Peak versus residual interface strengths for landfill liner and cover design

The rationale for using peak, residual, or a combination of these shear strengths for the analysis of geosynthetic-lined slopes and design recommendations for landfill liner and cover systems is presented herein. Landfill liner systems using geosynthetics that contain sideslopes are recommended to be designed using the methodology presented by Stark and Poeppel: (1) assign residual shear strengths to the sideslopes and peak shear strengths to the base of the liner system and satisfy a factor of safety greater than 1.5; and also (2) assign residual strengths to the sideslopes and base of the liner system and satisfy a factor of safety greater than unity. The authors recommend that the stability of landfill cover systems be analysed using peak shear strengths with a factor of safety greater than 1.5 because of the absence of large detrimental shear displacement along the weakest interface. If, for some reason, the slope angle of the cover system exceeds the friction angle of the weakest interface, or large displacements such as construction-induced displacements or seismically induced displacements are expected, a residual shear strength with a factor of safety greater than unity should be used for the cover design. In both liner and cover designs a peak composite failure envelope that describes the weakest interface should be used to represent the peak shear strength, and the residual failure that corresponds to the peak composite failure envelope should be used instead of the lowest residual failure envelope.