Published in Ground Engineering Magazine’s Talking point in November 2013, here is the full article from geogrid expert Nigel Wrigley. He discusses why the industry has two different test methods to assess some aspects of the performance of Geogrids for groundworks.
The CE marking legislation brought in on 1 July 2013 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 been 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. This is not helpful for the designer in achieving a like-for-like specification.
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, in the absence of a hEN, a European Technical Assessment (ETA). This is a major change as previously CE marking was voluntary in the UK. Geogrids come under this requirement in the category Geotextiles, Geomembranes and Related Products.
The harmonised standards, including for example BS EN 13249 which covers the use of geosynthetics in the construction of roads, have within them a test for tensile strength where the geosynthetic product is being used for a reinforcement function. It is these harmonised standards and the consequential CE marking requirement to test and declare the tensile strength characteristics relating to maintaining the reinforcing function in the ground structure that appears to have been the catalyst for a polarisation in geogrids.
As required in BS EN 13249, proven square or rectangular grids such as E’GRID geogrids from Wrekin Products 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 ETA’s based on EOTA Technical Report TR 041.
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 the common method of 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”. Internationally, “reinforcement” by a geogrid is defined in BS EN ISO 10318 as being “use of the stress-strain behaviour of a geosynthetic material to improve the mechanical properties of soil or other construction materials”.
Stabilisation in EOTA TR041, the alternative technical approval used for triangular or hexagonal geogrids 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. Now, 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 well understood term ‘reinforcement’ to ‘stabilisation’? The reason behind the split in testing methods 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.
This situation was pre-empted in my paper ‘The confinement effect of different geogrids: 4 : the development of an index test’ which I presented at the EuroGeo5 Geosynthetics International Conference held in Valencia in 2012*. This proposed an alternative test that measures omni-directional strength. This better replicates the stresses put on geogrids under load in actual use in roads. It is a modified form of the geomembrane and geotextile tests covered by DIN 61551 and ASTM D5617. The test takes a large circular sample that’s stressed by inflating a sub-membrane thereby stretching the grid equally in all directions. The test results show the test methodology is well developed and can be used to consistently measure tension at 2% strain, a key index property linked to geogrid performance in the ground. This test is applicable equally to geogrids with any shape of apertures. There is a useful table for comparing square and triangular/hexagonal grids based on results of this test method at the foot of this page.
Specifiers and designers need to have the ability to compare different products and make an informed choice. This is a primary function of a standard. The current situation is that a parallel methodology to that which was previously universally accepted has been created that is only applicable to a specific grid pattern arrangement. I believe it would be very beneficial for the industry if one common testing route could be adopted that permits that informed choice between different geogrids. After all what specifier and designer wants a choice of one?
*The initial test results were part of my paper at the EuroGeo5 Geosynthetics International Conference and a copy of the proceedings is available at www.eurogeo5.org.