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Geological Society, London, Special Publications; 2007; v. 284; p. 41-55;
DOI: 10.1144/SP284.4
© 2007 Geological Society of London

Articles

Velocity survey of an excavation damaged zone: influence of excavation and reloading

J. Damaj1, C. Balland1, G. Armand2, T. Verdel1, D. Amitrano3 & F. Homand4

1 LAEGO-INERIS, Ecole des Mines de Nancy, Parc Saurupt, F-54042 Nancy Cedex, France (e-mail: cyrille.balland{at}ineris.fr)
2 ANDRA, CD 960, Voie Gasselle, 55290 Bure, France
3 LIRIGM, Université J. Fourier, BP 53, 38041 Grenoble, France
4 LAEGO-ENSG, bât. E, rue du Doyen-Marcel-Roubault, 54501 Vandœuvre-les-Nancy cedex, France

During gallery excavation, regardless of the method chosen, the surrounding rock is mechanically disturbed in the case of underground disposal of chemical or radioactive wastes, and such mechanical changes to the rock state can create preferential pathways for the release of material from the waste inside the excavation (e.g. chemical waste gases, brine or dissolved radionuclide) up to the biosphere. The mechanical characterization of this disturbed zone is thus essential in assessing the rock capacity necessary to form an impermeable geological rock barrier. The key to this effort lies in determining the nature, extent and change to this disturbed zone. A survey by ultrasonic wave analysis is particularly appropriate for these purposes, as it provides information on the zone left undisturbed by the boreholes. The main aim of this paper is to describe a recent in situ experiment (called EZ-A) conducted in Opalinus clay at the Mont Terri Rock Laboratory in Switzerland. This experimental campaign successfully studied the possibility of improving the properties of rocks forming the excavation damaged zone (EDZ). The experiments consisted of three stages: the first entailed locating the EDZ caused by the construction of gallery EZ-A at the Mont Terri laboratory; the second traced the evolution of the EDZ arising from a 20 cm thick and 150 cm deep slot excavation in the EZ-A gallery floor; and the third stage focused on the P-wave velocity evolution surrounding the slot during a pressure loading at the slot walls, which serves to characterize the EDZ evolution in the slot. These three stages were carried out using seismic tomography and then recording the wave propagations during the excavation and reloading stages. The measurement of velocity before and after slot excavation reloading, together with the survey during slot excavation, showed a decrease in P-wave velocity underneath the slot floor down to a 0.2 m depth. Reloading clearly improves P-wave propagation along the sidewall and slightly decreases it under the slot.