Evaluation of creep behavior of geosynthetics using accelerated and conventional methods

Geosynthetics are susceptible to creep, which leads to time-dependent strains and potentially induces deformation of the structural systems. In the design of geosynthetics, one of the major issues is to apply the appropriate creep reduction factors. To evaluate the creep behavior of geosynthetics, four creep test methods were utilized in this study: Stepped Isothermal Method (SIM), Time-Temperature Superposition (TTS), Time-Temperature-Stress Superposition (TTSS), and a conventional method. SIM and TTS are accelerated creep methods by using elevated temperatures instead of a long testing duration. SIM particularly uses a single specimen throughout a sequence of elevated temperature tests and thus, material variability can be avoided in contrast to TTS. The procedure to generate a creep master curve in SIM was modified from that recommended by ASTM to account for thermal expansion of geosynthetics. TTSS imposes the stress effect to TTS and is suitable for polymers that have limitations in adopting TTS. In this study, three types of geosynthetics were tested: drainage components, i.e., high density polyethylene (HDPE) geonet and geocomposite, the expanded polystyrene (EPS) geofoam, and the polyethylene-terephthalate (PET) and HDPE geogrids. For the geonet and geocomposite, the tests were performed under compressive loads at different inclined angles to simulate the application in the side slope of the landfills. The results showed that the creep strains of the drainage components increased with inclined angles for both geonet and geocomposite. For the geonet, the secondary creep stage was found to coincide with the roll-over of upper ribs, indicating that the geometry of geonet had a strong influence to its creep behavior. Furthermore, the onset time of the secondary stage decreased as inclined angles increased. The creep behavior of the geocomposite was substantially different from that of the corresponding geonet, showing only primary creep stage. The absence of the secondary creep was due to the localized interface friction between the needle-punched nonwoven geotextile and the ribs. The friction prevented the abrupt roll-over phenomenon in the geonet. The compressive creep behavior of the EPS geofoam was investigated. A bilinear relationship between compressive strength and temperature with transition at 43oC had direct impact on the results of SIM and TTS. A premature secondary creep stage in comparison with a conventional method data and the change of activation energy were observed at test temperatures above 43oC. The alternative accelerated creep test, TTSS, was conducted at temperatures below 43oC and was found to be the most appropriate method for this geofoam. The tensile creep behavior of the PET and HDPE geogrids were evaluated, and the creep strains of the PET geogrid were much less than the HDPE geogrid at the same percentage of ultimate tensile strength. Also, the HDPE geogrid went through the primary, secondary and tertiary creep prior to the rupture, whereas only primary creep and rupture were detected in the PET geogrid. The activation energies of the PET geogrid were consistent regardless of the types of accelerated creep test methods. Contrary, higher activation energies were resulted from the short-term accelerated tests in comparison to the long-term tests for the HDPE geogrid. In order to develop the constitutive relationship, the Weibull model was modified. The results of model were well correlated to the experimental data.