Ultrasonic sounding and monitoring of the excavation damaged zone in a soft supported gallery
Résumé
Under high in situ stresses and high anisotropic stress ratios, the excavation of underground openings generally causes the creation of a disturbed (EdZ: Excavation disturbed Zone) and/or damaged (EDZ: Excavation Damaged Zone) zone, resulting from the initiation and growth of cracks and fractures and by a redistribution and rearrangement of the initial stresses. Due to the modifications of mechanical and hydromechanical properties in the EdZ/EDZ, the latter constitute a potential risk for the efficiency of the geologic and/or engineered barriers. To estimate the performance of a site of radioactive waste storage, it is essential to know the geometry, the extensions of the EdZ/EDZ, the density of cracks, their connectivity and the variations of permeability which can result from it. In particular, their extension depends on numerous factors among which the nature of the rock, the pre-existent fractures and their reactivation, the initial stress field (magnitudes and/or rate of anisotropy), the time, the geometry and the techniques of excavations etc.. In addition, the evolution of EdZ/EDZ properties in the medium term is little or poorly known, particularly under the influence of environmental conditions such as the reconfining by near field rock creep in contact with a rigid structural support or the evolution of hydric conditions (desaturation and resaturation). Ultrasonic experimentation under the OHZ experiment (one of the numerous in situ experiences carried out in the Meuse/Haute-Marne Underground Research Laboratory) has been implemented to characterize the EDZ extension and its evolution in time according to the structural support type (soft or rigid) and the environmental conditions present in the laboratory. This study consists of two experimental components : (1) the prior auscultation of the sole and face of the gallery by ultrasonic transmission tomography, (2) the monitoring the EDZ and the analysis of measurable changes in the propagation of ultrasonic waves in the medium term. Mechanical modelling of the gallery GCS (gallery with soft support and oriented parallel to the horizontal major stress) has predicted an extension of EDZ between 1.4 and 1.9 m according to the front advancement, the position around the gallery (roof, bottom and gallery side) and the rock characteristics. Moreover, a former experimentation showed that below the bottom of a gallery with this same orientation, ultrasonic velocity falls related to the damage were significant up to 2 m deep. The area of interest of experimentation OHZ encompasses at least this area with in addition one reference measurement in the undisturbed area. To escape heterogeneous variations of clay layers and the main anisotropy effect, two devices were selected with a triplet of horizontal drillings in the siding and a doublet of vertical drillings in the bottom of the gallery. P-wave tomographic data allowed rebuilding the images of the velocity on the 4 plans defined by the drillings. These images (figure 1) have shown a significant perturbation of the velocity field over 1.8 m deep in the gallery side. This area has been associated with the EDZ whose extension is of the same order of magnitude as which had been modeled during the blind predictions. Furthermore, there is an area of 0.4 m in depth on this siding with very low velocities or disappearance of ultrasonic rays in the 3 plans. That can be associated with fracturing of argillites. This very low velocity zone is not visible at the bottom, where velocity perturbations are much lower, with constant velocity over 1.6 m. The containment of the concrete floor, which is particularly thick at this location, might play a role in this observation. Finally, some anomalies of velocity are still visible beyond the end of the zone associated with the EDZ. They are probably caused by isolated fractures that occurred during excavation of the gallery and identified on extracted cores from the drillings. The magnitude of these anomalies would suggest that these fractures are closed. A program of inversion of these continuous in situ measurements of five elastic wave velocities (i.e. VP(0°), VSh(0°), VP(90°), VSv(?) and VP(?), where ? is off-axis) has been developed under Mathematica. Then, the five dynamic elastic cosntants for the assumed transverse isotropic character of the argillites are derived as a function of time and the distance from the gallery wall (Balland and Souley, 2011). From these elastic constants, some indicators of damage rate were proposed with respect to the space and the time (Sayers and Kachanov, 1995). Finally, in addition to the experimental data analyses, synthetic data (generated from the given elastic constants and velocities derived from the theoretical formulation) as well as data from literature (Sarout et al, 2007) (measurements of ultrasonic velocities in the laboratory on samples) allowed us to successfully evaluate the consistency of the results obtained with this program. The next step is the implementation of the same ultrasonic experimentation around the GCR Gallery which presents a rigid structural support, with the same orientation of GCS.