance

construction QA is performed on the geotextile delivered to site to ensure it meets the project specification.

the cost of construction QA is very small when compared to purchase price of the geotextile or the cost of failure.

construction QA is recommended for any large project or where consequences of failure are high, such as with hazardous landfills.

documented test results are a critical part of the communication between manufacturers and engineers. both parties need to understand the basis for the result. for example, a manufacturer should state their method and frequency of testing.

there are Australian and international standards for geotextile testing. these provide the framework for conversations between engineers and manufacturers. a number of laboratories are available to perform construction quality assurance.