Abstract:

The back pressure saturation method, a widely adopted and efficient technique for enhancing soil saturation, can nonetheless introduce notable deviations in soil strength parameters. Standard spherical glass bead sand was utilized for conducting benchmark consolidated undrained (CU), consolidated drained (CD), and dry sample tests. Real-time accurate measurements and comparative analyses of deviatoric stress and pore pressure (or volumetric deformation) data were performed. Utilizing the p'-q stress path diagram, the influence of back pressure application on soil mechanical properties was significantly demonstrated and quantitatively analyzed, thereby preliminarily elucidating the mechanism of back pressure influence. The setting of back pressure significantly impacts the results of CU tests, where the shape of pore pressure development governs the shape of deviatoric stress development, ultimately influencing the determination of strength parameters. However, the stress path remains constrained within the framework of the revised Cam-Clay model. The mode and rate of pore pressure development are primarily constrained by the magnitude of the back pressure setting and the relative density of the sample. As back pressure increases, the potential change in pore pressure also increases, resulting in a greater amplitude of deviatoric stress change. Similarly, a higher relative density leads to a faster development rate of pore pressure and an increased rate of deviatoric stress. Under identical initial conditions, the development of pore pressure in CU tests exhibits high consistency with the development of volume deformation in CD tests, revealing the common essence of the sample’s volumetric deformation potential across different boundary conditions. A quantitative prediction formula for the residual strength of CU tests at the critical state is presented. The residual pore pressure value can be initially quantified based on the relative density and back pressure measurements, subsequently leading to the determination of the residual strength of CU.

Key words: back pressure, deviatoric stress, consolidated undrain (CU) strength, effective stress path, spherical glass sand