The load-displacement response of axially loaded structural inclusions in soil, which transfer the load to the soil along their shafts, is of growing interest in geotechnical engineering. The load-displacement response of a shaft interface is characterized by nonlinear and inelastic behavior. Surface roughness of the inclusion and stresses and deformation characteristics (stress-strain response, dilation, or contraction) of the soil element surrounding the inclusion are significant aspects of the interface mechanism. Localized shear displacement at the soil-shaft interface necessitates use of a constitutive model specifically developed for the interface. To verify theoretical models and modify and improve them, laboratory tests are performed. In recent years, there has been increasing emphasis in measurement of small-magnitude local strains to define stiffness at low strains similar to those encountered in the field in stress-path testing. To verify the theoretical model of the shaft-soil interface, a special test rig was developed that uses a cylindrical soil specimen with an inclusion in its center. The specimen represents a soil element surrounding a structural inclusion. The shaft-sand interface was investigated by small-magnitude local measurement of interface strains and thereby interface stresses and displacements during axial loading of the inclusion. The results are compared with those inferred from global measurements of interface variables. The advantages of the small-magnitude local strain measurements in determining the interface model parameters are presented. The use of the measured quantities in an elasto-plastic interface model is demonstrated by capturing the effects of confining stresses and boundary conditions of the soil specimen surrounding an axially loaded inclusion as well as the surface roughness of the inclusion.
