Reading the trade press, it would seem that civil engineers are finding a new use for geo-synthetics on an almost daily basis. Although the term 'geo-synthetics' is used here, this group of ground engineering products includes geo-fabrics, geo-membranes, geo-textiles, landscape fabrics, geo-grids, and geo-matrices. Basically, they are all sheet products of synthetic or natural origin that are used in some way to 'improve' the natural ground or earthworks.

Some definitions

Geo-synthetics is a 'catch-all' term used to describe sheets of a man-made material that may be knitted, woven or non-woven.


Geo-textiles:

geotextile

Geo-textiles are woven, non-woven or knitted, permeable sheets, usually, but not exclusively, non-biodegradable.

Landscape fabrics are an example of a light-use geo-textile commonly used in the horticultural and soft-landscaping trades.


Geo-membranes:

geomembrane

Geo-membranes are impermeable and usually non-woven, ie a film or solid sheet.

Typical use would be as a vapour or damp barrier.


Geo-composites:

geo-composite

Geo-composites normally refers to a sheet that combines two or more layers of geo-synthetics.

For example, a drainage composite usually combines a permeable geo-textile with an impermeable geo-membrane.


Geo-grids:

geogrid

Geo-grids are tough, non-woven synthetic sheets with large (100mm+) rectanglar holes.

Used for ground stabilisation. They are also known as geo-nets.


Geo-matrices/Geo-webs:

geomatrix

Geo-matrices can be thought of as 3-dimensional geo-grids, in that they are cellular and used to stabilise surface soil, gravels etc.

Often used as a form of Grass Paving

See also Cellular Paving Systems page.


Many types of materials may be used to manufacture geo-fabrics, depending on the end use. The two main distinctions are between woven and nonwoven products.

Geo-synthetics are manufactured from synthetic materials, but there are some applications that call for biodegradable, natural materials such as coir or jute. Polypropylene is a popular choice for geo-grids or geo-matrices, as it is strong and chemically inert. Polyester is chosen for its strength and flexibility.

There are permeable woven geo-textiles commonly used for filtration purposes and impermeable membranes to resist mud-pumping. Some fabrics offer high puncture resistance, an important consideration in road and rail construction projects, or where the integrity of the sheet is essential, as in landfill sites.

Uses

Geo-synthetics have three main roles in the construction industry. They are used to provide...

  • Separation
     
  • Filtration (liquids and gases)
     
  • Reinforcement
     

...either singly or in combination.

Geo-synthetics have literally hundreds of uses, but the most common include...


Roads and Pavements

Separation:

  • - to prevent bedding layers disappearing into sub-bases/subgrades
  • - to keep sub-bases from mixing into sub-grades
  • - to minimise/prevent soil-pumping
maintaining bedding
Maintaining sub-base integrity

Filtration:

  • - as drainage composites or lagging to filter drains

Reinforcement:

  • - to inhibit reflective cracking in asphalt courses
  • - strengthening of poor sub-grades

Railway Construction

Separation:

  • - similar uses to those in road construction
  • - separation of ballast from sub-grade to minimise 'pumping'
pumping

Filtration:

  • -track-edge drainage

Reinforcement:

  • -sub-grade and sub-base reinforcement

Ground Engineering

Separation:

  • - weed control; root barriers
root barrier
Root Barriers protect drainage and foundations

Filtration:

  • - usually as a drainage composite but also as a filter membrane for land drains

Reinforcement:

  • - used to stabilise topsoils
  • - to stabilise earth banks

Other Uses

Erosion Control:

  • - to prevent coastal or fluvial erosion
  • - to counter scour by wind and rain and surface run-off
  • - may be biodegradable and intended to last only until vegetation has established itself

Vapour and/or Gas Membranes:

  • - impermeable to protect part of a structure
  • - damp-proofing; radon barriers

Landfill Sites:

  • - containment and filtration
  • - leachate and gas collection


Specification

Separator geo-fabrics are the most commonly specified geo-synthetics used in the construction of roads and pavements. These are often non-woven, permeable materials primarily used to prevent the various layers of a pavement mixing or disappearing into lower layers.

DTp Clause 609 lists four essential qualities for separator geo-textiles:

  • Pore size O90 - a measure of average hole size - lower figure means finer particles trapped
  • Permeability, as measure in litres per m² per second
  • Tensile strength - usually an abitrary minimum of 10 kN per m²
  • Puncture resistance (CBR)

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