Stress relaxation of smooth HDPE geomembranes

Abstract

The stress relaxation behavior of the HDPE part is dependent on formulated density and melt flow index. Since 1990, the multi-axial tension test has been regularly used to assess the ability of geomembranes to undergo out-ofplane deformations as in cases of rising gas bubbles or differential settlements. This multiaxial deformation is simulated in ASTM D5617 Standard Test Method for Multi-Axial Tension Test for Geosynthetics, at a constant rate of 6.9 kPa/min. While such a rate appears reasonable for index properties of most geomembrane types, semi-crystalline geomembranes like high density polyethylene (HDPE) might be better represented at much slower rates. This experimental study was undertaken to investigate this possibility. Using eight sequentially slower pressurization rates from the default value down to 0.000157 kPa/min the strength values were not markedly changed (yield was unchanged and break increased slightly). However, the elongation values changed considerably (yield increased by 200% and break increased by 400%). Thus, it is felt that the field performance of HDPE geomembranes can best be simulated by conducting the multi-axial test at very slow pressurization rates. While such rates are not practical in the context of an index test method the authors feel that some type of commentary to this effect could well be included in the respective test method standards.