Development of a P-Wave Measurement System for Laboratory Specimens

A new system, capable of generating and receiving P-waves in the laboratory specimens, is developed. P-waves were generated using a multi-layered piezoelectric actuator made of ceramics placed above the top cap and were received at two different levels at the side of specimen using two piezoelectric accelerometers. Triggering to generate P-waves was made by employing single pulse, single sinusoidal or continuous sinusoidal waves. Dynamic vertical Young's moduli (E_d) were computed from the travel time of the waves between two accelerometers and the corresponding travel distance between them. E_d obtained from this system were compared with the statically measured vertical Young's moduli (E_s) evaluated by conducting small unloading/reloading cycles on the same specimens. The stress level was kept constant to make both measurements just one after the other. For this study, four tests were conducted using medium and large scale triaxial apparatuses. The testing material included Toyoura sand (Uniformly-graded sand from Japan) and Chiba gravel (Well-graded sandy gravel from Japan). As a result of this study single pulse or single sinusoidal triggering was found better an continuous sinusoidal triggering. Moreover, no significant effect of specimen size was observed in E_s and E_d for Toyoura sand specimens. Next, E_d obtained was larger than E_s in both types of materials. Lastly, the difference between E_s and E_d increased with the increase in particle size of specimens. Such difference was about 15–25% in Toyoura sand specimens and it was about 100% in loose Chiba gravel specimens.