Deformation behaviour of piezoelectric elements for acoustic dispersion measurements in soils

Piezoelectric plates, including bender elements (BEs), extender elements (EEs) and piezoelectric discs (PDs), are widely used to study acoustic wave dispersion in geotechnical materials. However, varying excitation frequencies alter piezoelectric plate deformation, creating clutter that complicates velocity detection, especially the near-field effect before the S-wave signal in BE tests. Numerical simulations reveal that ideal waves are generated only when the excitation frequency is below or equal to the first resonance frequency; arbitrary choices lead to misinterpretation of dispersion results. The traditional BE thus applies only to low frequencies, limiting dispersion testing, whereas the PD effectively broadens the usable frequency range for S-waves. To validate, EEs, BEs and PDs were fabricated and calibrated with standard materials, then tested on silty sand. Results show clear waveforms without clutter, and the near-field effect disappears. Measured data align well with model predictions: P-wave velocity exhibits significant dispersion due to the presence of gas, while S-wave velocity shows little dispersion within the tested frequency band.