Soil-geosynthetic interaction: interface behaviour

Knowledge of soil vs. geosynthetic and geosynthetic vs. geosynthetic interface shear behaviour is of fundamental importance to designers. This paper considers factors influencing measured behaviour, summarizes methods of measurement including specifications, presents data that quantifies variability, details methods for obtaining characteristic interface shear strength parameters for use in design and defines key questions to be answered by engineers. It is shown that the design of direct shear apparatus is the main reason for observed large variability of measured interface strengths from inter-laboratory comparison testing pro-grams. The need to carry out repeat tests at each normal stress is established. The use of global databases of measured interface strengths to inform selection of strength parameters is discouraged. A recommendation is given to use the results of repeatability testing programs to support calculation of characteristic interface strength parameters. Using the example of landfill lining design, guidance is provided on selection of strength parameters in conjunction with relevant factors of safety, consequences of failure, selection of the controlling interface and minimizing interface displacements.