Robustness of the P-RAT in the Shear-Wave Velocity Measurement of Soft Clays

Shear-wave velocity, $V_s$, is a mechanical geotechnical parameter required for the dynamic response of geomaterials. This property can be advantageously assessed both in the field and in the laboratory. Although field tests may offer the most precise methods to obtain $V_s$ of a certain soil, they, however, do not permit conditions different from those encountered in the field to be readily investigated. Several laboratory techniques of $V_s$ measurement have been developed, most notably the resonant column and the piezoelectric bender elements. Although the latter is widely used, it leads to a number of difficulties that cannot be denied, such as uncertainties in first arrival’s detection, near-field effects, and mixed radiation of primary and shear waves. In an ongoing effort to minimize/eliminate these difficulties, the geotechnical group at the Université de Sherbrooke (UdeS) developed the piezoelectric ring-actuator technique (P-RAT), which can be easily incorporated into traditional geotechnical apparatus, an advantage that facilitates its utilization in other geotechnical laboratories as a powerful tool in $V_s$ measurement. This paper summarizes the results of parallel tests of P-RAT installed in typical oedometer cells to measure the $V_s$ of soft sensitive clay samples extracted from two different sites in Québec, Canada. These tests were carried out at two different institutes, the UdeS and the École de technologie supérieure (ÉTS). The main purpose was to examine the reliability of the P-RAT test results by applying the same test procedures to similar test materials. The results show that the change in the used sensors, input signals, data acquisition system, and technical operators between the two laboratories has no practical effect on the measured $V_s$ values of all tested samples, which confirms the robustness of the technique and promotes its incorporation in other geotechnical apparatus and laboratories. Results obtained at both institutions were also used to present consolidation curves of the tested soft clays in terms of their shear-wave velocities and establish unique correlations between the stress-normalized shear-wave velocities, $V_{s1}$, with the overconsolidation (OCR) and void ($e$) ratios for each site investigated.