Undrained triaxial test stress path shape as an indicator of sample quality

When determining soil strength parameters from laboratory tests for use in geotechnical safety evaluations, knowing the quality of the test sample is important. The quality of a triaxial test sample is commonly assessed by evaluating Δe (the change in void ratio during consolidation to the initial in situ stress)/e₀ (the initial void ratio). However, the type of behaviour of a sample during advanced testing can also provide information about the sample quality. We have analysed the stress paths of 89 active and passive anisotropically consolidated undrained triaxial tests on high quality block samples of clay with regard to shape, average dilatancy, and strain at failure and visualized the results in plots of the normalized shear stress against the overconsolidation ratio. From those plots, typical regions where points with a certain stress path shape, average dilatancy, or strain at failure can be identified as well as outliers. Our analysis indicates that outliers typically are points with a lower sample quality.