To simulate the complex multi-axial stress conditions of rock, a customized GDS True Triaxial Apparatus (GDSTTA) for rock (as shown in Fig 2) was manufactured for Northeastern University, combining the design requirements of the customer and Earth Products China (EPC).
The GDSTTA consists of two vertical and two horizontal servo-motor actuators for applying σ1 and σ2, and two high-performance pressure controllers to apply confining pressure and pore pressure respectively. The axial stress (σ1), horizontal stress (σ2), confining pressure (σ3) and pore pressure can be controlled independently.
For the GDSTTA, the stiffness of the loading frame is evaluated as 1GN/m, and the actuators are designed to produce a force up to 32kN. The confining pressure and pore pressure are both 2MPa. The dimension of the rock specimen is 20mm×5mm×40mm (Fig 3). Four LVDT's are used to monitor the displacement of the actuator and specimen deformation.
Fig 2. GDS True Triaxial Apparatus of Rock (GDSTTA)
Four internal encoders are configured to provide maximum accuracy and resolution, which can also be used to verify the sample LVDT's for additional test confidence. In the advanced software module, the synchronous off-centre loading function is able to accomplish the various stress path tests under load/displacement/stress/strain control modes, including uniaxial and biaxial, conventional triaxial and true triaxial compression. The triaxial cell has a large transparent plexiglass visual window, which is used in conjunction with the existing microscope, so as to facilitate real-time observation of the rock fracture process under uniaxial and biaxial compression conditions.
The apparatus uses two paired actuators to synchronize loading in the same direction at the same time, which can effectively avoid the off-centre problem during testing. The loading method requires extremely high servo control precision. Figure 4 shows a comparison between single actuator loading and synchronous loading under biaxial compression.
Fig 3. Installation of rock specimen on the GDSTTA equipment