Variability and Correlation of Strength Parameters Inferred from Direct Shear Tests

This paper presents a theoretical approach to evaluate the uncertainties and the correlation of the shear strength parameters c and t (cohesion and friction coefficient), obtained in direct shear tests. The analysis is based on the hypothesis that the soil is statistically homogeneous, with shear strength normally distributed having an expected resistance which is linearly dependent on the effective normal stress. With regard to the scatters of the population of the shear strength, two further statements have been formulated for the standard deviation: (1) a constant value, independent of the effective normal stresses and (2) a value which is linearly dependent on the normal effective stresses as a consequence of a constant coefficient of variation. The investigation shows that the strength parameters are negatively correlated and the coefficient of correlation and the coefficients of variation of cohesion and friction angle are highly dependent on the number of shear tests and the normal stresses used in the tests.