A true triaxial apparatus for soil testing with mixed boundary conditions

The mechanical description of a new true triaxial apparatus for soil testing is presented. The design took into consideration flexibility in accommodating different specimen sizes, easy assembly procedure, and well-controlled boundary conditions. The apparatus can perform stress-controlled and strain controlled experiments. It is well instrumented with load, displacement, and pressure sensors and has the capabilities to capture strain localization and shear band development. Verification experiments were conducted on F-75 Ottawa sand to study the influence of b-value (b = (sigma(2) - sigma(3))/(sigma(1) - sigma(3))) on stress-strain and volumetric behavior of sand. The results show that the specimen stiffness increases, and the amount of post-peak softening increases as b-value increases. The peak and critical state friction angles and the rate of dilation increase as b-value increases from 0 to 0.25, followed by a smaller increase in the friction angles and no change in the rate of dilation as b-value increases. Specimens failure is characterized by nonuniform deformations that initiate during the hardening regime before the peak stress; however, shear bands become visible on the specimen surface during the post peak softening at which specimens' volumetric strain changes from dilative behavior to the constant volume condition.