Large-Scale True Triaxial Test on Stress-Strain and Strength Properties of Rockfill

Limited by the particle size requirements of testing apparatus, little true triaxial test (TTT) research on rockfill has been conducted. Using a large true triaxial apparatus (TTA) developed independently by the Changjiang River Scientific Research Institute (CRSRI), tests with different intermediate principal stress ratios {Ratio $b[=({\sigma }_2-{\sigma }_3)/({\sigma }_1-{\sigma }_3)]$} and plane strain tests were carried out on sandstone rockfill. The results show that, with the same Ratio $b$, the initial slope in stress-strain curves and peak stress increased with rising ${\sigma }_3$; with low ${\sigma }_3$, volumetric strain first contracted and then transitioned to dilatancy; with high ${\sigma }_3$, it continued to contract throughout the deformation process. With stable ${\sigma }_3$ but increasing Ratio $b$, softening and brittle failure after peak stress were obvious and the strain corresponding to peak stress slightly lagged behind the strain bending point in the ${\epsilon }_1–{\epsilon }_v$ curves, which reflected deformation and failure of the specimen force chain. The effect of Ratio $b$ in increasing strength was significant, especially in the range 0–0.25, where strength showed significant growth. The Lade–Duncan failure criterion was more suited to revealing strength evolution than were other criteria, but both ${\sigma }_3$ and Ratio $b$ induced significant particle breakage. The actual tested strength was always lower than the estimated strength obtained by the Lade–Duncan failure criterion. Under plane strain, when deviatoric stress approached its peak value, Ratio $b$ was around 0.17–0.19 in all tested specimens. The contribution of ${\sigma }_2$ in increasing specimen strength was basically reflected. This means that, when test conditions are limited, the plane strain test can be used to make a rough estimate of the strength index of rockfill under complex three-directional stress.