The time to rupture of a 1.5-mm-thick high-density polyethylene geomembrane aged in a typical landfill composite liner configuration is investigated under a pressure of 250 kPa at 85 degrees C. The geomembrane was underlain with a geosynthetic clay liner and had a 560 g/m(2) needle-pinched nonwoven geotextile protection layer separating it from 50 mm of drainage gravel containing leachate. Seventeen (0.6-m-diameter) tests were conducted. In addition to 9 months required to deplete antioxidants (Stage I), the tests indicated a lag period (Stage II) of 5.5 months and a time from the start of degradation to rupture (Stage III) of 20 months, giving a total inferred time to rupture of 34.5 months (2.9 years). There were up to 61 brittle ruptures per sample (i.e. >2 million cracks per hectare). The ruptures were predominately oriented in the machine direction and located (1) directly beneath a gravel contact, (2) at the side of a gravel indentation, or (3) between gravel indentations. The ruptures between gravel indentations were the least frequent but largest. The calculated strains perpendicular to the rupture direction were 24 +/- 6%. Rupture occurred, although the average stress-crack resistance for all ruptured geomembrane samples still was approximately 760 +/- 200 h, with a minimum of 360 h. These results indicate the importance of minimizing tensile strains in the geomembrane in the design of a liner system. (C) 2014 American Society of Civil Engineers.