ARRHENIUS MODELING TO PREDICT GEOSYNTHETIC DEGRADATION

A methodology for the determination of the degradation of polymeric materials used in long-term applications such as landfill liners and covers, vertical and lateral expansions of solid waste facilities, retaining wall and slope reinforcement, etc., is the focus of this paper. Attention is placed on geomembrane barriers, but geotextile filters, geonet and geocomposite drains, geogrid and geotextile reinforcement and plastic pipe might also be involved. The predictive technique most widely used for polymer degradation (and explained throughout this paper) is based on a time-temperature superposition principle, and is called 'Arrhenius modeling' in this paper. It uses high-temperature incubation of the polymeric material in question followed by some type of laboratory testing, in order to extrapolate the experimental behavior to the site-specific, and lower, temperature. The tacit assumption involved is that the material's behavior within the high-temperature incubation range (as indicated by the activation energy, 'E(act)') is constant within this range and can be extrapolated to the lower temperature behavior of practical interest. Assuming this to be the case (it is obviously a critical assumption and promises to be difficult with oriented geosynthetic products), the nature of the experiments used to identify the amount of degradation becomes very significant. We have outlined 13 candidate tests (there are undoubtedly more) for the evaluation of polymers commonly used to manufacture geosynthetics. They are grouped in the mechanical, physical and chemical test method categories. Each test method is described in some detail. Furthermore, an attempt has been made to predict how the various responses might behave in their elevated temperature test environments. Using these trends, an Arrhenius plot can be generated, and a prediction of degradation to a site-specific temperature should be possible to obtain. It is hoped that this paper will stimulate discussion (and even controversy) and, if warranted, an intensive research effort in this area. With a strong enough effort, a single geosynthetic material can be investigated by a number of different testing techniques to see if a consensus regarding degradation times is reached. At the minimum, such a broad-based approach should identify which testing method is preferred for use in Arrhenius modeling. At this point, that particular method(s) can be used on all of the different polymer materials used in a particular application to see which one possesses the best resistance to a specific type of degradation.