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Workreport 2018-14



ONKALO POSE Experiment - Integrated Analyses of True-triaxial Tests


Behrestaghi, M., Sehizadeh, M., Young, R.



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The Posiva Olkiluoto Spalling Experiment (POSE) was conducted at the ONKALO facility in western Finland. The rock mass in the vicinity of the experiment is a heterogeneous mix of anisotropic veined gneiss (VGN) with well-developed micaceous foliation, and relatively more homogeneous granitic pegmatoid (PGR). Seventeen cube-shaped true-triaxial tests were undertaken by the University of Toronto on specimens of VGN and PGR to explore the mechanics of damage development under a stress path representative of in situ conditions experienced during POSE.

The three research objectives of the true triaxial tests were:

1.     To test specimens under representative stress conditions to evaluate the effects of the angular relationship between the main principal stress directions and the strike of the foliation planes to strength and, further to study the damage process in grain scale by post mortem analysis.

2.     To investigate the effects of the angular relationship between the σ2 principal stress direction and the strike of the foliation planes in the weakest orientation for VGN on observed strength.

3.     To compare the evolution of acoustic emission (AE) activity and seismic velocity measured at the POSE field site for the PGR with its laboratory specimen counterpart that was subjected to a similar stress path to that experienced in situ.

The analysis of the temporal and spatial AE activity, construction of 3D damage fracture network images from X-ray CT, and thin section analysis performed on the rock specimens were used to investigate the effect of the orientation of foliation planes on the strength, deformation and seismic signature, and also the chronological occurrence of the damage fracture networks within failed specimens. The induced fractures preferably initiate and propagate within the foliation zones characterized by plate-shape minerals of biotite and muscovite, and predominantly avoid quartz-feldspathic granular types of mineral zones. A sympathetic angular relationship exists between the induced fractures for the 0° and 30° – 45° VGN specimens; this relationship is not as strong for the 60° VGN specimens. The 90° VGN and the PGR specimens survived the maximum applied stresses.

VGN samples with a 30° foliation angle relative to the maximum loading direction, and the minimum loading direction perpendicular to the strike of foliation, exhibited the lowest strength of 94 MPa and 111 MPa but the failure initiated at 55 MPa in the second sample. This value corresponds to the stress state at the POSE experiment hole wall when the first damage was observed. It is also close to the crack initiation strength 53 MPa defined from uniaxial tests from the experiment area. When confining stress conditions are rotated 90° so that the intermediate stress σ2 is perpendicular to the strike of foliation, the failure strength can increase by 15 %, which is less than what the classic shear plane equation yields. The strength of the weakest member of VGN can increase significantly when a non-compliant pegmatitic inclusion is aligned normal to the dip direction of the VGN foliation planes. The degree of foliation and heterogeneity is remarkably self-similar at different scales, from in situ tunnel exposures at a scale of 10 m to specimens of 0.1 m used in the true-triaxial tests, providing confidence that laboratory results are applicable to the field scale.

Comparison of the evolution of AE activity and seismic velocity measured at the field site for PGR with its counterpart laboratory specimen was found to be in good agreement. PGR is the strongest member of the rock mass and displays significant AE activity for specimens subjected to the simulated in situ loading without the development of macroscale fractures. The PGR specimens reached a failure strength over 210 MPa with 1 MPa/20 MPa confining stresses, being significantly higher than previously measured 123 MPa and 179 MPa values with 3 MPa confinement.


ONKALO, POSE, True-triaxial tests, repositories, Acoustic emission, Seismic wave velocities, Foliation planes, Pegmatite granite, Micaceous gneiss, Stress path, micro CT X-ray, thin section analysis, Damage, Micro-fabric


WR 2018-14_web (pdf) (12.7 MB)


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