Designers of polyethylene geomembrane applications need to know that they can obtain a material which offers the performance characteristics they seek and that a suitable geomembrane has consistent quality.
Performance-related tests show, to greater and lesser degrees, how a geomembrane performs in its intended application, while index tests are primarily intended to check product quality and consistency.
The results of index tests are not suitable for demonstrating performance in specific design situations but show whether the geomembrane complies with certain physical, mechhanical and chemical prop- erty criteria, such as thickness, puncture resistance and carbon black content.
Few performance-related tests are presently in common use. The Geosynthetics Research Institute (GRI) at Drexel University, USA has a continuing programme of developing such tests. Closer to home, CEN (the European Committee for Standardisation) is in the process of preparing a number of performance- related tests which will become EN or ISO standards in due course. Much work has also been done by GRI, ASTM and CEN to develop index tests of direct relevance to geomembranes to replace older tests borrowed from other applications.
Geomembrane manufacturers and suppliers need to keep abreast of these developments, with typical results of these newer tests on their products being available to designers, specifiers and regulators.
Designers and specifiers of geomembrane applications should understand the characteristic properties of the geomembrane relevant to their projects. They need to be aware of which tests are important (and to which standard) to include in the specifications. And they must know the acceptable test value to specify for each test, and which properties require quality assurance testing.
At present, there is still a confusing plethora of tests and standards (ASTM, GRI, ISO, FTMS, DIN, EN and BS) used by different manufacturers and their methods of reporting product test data are also variable. This situation should im- prove over the next few years as the new EN standards are published and as ISO standards are agreed.
The most common standards in use in the UK are from ASTM, rightly so as they are in English and readily available in the UK (eg ASTM 1996). They benefit from an active test method development and reappraisal programme specifically for geomembranes.
As CEN develops a range of equal or better test standards, then the changeover to EN standards should take place with a period of parallel use of EN and ASTM standards until the new tests are better understood. Prior to that, the use of test standards other than ASTM (or GRI standards which often are precursors to ASTM standards) only serves to confuse.
It follows that manufacturers wishing to sell geomembrane into the UK should currently present their product test data to ASTM standards. It is incumbent on designers, users and regulators that they understand the tests in a project specification and they should resist requests to consider use of geomembranes which are supported by product data based on unfamiliar test standards.
Care must be taken in assessing manufacturers' product data sheets as the values given are commonly 'typical' values and not 'minimum' or 'minimum average roll values'. There is no set definition of typical and it may represent the median value in which case 50% of samples will have properties less than the typical value.
In selecting acceptance test values for a project specification, designers must be satisfied that the values for the tests in their specification can all be reasonably met by one or more available geomembranes. Preferential sel- ection of test values of the best properties drawn from a range of products will merely result in the specification being unachievable by any available geomembrane.
There is little point in specifying tests that do not provide useful information for the geomembrane application or for product consistency. But which should be selected as appropriate for a specification from the long list of possible tests (eg Table 1)? It is likely that for most applications, many of these tests will not produce useful information.
Oxidative Induction Time and Melt Flow Rate are important to geomembrane manufacturers but are usually of little relevance in project specifications. While stress crack resistance is a very important property for HDPE (but not for LLDPE, LDPE or VLDPE), the Bent Strip Test (ASTM D1693) is not an appropriate test as the polymer resins in use today quickly stress relax under constant strain and never fail. Dimensional Stability and Low Temperature Brittleness are unlikely to be relevant in most cases and where UV resistance is sought, the two carbon black tests usually suffice. If the geomembrane is to be buried, then Carbon Black Content is not a priority test although the effectiveness of the mixing process during manufacture shown by the Carbon Black Dispersion test is often considered useful.
Guidance on the allowable values for the selected tests can be found in NSF 54 (1994) and, more recently in the GRI standard GM13 (GRI, 1997). If a material property is sufficiently important to be specified then the specifier should require manufacturer's quality control (MQC) testing and, as part of the construction quality assurance (CQA) programme, sufficient conformance testing so that the delivered material is shown to comply with the specification. The GRI GM13 standard also gives a minimum testing frequency for MQC. The specification and/or CQA plan should define the frequency of conformance tests.
Textured geomembrane can be used where extra frictional resistance is required to prevent the geomembrane sliding on the underlying surface or the overlying material slipping on the geomembrane. Texturing can be formed by several manufacturing techniques and the variation in roughness of the textures from one manufacturer to another can be very marked with quite different frictional resistance characteristics.
Where frictional resistance is a key design factor such as on landfill side slope liners and soil covers over sloping geomem-branes, friction tests in large shear boxes (300mm square) to assess the frictional resistance of interfaces between layers of geosynthetic materials and soils are important.
The available frictional resistance between geosynthetic materials and at soil/geosynthetic interfaces varies widely. As an index test, a direct shear test method using a standard sand which is currently out for public comment (pr EN ISO 12957-1: 1998), would show the influence of different surface roughnesses on shear resistance. Performance-related shear tests should include the actual geosynthetic layers and soils to be used on site and the test designed to replicate as far as possible the site conditions. Such tests provide the essential means of assessing the shear resistance between geosynthetics layers and at interfaces with soils.
Selection of an adequate geotextile to protect a geomembrane from damaging strains caused by granular layers and overlying loads can be based on the results of another perfor- mance-related test called the 'cylinder test' to a standard method rec- ently developed by the Environment Agency.
Despite improvements in manufacturers' quality control and the fact that many producers operate to recognised QA procedures (eg ISO 9000), significant inconsist- encies in product quality do occur. As geo- membranes are often the most critical element in a design, it is important that they are specified with care and that conformance to the specification is confirmed by testing.
Adrian Needham is director of Weeks consulting engineers and environmental scientists
ASTM (1995 ) ASTM Standards on Geosynthetics, 4th edition.
GRI (1997) Test Properties, Testing Frequency and Recommended Warrant for HDPE Smooth and Textured Geomembranes. Test Method GM13.
National Sanitation Foundation (NSF) Standard 54 - 1993, Flexible Membrane Liners.