Service Lifetime of HDPE Geomembranes

A temperature of 40 °C, quite unusual for geotechnical applications, was chosen within the BAM certification because geomembranes in landfill liners may be exposed to elevated temperatures when the waste body of the landfill contains biodegradable materials and especially when it is run as a bioreactor landfill. 

Such a calculation shows that the service life of a geomembrane is predominantly determined by the average stress level as indicated by the master curve. A high initial stress does not substantially shorten the service life as there is only a 10 % difference in the calculated service lives between the relaxation histories (1) and (2) in Fig. 5.23. In both cases the service life clearly exceeds 100 years. On the other hand, the service life drastically decreases if the average stress level is increased. 

However, none of these experiments were pursued to the end of the depletion process and oxidative degradation was not actually achieved, since this requires very long test times even at high temperatures. Therefore, the depletion rates have to be considered with caution because they have been estimated from short-term experiments, and the assumption that the antioxidant depletion time is the most relevant part of the service lifetime, needs experimental justification. 

Oxidative degradation and the importance of antioxidant additives have already been discussed in detail in Sect. 5.2.2. T q)ically, phosphite and a hindered phenol are used as the main components of the antioxidant package for most of the HDPE geomembranes available on the market. Normally, carbon black is added as UV stabiliser. The concentration of the antioxidant components is t3 ically in the range of a few thousand ppm (mg/kg). Depletion of these antioxidants was identified as the relevant 

Parameters A and B differ significantly for various materials and measurement conditions. 

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Sufficient data for HDPE geomembrane materials are not usually available from pipe pressure tests which would enable a direct extrapolation of the stress-rupture curve at 23 °C or 40 °C according to the instmctions of ISO 9080 for small stresses and longer times. The emphasis is on the brittle branch of the stress-rupture curve which results from brittle failures and determines the failure behaviour at small stresses and extremely long service lifetimes. However, one can use the experience accumulated with HDPE materials over many decades (Schulte 1997 Krishnaswamy 2005) and fall back on the extrapolation factors of DIN 16887 or ISO 9080 which were checked by measured stress-rupture data of polyethylene pipes. These factors Ke give the time limits of a permissible extrapolation of the stress-rupture curves in the following sense. Let us assume that hoop stress versus times-to-failure data are measured at several higher test tempera-... 

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