Design and Fabrication of End Platens for Acquisition of Small-Strain Piezoelectric Measurements During Large-Strain Triaxial Extension and Triaxial Compression Testing

A triaxial testing device was integrated with piezoelectric transducers to measure small-strain (<10(-3)%) dynamic soil properties during large-strain (15 %) triaxial testing. To incorporate the technology into existing equipment, end platens were designed and fabricated to facilitate direct contact between the transducers and the soil specimen. The platens protected the sensitive electronics while providing a seal between the confining fluid and the pore fluid. Two types of transducers were incorporated into the apparatus, bender elements and bender disks, used to measure shear wave and compression wave velocities, respectively. The 3.81-cm (1.5-in.) diameter acrylic end platens were designed to house the transducers, a porous stone, and openings to facilitate pore fluid drainage and wiring for the transducers. The top platen included a vacuum attachment and piston mount that enabled triaxial compression and triaxial extension testing. Removable stainless steel inserts were designed and fabricated to house and secure the transducers. These stainless steel inserts were used to ground the apparatus and allowed for maintenance and, if necessary, replacement of individual transducers. To ensure that the transducers were not damaged when subjected to the pore fluid, the transducers were waterproofed. Accurate readings of shear wave and compression wave velocities were obtained via proper design, fabrication, calibration, and implementation of the integrated small-strain components. Accurate readings of axial deformation, shear stress, and confining stress were also obtained via proper design, fabrication, and implementation of the vacuum connection components. Calibration results, as obtained from tests on specimens of medium-dense, dry, Ottawa sand, are presented and discussed. The system time delay was determined to be 5.67 x 10(-5) seconds for the bender elements and 3.50 x 10(-5) seconds for the bender disks. Measured shear wave velocity values ranged between 178 and 251 m/s and the corresponding compression wave velocity values ranged between 291 and 451 m/s.