To properly understand and assess the long-term behaviour of geosynthetic materials it is necessary to investigate the various types of possible degradation mechanims. This includes both chemical and mechanical behaviour, and sometimes even their interactions with one another. Clearly, chemical degradation of geosynthetics depends on the polymer type. For example, polyolefins are vulnerable to oxidation; polyesters are susceptible to hydrolysis; and plasticizers can leach from polyvinyl chloride. This paper describes the concept of these three types of degradation, but focuses on the oxidation of polyolefins since the majority of the geosynthetics is made from this type of polymer. The methods used to predict the lifetime of antioxidants and service life of the geosynthetic material will be illustrated. Furthermore, the influence of temperature, pressure, and ultraviolet light on the service life are also demonstrated. Finally, the current specifications targeting the longevity of different geosynthetics are presented.
Regarding mechanical degradation, the paper mainly focuses upon the creep deformation of geogrids and stress crack resistance (SCR) of polyethylene geomembranes and geopipe. The method to assess stress crack resistance is described, and the microscopic mechanisms that lead to such failure are explained. For creep evaluation, different acceleration tests are presented and their applicability with respect to the different types of polymers is illustrated. In addition, the long-term shear behaviour of geocomposites and geosynthetic clay liners is presented.
A report is presented on the development of a test for determining the long-term oxidation resistance of polyolefin materials, which is accelerated not only by elevated temperatures but also by increased oxygen pressure and a stirred aqueous medium. The method permits durability tests to be carried out at markedly lower temperatures with reasonable testing durations and significantly reduced diffusion limited oxidation effects and allows for physical and chemical impacts of aqueous media. Oxidative durability can be assessed by autoclave immersion exposures at three different temperatures (60, 70 and 80 degrees C) and 50 bar oxygen pressure and at 80 degrees C and two different oxygen pressures (usually 10 and 20 bar). Accelerated test data are evaluated by modified Arrhenius equations with the application of a three-dimensional regression analysis. The application of the test method to various materials, including PP non-woven materials and geosynthetics is demonstrated. 10 refs.