Geogrid vs geotextile – What are the differences

The word geosynthetics combines geo (“soil or earth”) and synthetics (“man-made”). Geosynthetics are man-made polymer products used with soil, rock and aggregate.
Because the polymers are inert and do not rot under fungal or bacterial attack, geosynthetics last in the ground and are used to improve soil behaviour.
They come in several forms. The main types are:

• Geogrids
• Geotextiles
• Erosion control products
• Geocells
• Geomembranes

This article explains the differences between geogrid and geotextile, which are often confused when choosing a material for a road, wall or hardscape job.

What exactly is Geogrid?

A geogrid is an open mesh of polymer ribs joined at the crossings. Geogrids are made from polyester, polyethylene or polypropylene.

They are used mainly to reinforce and stabilise soil and aggregate, for example in retaining wall backfill, steep slopes, embankments and road bases over soft ground.

Geogrids are chosen for heavy loads, where a material needs high tensile strength and stiffness. Most geotextiles are not designed for this.

Geogrids carry load at low strain, so they are specified by their tensile strength at 2% and 5% strain as well as their ultimate strength (ASTM D6637). There are three main types: extruded, woven and bonded.

a. Geogrids that have been extruded
These are made by punching a pattern of holes in an extruded polymer sheet and then stretching it in one direction (uniaxial) or two (biaxial).
Extruded geogrids are stiffer than woven and bonded geogrids.

b. Woven Geogrids
These are made by knitting or weaving high-tenacity polyester yarns into longitudinal and transverse ribs.
The junctions are tied by the knitting or weaving, and the whole grid is then coated (usually with PVC or bitumen) to protect the yarns.

c. Geogrids that are bonded
Bonded geogrids are made by welding or bonding two or more sets of straps or strands together at right angles.

What exactly is a Geotextile?

A geotextile is any permeable textile used with soil, earth or rock in a geotechnical project, structure or system.

It is the most widely used geosynthetic. Geotextiles come in rolls up to about 18 feet (about 5.5 metres) wide and 160 to 500 feet (50 to 150 metres) long.

Geotextiles are also called geo fabric or filter fabric. They are made from polyester or polypropylene and are classed as woven or nonwoven, depending on how the fibres or yarns are arranged.

a. Geotextile (woven)
Woven geotextiles are made by interlacing yarns or tapes in two directions, like cloth. They have a regular, visible pattern and a defined opening size.

They have high tensile strength and low elongation under load (usually up to 15%), so they are used for separation, load distribution and reinforcement, and for filtration where the soil suits their opening size.

b. Geotextile (non-woven)
Nonwoven geotextiles are felt-like with no visible pattern. They are made by bonding a random web of fibres by needle punching, heat bonding or resin bonding.

Nonwoven geotextiles stretch much more than woven ones under load (about 50 percent).

So they are used for filtration, drainage, separation and as a protection layer (for example under a geomembrane), but not for soil reinforcement such as retaining walls.

The following are examples of geotextile separation applications:

• Flexible or unsurfaced pavements for car and truck traffic
• Site access roads and working platforms
• Caps for sludge ponds or industrial waste lagoons
• Railway track beds
• Container storage yards
• Flexible or rigid airfield pavements
• Fill pads and embankments of moderate height

Geotextiles and Geogrids: What’s the Difference?

• Geotextiles are used for separation, filtration and drainage; geogrids are designed for reinforcement.
• A geogrid does not separate materials. Its apertures are open, so when the subgrade gets wet, fines can pump up through the grid into the aggregate. A geotextile lets water pass between the soil and the aggregate layer while holding the soil particles back.
• Geogrids usually reinforce better, because the aggregate interlocks in the apertures. A geotextile separator keeps the aggregate layer clean, so the layer keeps its thickness, strength and service life. On soft ground the two are often used together.
• A geotextile reinforces through friction with the soil; a geogrid works mainly through interlock and passive bearing on its transverse ribs, plus friction.
• A separation geotextile is laid directly on the graded, prepared subgrade, with overlaps between rolls, before the aggregate is placed.

Each stage of the sequence below is covered in more detail in our step-by-step geogrid installation guide, from foundation preparation to capping, with a short site checklist.

The installation of a geogrid entails the following steps:

For a geogrid-reinforced retaining wall, the usual sequence is:

• Design the wall, including geogrid strength, length and vertical spacing
• Excavate to the design levels
• Prepare and compact the sub-base and base
• Lay and level the gravel levelling pad
• Build the wall facing course by course, laying each geogrid layer flat and taut over compacted backfill at the design levels
• Place and compact backfill over each layer before the next

For more on geogrid or geotextile selection and installation, contact the Ocean Geosynthetics team.