Laboratory measurements of the longitudinal and transverse wave velocities of compacted bentonite as a function of water content, temperature, and confining pressure

Abstract

Bentonite is a material commonly included in the design of the sealing liners of radioactive waste repositories. Understanding the evolution of its elastic properties during the repository lifetime could provide a powerful aid to the development of nonintrusive monitoring techniques based on tomographic methods. In this contribution, we present the results of a series of laboratory experiments aimed at measuring the elastic properties of a compacted bentonite at different water contents, temperatures, and hydrostatic confining pressures similar to those expected in a realistic radioactive waste repository. We prepared four sets of samples at different water contents (10, 20, 35, and 52%); we measured that VP and Vs (and the derived bulk and shear moduli) (i) increase significantly (up to 60%) with increasing water content, (ii) decrease up to 15% with increasing temperature in the range of 30 to 80°C, and (iii) increase significantly with confining pressure following different paths highly dependent on the water content (with drier samples showing stronger dependence on confining pressure). Further, we provide empirical relations between ultrasonic velocities and temperature, and ultrasonic velocities and pressure. We also measured the effects of dehydration at temperatures up to 160°C, which led to the structural failure of samples. Finally, we modeled our results according to two widespread effective elastic media models, Hashin‐Shtrikman bounds and modified Voigt average, achieving only partial success in describing our results. In the light of these results, the increase of seismic wave speeds as function of increasing confining pressure is interpreted as an effect of the pore collapse.