Bentofix® Geosynthetic Clay Liners (GCLs) are commonly reinforced with polypropylene (PP) fibres to improve their resistance to shear loads. Since the shear resistance is often used in the design of slopes, the long-term properties of the reinforcement can be the determining factor affecting the service lifetime of the slope. Therefore, it is important to understand the long-term behaviour of reinforced GCLs. Two possible failure mechanisms for the reinforcing fibres are creep rupture and oxidation.
For this reason NAUE started a research program in 1999 to investigate these issues. One research part was carried out by the BAM (Bundesanstalt für Materialforschung und -prüfung [federal agency for materials research and testing]). The results from the BAM experiments on the > 200 year functional lifetime of Bentofix discussed in the NAUE News issue No. 22 were independently complimented by a series of ovenaging experiments designed to measure the oxidation rates of the fibres when exposed to air in a simulated landfill environment. The second research project was carried out at TRI, a testing and research institute in Austin/Texas. This study involved using Arrhenius extrapolation methods to determine the oxidation rate of fibres when exposed to air at temperatures of 100°, 90°, 80°, 70°, and 60°C. The material tested was a needle-punched, nonwoven polypropylene Secutex® geotextile made from fibres used in the Bentofix® GCL. The specimens for oven exposure were approximately 5 cm x 15 cm. Over 500 specimens were cut and then were shuffled to try to minimize the effect of thickness variations in the material.
Test specimens were exposed in forced-air ovens at temperatures of 100°C, 90°C, 80°C, and 70°C.
The specimens were hung under racks with unfolded steel paper clips. The specimens were spread out evenly around the oven and they were not touching each other. Three different ovens were used to perform the four sets of exposures. The unexposed and exposed test specimens were evaluated by a strip tensile test. The test grips were 2.5 cm x 10 cm, the strain rate was 10 cm/min, and the initial gage length was 7.5 cm. All tests were taken to failure and the maximum load and the strain at the maximum load were recorded.
The aim of the study was to propose a generally accepted requirement that the tested geotextile should maintain over 50 % of its strength when exposed to the tested condition, which was also the basis for the extrapolation. When these data were used to extrapolate, it was found that if the textiles were continuously exposed to fresh air in a high air-flow environment, the predicted service lifetime would be about 17.8 years at 15°C. However, since these were extreme and not realistic conditions the results were compared to oxidation rates found in 8 % oxygen, which is believed to be the maximum concentration one would find in a buried application. In this case, the oxidation rate was 21 times slower than the rate found in air (21 % oxygen).
This means that the 17.8 year service lifetime would actually be 373 years in a buried application; agreeing very well with the results from the BAM. The results for these two independent studies clearly show the long term performance capabilities of Bentofix® GCLs. However, one can be less conservative and assume that a remaining longterm tensile strength of 25 % or even 10 % would be sufficient. In this case the lifetime prediction for Bentofix® would increase to 560 years, resp. 672 years. The full study will conclude shortly.
