When it comes to providing tensile soil reinforcement, nothing does it better than Geogrids. These are geosynthetic material made from polymers such as polypropylene, polyethylene or polyester and are used widely in Civil Engineering applications.
They are in the form of open grids so that soil can strike through the apertures and the two materials interlock together to give composite behaviour. Geogrids are commonly applicable in the construction of retaining walls, steep slopes, roadway bases and foundations.
Geogrids work by interlocking with the granular or soil material placed over them. The apertures allow for strike-through of the cover soil material which then interlock with the ribs (flat straps/bars) Providing confinement of the overlaying granular/soil material due to the stiffness and strength of ribs.
Importance of geogrids
The main function of Geogrids is reinforcement. Depending on the application under consideration, Reinforcement could either be uniaxial (strength in one direction) or biaxial (strength in all directions). They are graded by a number of performance properties for instance tensile strength, junction efficiency or trade name and grade for instance E’GRID 3030.
In Africa, the use of geogrids as reinforcement is quickly gaining pace. However, most of the geogrid technology is imported.
Mantis Earth Limited is among several companies based in Kenya that imports geogrids. The firm is the official distributor of geogrids from Tensar International a leading UK based solution provider for soil reinforcement and ground stabilization. Liz McKee who is the director at Mantis Ltd says that Geogrids are especially useful when working on strict construction deadlines.
She says this is made possible by the fact that with Geogrids less aggregate is required. And by and large the construction cost is reduced due to reduced transport costs to deliver aggregate or remove excavated soil.
With geogrids the amount of usable land on a site is increased because it enables construction of steep slopes or walls, enabling construction of a road over poor ground conditions or decreasing the thickness of fill required to construct a road.
“By incorporating Geogrid, the designer has an opportunity to reduce construction costs and also reduce maintenance costs by increasing design life,” she explains. In the long run Liz asserts, there are fewer repairs due to higher performance of a structure or road.
Geogrids, says Liz have been used in many countries in the world under a variety of climates and soil conditions, and have frequently been used to solve challenging design or construction problems.
The manufacturing process produces a hexagonal geogrid structure consisting of high strength junctions and stiff ribs forming equilateral triangular apertures. The ribs present a thick square leading edge to the aggregate, which allows the geogrid ribs to get a good grip on the aggregate particles beneath, resulting in an extremely effective mechanical interlock.
This ultra-efficient interlock helps to control lateral movement and dilation of aggregate particles, so that a highly effective angle of shearing resistance is achieved. This effect is also referred to as confinement because interlock efficiently confines and restrains the aggregate particles.
Categories of geogrids
The first, and ‘original’, geogrids are commonly referred to as ‘punched and drawn’ geogrids. A sheet of either HDPE or Polypropylene has holes punched into it in a regular pattern and the sheet is then ‘drawn’ or ‘stretched’ into the finished product.
The second category of geogrids is ‘coated yarn’ types. They are, in fact, technical textiles in the form of grids and use bundles of fibres (most commonly Polyester) as the reinforcing component that is then coated to provide protection during installation and in service. The grid structure is formed by knitting or intertwining the transverse and longitudinal bundles of fibres.
The third category of geogrids is made by laser or ultrasonically welding together polyester or polypropylene rods or straps (as used in packaging / shipping) in a grid like pattern.
Qualities of good geogrids
Understanding important qualities of a good geogrid, Liz advices, is a major prerequisite while choosing the product.
The key attributes to consider in selecting a geogrids are the height and thickness of the rib, the aperture area, the tensile loads at 2% strain, the junction efficiency, the open area percentage, the carbon black content, the quality of the raw material used, whether the product has a CE Mark with full traceability, and whether it has been manufactured under a certified ISO 9001 system.
Geogrid are mainly applicable in Road Infrastructure, airport pavements, dock pavements, safe working platforms, housing and railway trackbeds.
“Wherever weak ground needs stabilization, grade changes require retaining walls or slopes or asphalt pavements needs reinforcing for better performance, geogrids can be very helpful,” says Liz.
Alex Ngove a sales consultant at Ragged Mercantile another firm that distributes geogrids in Kenya , says that the market for the products is fast expanding in the country. He observes that geogrids are important for base reinforcement, earth retaining wall construction including veneer stabilization, the segmental retaining wall and embankment reinforcement.
For Alex when choosing geogrids ensuring that they have very high strength at low strains is important. Ability for immediate interlocking with cover aggregate is also essential. But most of all, geogrids should have High resistance against installation damage and high resistance against biological and chemical degradation. It should also be quick and easy to install.
Yet the users of geogrids in Africa continue to face major challenges while using the product. Sugandha Dalmia from Strata India a firm that specializes in geogrids reckons that one of the greatest challenges is lack of knowledge on design standards, material properties and construction quality requirements of geogrids.
He says conducting various knowledge sharing programmes for engineers jointly with educational institutes can go a long way to create the much needed awareness on geogrids. To enhance demand, Dalmia urges for intensive campaigns on the benefits of using geogrids.
He urges contractors to observe extreme care while using geogrids.”Handling and placement must be as per construction drawings and rigid quality standards. There must be zero tolerance towards quality and error,” cautions Dalmia whose firm manufactures Polyester knitted geogrid using high tenacity yarn and promises design life of 100 to 120 years.
It is a challenge that is echoed by Philippe Grimmelprez of Mattex Geosynthetics who says that for them, trainings, seminars and practical help at installation stage has paid off.
“Thanks to intensive awareness programme that we have undertaken on our soil reinforcement products, customers know most of our products including MattexGeotextiles and drainage Matts.”
The firm specializes in Geosynthetics which they say give a significant advantage with regard to sustainability and saving on cost.