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Title Characterisation of rock mass crack damage using ultrasonic surveys

Authors J.M. Reyes-Montes W.S. Pettitt J. Andrews R.P. Young

Publication Reference Euradwaste '08, Seventh European Commission Conference on the Management and Disposal of Radioactive Waste, 20-22 October 2008, Luxembourg

Abstract Medium and high level radioactive waste requires safe storage isolated from the environment over geologic time periods. Disposal in deep underground geological formations is one of the likely options to dispose of this waste. A key element in the evaluation of the long-term safety of an underground disposal is the study and control of the damaged zone (DZ) surrounding the infrastructure. The evolution of the DZ is affected among other factors by excavation induced stress changes, pore pressures and the heating-cooling cycle of the decaying waste. Ultrasonic monitoring provides a unique means to non-destructively monitor the evolution of fracturing around the DZ. In particular, the monitoring of waveform attenuation and changes in propagation velocity can be used as an estimate of the evolution of crack damage. In this paper, two different approaches are presented in order to interpret the evolution of rock mass properties related to its capability for the transportation of fluids. These techniques were initially developed as part of the OMNIBUS project, part funded by the EC under the 5th Euratom framework and will be further extended to study the impact of thermal-induced stress changes on the host rock as part of the TIMODAZ project (in the 7th Euratom research framework programme). Firstly, monitored full waveforms from laboratory compression tests and in-situ experiments were correlated with a suite of tests in finite difference models with variable fracture density, size and fluid content (Pettitt et al., 2004). The correlation of amplitude ratios and phase differences through the models with the empirical data provided a tool to interpret the rock disturbance in terms of rock mass properties. A second approach uses the non-interactive crack effective medium theory, which allows the derivation of anisotropy and splitting of elastic waves from modelled crack density, aspect ratio and fabric orientation for moderately jointed samples. For the purpose of this investigation, the isotropic case is considered, for which the method can be used to invert elastic wave velocities to infer the evolution of crack density and aspect ratio. Inversions performed using this approach have been shown to be very stable and gave coherent results in terms of crack density and aperture evolution, mean crack distribution orientations and porosity evolutions (e.g. Benson et al., 2006) These parameters are key to characterise the rock’s permeability.

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Link ftp://ftp.cordis.europa.eu/pub/fp7/euratom-fission/docs/euradwaste08_prog_abstracts_wv_en.pdf



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