Strain-Softening Behavior of Waste Containment System Interfaces

Laboratory test results indicate that many geosynthetic interfaces in liners and covers for waste containment exhibit strain-softening behavior. A peak shear strength is mobilized at small displacements, but post-peak strength reductions occur with increasing displacement. Mechanisms that contribute to strain-softening include clay particle reorientation, geosynthetic polishing, and geosynthetic failure. Test results obtained from direct shear and torsional ring shear devices are summarized to illustrate these mechanisms. The design implications of strain-softening behavior are then investigated using analytical and numerical models. Deformations in the waste under gravity are sufficient in many cases to limit the available shear resistance along an interface to less than the peak strength. It is recommended that a factor of safety greater than one be achieved in all containment system slope designs, assuming that residual strengths are mobilized along the base of the entire containment system.