Compression of “Free-Ends”During Triaxial Testing

When lubricated platens (free-ends) are used to load particulate media there is an associated error in any measured axial deformation due to compression and distortion of the rubber/grease layer. This extraneous deformation is termed the bedding error and it may account for as much as 80% of the total axial deformation recorded during a constant R stress path. Two independent methods of determining the bedding error have been used. In the first method, additional sets of free-ends are incorporated into samples during their preparation. In the second method, samples with various heights are employed. For a Constant R stress path the bedding error is linearly proportional to log σ₁¹ for the whole of the stress range investigated. This linear relationship is true regardless of sample porosity, particle shape and particle size, however, the magnitude of the bedding error varies non-linearly with particle size and the value for a sandy gravel is approximately 30 times the value for a fine silt.