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Talking Point with Nigel Wrigley

Use of two different test methods to assess the performance of geogrids for groundworks as a result of CE marking is complicating specifications

CE marking legislation should give specifiers and purchasers of construction products a more level playing field for product comparisons. Most product areas have managed this process easily, but for geogrids it’s more complicated.

The industry is in a situation where different geogrids are providing the same function of mechanical stabilisation effectively through reinforcement, but their tensile strength and strain properties are tested using alternative methodologies that make comparisons impossible.

From 1 July 2013, under the Construction Products Regulation 2011, it became mandatory for companies bringing products to market to apply CE marking to any of their products either in accordance with a harmonised European standard (hEN) or a European Technical Assessment (ETA).

Proven square or rectangular grids are tested in accordance to BS EN ISO 10319 for tensile strength to demonstrate and measure their reinforcing properties. However, newer triangular or hexagonal geogrids avoid this test by being tested for “radial stiffness” because they are defined as providing non-reinforcing “stabilisation” under brand new specifically introduced ETAs based on EOTA Technical Report TR 041.

The term “stabilisation” has carried over to describe reinforcement against dynamic loads as “mechanical stabilisation”.

So what is the difference between stabilisation and reinforcement? Historically soil stabilisation was often achieved using cement or lime mixed with the soil to improve its properties chemically. More recently geogrids have become more common in stabilising foundations for roads, rail tracks, and other trafficked areas and the term “stabilisation” has carried over to describe reinforcement against dynamic loads as “mechanical stabilisation”.

Stabilisation in EOTA TR041 is defined as the beneficial consequence on the serviceability of an unbound granular layer via the inhibition of the movement of the particles of that layer under applied load. This is the result of the mechanical effect of confinement on an aggregate layer, resulting from the mechanism of interlock provided by a stiff geogrid structure.

The stiffness of a geogrid is a direct measure of its “stress-strain behaviour”. Therefore EOTA TR041 requires an improvement of the mechanical properties of a granular soil or other construction material layer by the use of the stress-strain behaviour of a geogrid. This is clearly “reinforcement” as defined in BS EN ISO 10318.

So why is there this polarisation in testing methods and a move away by some from the previously understood term “reinforcement” to “stabilisation”? The reason possibly stems from the fact that the test method for tensile strength defined in BS EN ISO 10319 is not entirely appropriate for triangular or hexagonal geogrids. However, the test method for “radial stiffness” adopted under EOTA TR041 is equally inappropriate for square or rectangular geogrids.

Specifiers and designers need to have the ability to compare different products and make an informed choice, but a common testing route needs to be adopted that permits that choice.

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