Multi-directional elastic wave measurements during triaxial compression on Toyoura sand using planar piezoelectric transducers

This paper presents the development and application of an automated multi-directional elastic wave measurement system using disk-shaped piezoelectric transducers. The system enables quick installation, reliable sealing, and non-invasive, automated elastic wave measurements in orthogonal directions throughout triaxial tests. The system was employed in triaxial compression tests on Toyoura sand to investigate the evolution of elastic wave velocities and their anisotropy up to large strains, under both drained and undrained conditions, as well as under saturated and dry conditions. The shear (S-) wave and compression (P-) wave velocities exhibited direction-dependent evolution during triaxial compression. For the saturated sand, the S-wave velocities corrected by a void ratio function aligned along a unified trend with stress states at the end of triaxial compression, regardless of the initial relative densities, drainage conditions, or wave propagation and oscillation directions. For the dry sand, a consistent relationship was observed throughout the tests between the P-wave velocity ratio and the square of the S-wave velocity ratio. These findings suggest that the evolution of elastic wave velocities and their anisotropy during triaxial compression are governed by the stress states and tend to converge toward a consistent residual state regardless of the initial relative densities or drainage conditions. This demonstrates the potential of multi-directional elastic wave measurements for capturing the changes in soil fabric during shearing.