Distribution and variability of interface shear strength and derived parameters

Knowledge of distribution type and quantification of variability are fundamental for evaluation of characteristic values for use in design as well as inputs for reliability analyses. The paper formally examines the assumption of normality to represent peak and large displacement interface shear strengths and the derived Coulomb strength parameters. Additionally, the variability and uncertainty for three generic interfaces are computed and compared using information obtained from global databases, inter-laboratory and repeatability testing programmes. Using subjective and objective statistical test methods, normal distribution is found suitable to represent interface shear strengths and their derived strength parameters especially when variability is small. When variability of the interface shear strength is high, a normal distribution with truncated tails at possible minimum and maximum values is recommended to avoid sampling of negative or extreme values. The variability of interface shear strengths computed using global databases are 3–5 times, and can reached up to 8 times higher for the derived parameters compare to repeatability datasets. It is concluded that variability and uncertainty computed using global and inter-laboratory datasets yield overly conservative outcomes when adopted in design.