Automated Flexible-Boundary True Triaxial System for Cohesive Soils

A fully automated flexible boundary true triaxial device with real-time feedback control system has been developed for testing cohesive soil. Using this device, a cubical specimen can be subjected to a wide variety of stress paths in three-dimensional stress space. Principal stresses are applied along each principal axis of the specimen using a Proportional - Integral - Differential (PID) based closed loop control algorithm and using electro-pneumatic transducers. 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. Use of flexible boundary conditions, the associated experimental issues, and flexible membrane compliance in strain computations are discussed. This paper describes the procedures involved in true triaxial testing such as preparing the remolded cubical specimens, assembling the specimen in the system, and saturating the specimen using backpressure. The PID coefficients used in feed-back control system show their significant influence in the measured stress-strain and strength behavior of clay. Therefore, for good quality testing, the values of PID coefficients have to be chosen carefully considering the influence of the variation in specimen stiffness during shear and pre-consolidation history.