Evaluation of Ultrasonic Pulse Velocity Measurements and Resultant Elastic Moduli of Hard Rock Core Specimens

Abstract

Material behaviour and strength characterization is a critical aspect of site investigation programs when designing engineered structures in rock. The measurement of Ultrasonic Pulse Velocities (UPV) in rock core is a straightforward non-destructive test used for estimating fundamental rock properties. UPV measurements emit high frequency acoustic waves through rock core specimens. The velocities of compressional and shear waves (P- and S-waves, respectively) are influenced by the unique microstructural characteristics of each rock sample. Upon calculating P- and S-wave velocities and bulk density, dynamic elastic constants can be calculated. A common problem with UPV derived elastic constants is that these measurements are often different than the static elastic parameters derived from destructive mechanical tests. Improving the understanding of why differences in elastic moduli occur between dynamic and static testing methods is investigated in this research. Factors including rock specimen lithology, density, core dimensions, saturation, pre-existing damage, and the impact of applied uniaxial and isotropic stress are investigated to evaluate their impact on P- and S-wave velocities and dynamic properties for commonly researched homogenous rock types. Further, the relationship between UPV core specimen measurements and in situ sonic borehole testing data is compared in a practical case study using new laboratory testing completed as part of this research, and published data from the Geological Survey of Canada. This study provides an opportunity for determining the impact of saturation and in situ stresses on measured wave velocities. The findings of this research demonstrate the significant impact of saturation, and uniaxial and isotropic stresses on P- and S-wave velocities and resultant elastic moduli on tested rock core specimens. Further, the non-linear axial stress-strain behaviours of UCS core specimens are related to the dynamic elastic measurements with and without compressive stress using UPV testing methods. Dynamic measurements of Young’s moduli taken at equivalent stress magnitudes where static Young’s moduli are measured are shown to better agree with static moduli data. Finally, axial and lateral P- and S-wave velocity changes under uniaxial compressive stress are analyzed with respect to critical damage thresholds, including crack closure, crack initiation, and crack damage for Lac du Bonnet core specimens.