High-resolution velocity field imaging around a borehole : Excavation-damaged zone characterization
Résumé
The excavation of a deep underground structure induces stress-field redistribution that creates an excavation-damaged zone (EDZ). The study of EDZ physical properties is of prime importance in the framework of nuclear-waste geologic storage, where EDZ constitutes a preferential pathway for storage materials toward the biosphere. A probe for measuring the ultrasonic velocity field around a cylindrical excavation zone has been developed. The probe uses PZT transducers operating at 150 kHz. To collect all required data with high accuracy and minimum effort, the probe is fully monitored for automatic data acquisition. Also, a numerical inversion method reconstructs a stress field using the measured velocity field. This method was partially validated in the laboratory on a multilayer test bed. Following laboratory tests and probe development, an in situ experiment was performed in the Meuse/Haute-Marne Underground Research Laboratory (France) to assess the safety of nuclear-waste storage in the Callovo-Oxfordian argillaceous layer. The damaged zone extends up to 0.175 diameter of depth, with an anisotropic damage pattern oriented according to the regional stress field. The results show the potential of the method to forecast damage of similar larger structures.
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