True triaxial testing system for Proportional-Integral-Differential clay with (PID) control

A flexible boundary electro-pneumatic true triaxial system has been developed for testing cohesive soil. The system is capable of applying principal stresses on each face of a cubical specimen using a Proportional-Integral-Differential (PID) based closed loop control algorithm. This device has the ability to measure both the internal and external pore pressures for a 102-mm cubical specimen and uses custom developed Butyl rubber membranes. Measurement of the internal pore pressure is accomplished using a needle piezometer. Appropriate software has been developed that can automatically perform saturation, isotropic or K-o consolidation, and shear testing under stress or strain control along various stress or strain paths. Using this system, results from isotropically consolidated triaxial compression and triaxial extension stress paths are presented for cubical kaolin specimens. Comparative tests using conventional cylindrical specimens using lubricated ends were also performed. Issues related to the interference of the flexible membranes, uniformity of strains and bifurcations, interpreted friction angles, and undrained shear strength are discussed.