Granular material radial deformation measurements with a circumferential extensometer in repeated load triaxial testing

Determination of both axial and radial specimen strains in a repeated load triaxial test is essential for properly characterizing the directional dependency of resilient granular material behavior. Radial deformation measurement is not yet included in the standard AASHTO test procedure. The method of measuring radial strains adopted in this study emphasizes the use of a contact-type specimen instrumentation technique with a circumferential chain extensometer. The cricumferential extensometer was successfully used in repeated lod triaxial testing to measure radial strains of three aggregate samples with different material types and properties. The accurcy of the measuring system was within 10^-5 (in strains) when the smallest strains recorded were on the order of 10^-4. Nonlinear stress-dependent models for properly charcterizing the anisotropic granular material stiffnesses were developed from measured axial and radial strains. The vertical/horizontal stiffness ratios in the triaxial sample consistently increased with increasing principa; stress ratios, which often occurs in pavements under wheel loading. In general, the repeatability in radial strain measurements was deemed to be satisfactory and more reliable strains were measured at low confining pressures.