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Long-term water flow capacity of geosynthetic drains and structural stability of their drain cores
(2008)
The German Federal Institute for Materials Research and Testing (BAM) has established a testing guideline to assess the suitability of geosynthetic or geocomposite drain (GCD) elements for final landfill cover systems. In an earlier paper the uniform procedure according to the BAM guideline for determining long-term water flow capacity was described, and the structural stability of drain cores under creep was investigated. The extrapolation of creep curves is permissible only when it can be shown, over the extrapolated period (at least 100 years), that oxidative aging does not invoke any relevant changes in the polyolefin material. This can be shown by measuring antioxidant depletion and mechanical degradation of the drain cores and filter/protection geotextiles in oven aging and water immersion tests. In this paper the results of such aging studies on three different GCD products are reported. Degradation of the tensile strength of HALS-stabilized polypropylene needle-punched nonwoven geotextiles (PP NWGTs) starts right from the beginning of the oven aging. This has to be taken into account for lifetime predictions based on residual tensile strength. The rate of oxidative degradation of NWGTs decreases significantly with increasing mass per area. Therefore the expected service life sets a limit on acceptable geotextile grammage. Antioxidant depletion in the drain cores, which are made of relatively thick PP strands or HDPE ribs, is about an order of magnitude slower than in filter and protection NWGTs, which are made of thin fibers. Service life values in excess of 100 years may be obtained for the products when the geotextiles and drain cores are properly stabilized and designed.
Geosynthetics are planar polymeric products, which are used in connection with soil, rock or other soil-like materials to fulfill various functions in geoenvironmental engineering. Geosynthetics are of ever-growing importance in the construction industry. Sealing of waste storage facilities to safely prevent the emission of wastewater, landfill gas and contaminated dust as well as the diffusion of pollutants into the environment and coastal protection against storms and floods and reconstruction after natural disaster are important fields of application. We will give an overview of the various geosynthetic products. Two examples of the material problems related to geosynthetics are discussed in detail: the effect of creep on the long-term performance of geocomposite drains and the numerical simulation of the interaction of soil with geogrids. Both issues are of importance for the use of these products in landfill capping systems. The various functions, which geosynthetics may fulfill in the protection of coastal lines, are illustrated by case studies. The geosynthetic market is evaluated and economical and environmental benefits, as well as environmental side effects related to the use of geosynthetics, are discussed.
Plastic products used in geotechnical engineering, so-called geosynthetics, are often made of polyolefins (polyethylene or polypropylene) and stabilized against oxidative degradation by antioxidants (AO). Three types of AO “packages” are used: (P1) phenols and phosphites/sulfides, (P2) hindered amines (HAS) with a marginal stabilization P1 of the basic resin, (P3) a combination of packages P1 and P2 with comparable amounts of phenol and HAS. We report about long-lasting oven aging and water immersion tests at 80 °C of 29 different polyolefin based products (geomembranes, geonets and geotextiles). Pronounced differences in the degradation behavior were found depending solely on the type of AO package. In case of P1, the oxidative degradation is prevented by the AO. Autocatalytic oxidation only begins after depletion of AO and may lead to catastrophic failure depending on the environmental conditions. In case of P2, degradation starts right from the beginning. However, it is decelerated and proceeds gradually. In case of P3, the degradation behavior seems to be a superposition of the characteristics obtained for P1 and P2. Plotting the fractional loss of the AO in P1 and P2 as function of the square root of aging time, we identified two regimes of AO depletion. We suggest to attribute this observation to the different mechanisms of AO depletion.