Creep, the tendency of materials to deform over time due to applied stresses, has been a focus of the geosynthetics industry since testing first began more than 25 years ago. Creep testing methods have evolved, but the accurate assessment of tension creep (along with transmissivity) remains essential to the product evaluations of organizations considering geomembrane/geocomposite drainage systems.
The Geosynthetic Institute’s 2014 whitepaper on the subject notes that, “While all types of geosynthetic materials can be evaluated for their tension creep behavior it is the reinforcing materials (geotextiles, geogrids, geostraps and geoanchors) that are usually the target for such evaluation. This is the situation since (i) limiting strains are required for safety of the structure, and (ii) a specific reduction factor is necessary for the design process.”
The institute recounts in its paper how early studies resulted in two parallel standardized test methods. “In both standards the test specimen is firmly gripped at its ends and then stressed mechanically or hydraulically. There is no lateral confinement against the creep testing specimens since Wilson-Fahmy, et al. (1993) have shown that when uncoupled from the lateral confinement there is no beneficial or detrimental effect. Deformations during creep testing are read by grip separation or alternatively by LVDT’s or lazer’s and are then converted to engineering strain using the original gauge separation distance. Data sets of instantaneous strain versus time are then plotted for different stress levels.”
While early creep testing was straightforward and inexpensive, it required a controlled laboratory environment and thousands of hours to complete. For example, during creep testing of Agru America’s 60-mil HDPE Drainliner, a compressive load of 15,000 psf was applied for 10,058 hours. A time-temperature superposition method shortened creep testing times, but required environmental chambers for each temperature. Finally, with the stepped isothermal test method adopted in 2007, strains could be measured as stresses were applied while increasing temperature in a single chamber. ASTM D7361 notes that, “This method can be used to establish the sustained load compressive creep characteristics of a geosynthetic that demonstrates a relationship between time-dependent behavior and temperature.”
The D7361 standard is recommended by the institute for measurements of drainage core creep (RFcr) done in conjunction with 100-hour transmissivity tests. Using equations from that standard and data from the earlier test, the RFcr factor for Agru America’s Integrated Drainage System (IDS) was determined to be 1.14. Actual RFcr values for Agru-IDS material in a capping system would obviously be less given the much lower normal loads.
Agru-IDS – as provided in the Drainliner, MicroDrain, and Super Gripnet final closure system components – provides excellent drainage of a final cover closure system for typical/aggressive slopes and fine-grained soil covers. The reliable and consistent high asperity spiked surface ensures optimum interface friction characteristics at any point on the sheet surface. The top surface is covered with a nonwoven geotextile to prevent soil infiltration and to create the drainage layer. The bottom surface with its high spikes and patterned texture (Super Gripnet product) provides maximum interface friction and high factor of safety against sliding.
Agru-IDS is manufactured on advanced flat cast extrusion equipment. Machined rollers provide the final structured surface with a minimum 3.3 mm (0.130 in) studded drain surface (Drainliner and Super Gripnet) on the top side and 4.45 mm (0.175 in) spiked friction surface on the bottom side (Super Gripnet). The finished product includes a smooth 12-inch edge on both sides of the roll to facilitate fusion welding as with standard textured geomembrane.