Estimating multidirectional stiffness of soils using planar piezoelectric transducers in a large triaxial apparatus

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Cohesionless granular soils display stiffness anisotropy, and accurately characterizing it is essential for numerous geotechnical applications. The inherent anisotropic behaviour of geomaterials is not comprehensively understood; hence, research on the development of novel geophysical testing methods for assessing multidirectional stiffness is important. This contribution presents an innovative arrangement of vertical and horizontal planar piezoelectric transducers to describe the stiffness anisotropy of soils. The transducer assembly can be used to measure nine different elastic wave velocities of a soil specimen in three directions of principal axes having a wide range of median particle sizes. To describe the performance of the developed transducer assembly, four different types of cohesionless granular materials: Toyoura sand, Kashima river sand, glass beads, and Oiso gravel were tested in a large-sized triaxial apparatus having a rectangular specimen of size 235 × 235 × 500 mm. All the tested materials displayed considerably stiffer responses in the horizontal direction as compared to the vertical direction. Except for the coarse gravel, natural sands and glass beads demonstrated isotropic behaviour in the horizontal direction. No significant influence of specimen dimension on the measured vertical wave velocities was detected when the large triaxial test results and medium-sized triaxial test results were compared. The paper also provides a comparison of the vertical stiffness evaluated using planar piezoelectric transducers with those measured statically by applying small-amplitude vertical cyclic loading.