Abstract
The EZ-B experiment has been completed in December 2007. The main results of this research project are summarized in the PhD of Salina Yong, which will be available to the public through the ETH Library e-collection and several international journal publications. The EZ-B experiment has delivered unexpected and fundamental new results about excavation induced perturbations in the Opalinus Clay at Mont Terri and the corresponding rock mass behavior. The results describe relevant in-situ mechanical behavior, which strongly differs from the behavior established from small scale lab testing on intact anisotropic Opalinus Clay samples. The experiment and numerical modelling have shown that a key element in controlling rock mass behaviour are thin tectonic shear zones, which occur frequently in the Opalinus Clay at Mont Terri. The investigation has also demonstrated that damage around an excavation can be assessed by considering the maximum and minimum principal stress ratios derived from 3D continuum modelling. These new insights differ from previous descriptions of the EDZ and the understanding of the rock mass response of the Opalinus Clay and have significant impacts on the design of repository excavations and seals.
The formation of EDZ damage was first studied in the entrance of the EZ-B niche, where insight into the excavation induced damage of the larger Gallery 04 was gained. From geological mapping, excavation induced fractures were found to be normal to bedding-parallel tectonic shears in the west wall and parallel with the wall of Gallery 04 in the east wall. The distinct difference between the two walls is the location of a second sub-horizontal set of tectonic shears. Numerical modelling demonstrated that mobilisation of the bedding-parallel shears dominated in the west wall whereas both the sub-horizontal and bedding-parallel tectonic shears were mobilised in the east wall. In turn, mobilisation of the tectonic shears elevated stress levels in the surrounding rock mass beyond a spalling limit (as defined by the ratio s3/s1) of 0.1. Hence, macroscopic fracturing normal to the tectonic shears were induced as the tectonic shears were mobilised.
The rock mass response ahead of the advancing face of the EZ-B niche, which strikes normal to bedding, illustrated the need to consider both the tectonic shears and distance from Gallery 04 for understanding the damage induced by the EZ-B niche excavation. Observations made during the niche excavation showed that roughly half the niche was located in the influence zone of the adjoining Gallery 04 and seismic perturbations reached as far as 1.6 m from the face. The tectonic shears provide a ready avenue for stress relief as the advancing niche face progressively reduces the kinematic constraint of the tectonic shears thereby allowing for deformations along the tectonic shears.
Radial boreholes drilled in the post-excavation phase allowed for a physical and numerical assessment of the redistributed stress conditions and corresponding rock mass damage. The perturbations of the rock mass in the sidewalls, floor and ceiling of the EZ-B niche mainly show strength reduction through microdamage (i.e. stresses above crack initiation) and only little induced macroscopic fracturing, as seen in drillcore mapping and optical televiewer images. Borehole breakouts in post-excavation boreholes only occur at greater depth, where rock mass strength properties and stresses have not been significantly reduced prior to drilling. Macroscopic fractures are mostly constrained to the first 20 cm and oriented parallel with bedding. The integration of geophysical data indicated that the zone of microscopic perturbation around the niche is direction dependant and on average 0.5-0.6m in the sidewalls, above the crown, and at the corners in the face. A larger zone was detected below the invert (1m) and in the centre of the face (0.7-0.8m). Stress ratios (i.e. s3/s1) from elastic numerical modelling showed that the zone of macroscopic fracturing corresponded with ratios of 0.1 or less. Thus, damage decreased with increasing stress ratios and distance from the near-field towards the far-field.
