Velocity-survey of an excavation damaged zone : influence of the excavation and reloading
Résumé
During gallery excavation, regardless of the method chosen, the surrounding rock is mechanically disturbed in the case of a an underground disposal (for ultimate chemical wastes or for radioactive wastes), such mechanical changes to the rock state can create preferential pathways for releasing 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 to assessing the rock capacity necessary for constituting an impermeable geological rock barrier. The key to this effort lies in determining the nature, extent and change to this disturbed zone. The survey by means of ultrasonic wave analysis is particularly appropriate for these purposes since it provides information on the zone left undisturbed by the boreholes. The main objective behind 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 (Bossart et al., 2002). This in situ experimental campaign successfully studied the possibility of improving the properties of rocks forming the EDZ (Excavation Damaged Zone). The experiments consisted of three stages: the first entailed locating the EDZ from the existence of gallery EZ-A at the Mont Terri laboratory; the second traced evolution of the EDZ arising from a 20-cm thick and 150-cm deep slot excavation at the EZ-A gallery floor; and the third stage focused on P-wave velocity evolution surrounding the slot during a pressure loading performed at the slot walls, which serves to characterize EDZ evolution in the slot. These three stages were carried out by applying the seismic technique of tomography and then recording wave propagations during the excavation and reloading stages. The velocity before and after slot excavation reloading, along 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 these same characteristics under the slot.