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Title Ultrasonic Monitoring of the Excavation Damaged Zone in Boom Clay in the PRACLAY Experiment

Authors Juan M. Reyes-Montes, Lou Areias, Xiang Ling Li, William S. Pettitt, Jonathan R. Haycox and R. Paul Young

Publication Reference Impact of Thermo-Hydro-Mechanical-Chemical (THMC) processes on the safety of underground radioactive waste repositories: An international conference and workshop in the framework of the European Commission TIMODAZ and THERESA projects. Luxembourg, 29th September – 1st October 2009. "Book of Abstracts" pp65.

Abstract The aim of the PRACLAY experiment is to characterise the large scale Thermo-Hydro-Mechanical-Chemical (THMC) response of the Boom Clay to the excavation of a disposal gallery and to an imposed large scale thermal load. The experiment simulates the thermal load imposed on the Boom Clay in a real repository of vitrified heat emitting high level waste. Ultrasonic monitoring is one of the methods used to characterize the damaged zone in the clay rock around the PRACLAY gallery. This non-destructive technique provides a unique means to monitor the evolution of fracturing around the excavation damaged zone (EDZ).The monitoring system consists of an array of 24 transmitters and 20 receivers installed both in the lining and the host rock to monitor changes in waveform attenuation and propagation velocity. Monitoring consists of daily measurements performed along raypaths crossing different regions within the EDZ. This investigation uses an inversion technique to analyze changes in transmission velocity based on the non-interactive crack effective medium theory. The technique allows the derivation of anisotropy and splitting of elastic waves from modelled crack density, aspect ratio and fabric orientation for moderately jointed samples. It considers the isotropic case to invert elastic wave velocities and infer the evolution of crack density and aspect ratio. This approach is very stable and gives coherent results in terms of crack density, aperture evolution, mean crack distribution and orientation as well as changes in porosity. This paper presents the first results for the passive and active monitoring during the period between September 2006 and January 2008, corresponding to the evolution of the EDZ following the excavation of the PRACLAY gallery and before the thermal load experiment begins.

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Link http://www.itc-school.org/index.php/Present-Courses/Impact-of-Thermo-Hydro-Mechanical-Chemical-THMC-Processes-on-the-Safety-of-Underground-Repositories.html



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