PID Controlled Combined Axial Torsional Testing System for Cohesive Soil

The present paper summarizes the details of a testing system developed for performing combined axial torsional loading on hollow cylinder cohesive specimens. The axial-torsional loading system used in this research employs a MTS loading frame with Testware-SX control. The frame is operated hydraulically through the use of a servovalve that feeds actuators which can independently control the axial load/displacement and/or torque/angular rotation. Stress paths ranging from triaxial compression to pure shear to triaxial extension can be obtained using this testing apparatus by rotating the fixed inclination of the major principal stress (β) with the axis of rotational symmetry. The ratio between the torsional shear stress increment and the vertical stress increment is maintained a constant value to achieve a desired β value. A new approach using PID control was implemented to achieve precise control. To implement this into the control procedure, calculated control channel was used throughout the test using updated specimen dimensions. Undisturbed hollow cylinder specimens were obtained using high water content kaolinite slurry, and custom built consolidometer. Typical test results for stress path corresponding to β = 30 degrees were presented for isotropically consolidated kaolin clay under undrained conditions.