Soil Response to Repetitive Changes in Pore-Water Pressure under Deviatoric Loading

Soils often experience repetitive changes in pore water pressure. This study explores the volumetric and shear response of contractive and dilative sand specimens subjected to repetitive changes in pore water pressure, under constant deviatoric stress in a triaxial cell. The evolution towards a terminal void ratio $e_T$ characterizes the volumetric response. The terminal void ratio $e_T$ for pressure cycles falls below the critical state line, between $e_{\min }<e_T<e_{cs}$. Very dense specimens only dilate if they reach high stress obliquity ${\eta }_{\max }$ during pressurization. The terminal void ratios for very dense and medium dense specimens do not converge to a single trend. The shear deformation may stabilize at shakedown, or continue in ratcheting mode. The maximum stress obliquity ${\eta }_{\max }$ is the best predictor of the asymptotic state; shakedown prevails in all specimens subjected to stress obliquity ${\eta }_{\max }<0.95·{\eta }_{cs}$ and ratcheting takes place when the maximum stress obliquity approaches or exceeds ${\eta }_{\max }\ge 0.95·{\eta }_{cs}$. Volumetric and shear strains can accumulate when the strain level during pressure cycles exceeds the volumetric threshold strain (about $5\times {10}^{-4}$ in this study). A particle-level analysis of contact loss and published experimental data show that the threshold strain increases with confinement $p_o^{\prime }$.