Generally, crystalline rocks have smaller hydrological permeability and higher mechanical strength and density, such as granitic rocks, compared with sedimentary rocks. In underground structures in granitic rocks, stress (including thermally induced stress increments) induced spalling is one of the reasons causing failures. To verify that earlier findings from the Canadian URL (where the rock is in principle fracture-free) could be applied to the rock in the Fennoscandian shield in Sweden (which includes fractures), SKB (the Swedish implementor), conducted the Äspö Pillar Stability Experiment (APSE) in order to determine the spalling/yielding strength of a granitic rock mass and to test how small confining pressures may affect the yielding strength (the stress state at which the rock starts spalling). The APSE was located at the 450 m level of the Äspö Hard Rock Laboratory in Sweden. The experimental layout consisted of a 7.5 m high, 5 m wide tunnel with an arched roof and floor, two 1.75 m diameter (6.5 and 6.3 m deep respectively) boreholes separated by a 1 m thick pillar of Äspö diorite containing fractures (Fig. 3).
The APSE was defined as Task B in the Decovalex-2011 project, aiming to increase our knowledge and modelling ability for the challenging issue of rock spalling for more reliable and realistic performance and safety assessments of radioactive waste repositories in crystalline rocks. The hydraulic and chemical processes were excluded due to the fact that very little water was observed during the APSE experiment and the test period was not long enough and the temperature was not high enough to cause any significant influence from hydraulic and geochemical processes. The problem was therefore a coupled thermo-mechanical one with challenges related to understanding and modelling the rock spalling processes at both the micro- and macro-scopic levels, which has been a difficult subject for rock mechanics studies in the past.
Seven teams studied the Task B problems: Institute of Rock and Soil Mechanics, Chinese Academy of Sciences, China; State Key Laboratory of Water Resources and Hydropower Engineering, Wuhan University, China; Institute of Geonics AS CR (UGN), sponsored by SURAO, Czech Republic; Fracom Ltd., Finland and Geomecon GmbH, Germany, Sponsored by Posiva, Finland; Japan Atomic Energy Authority (JAEA), Japan; Korea Atomic Energy Research Institute (KAERI), Korea; University of Alberta, Canada, sponsored by SKB, Sweden.
The modelling of APSE in Task B was divided into three stages.
Stage 1 aimed to calibrate computer codes in order to identify and represent the properties and failure mechanisms of the Äspö Diorite;
Stage 2 aimed to simulate the spalling processes of APSE as a time-dependent coupled TM processes through back-analysis;
Stage 3 aimed to simulate the entire excavation and heating phases of the experiment in order to determine the spalling strength, deformations and the spalling notch geometry, and to analyse result sensitivity among the teams’ models.
Main achievements and outstanding issues
The computer model and code calibration against the measured properties and failure mechanisms of the Äspö Diorite during Stage 1 was completed successfully by most of the teams. The final modeling results of Stage 2 matched measured data well but there was a general over estimation of the rock temperatures in the results. For Stage 3, the estimation of spalling strength by back calculations shown a close agreement with teh measured data, with a small standard deviation of 7 MPa, compared with the spalling strength of 120 MPa of the pillar, based on elastic models. These results are comparable with earlier findings on the spalling strength of granitic rock masses.
Modelling mechanical behaviour of crystalline rocks is a difficult challenge, as shown by the past experience of the earlier Decovalex project phases, when the rocks contain fractures of varying sizes, shapes, water-bearing status, mineral filling status, and displacement/damage histories. The main difficulty, and the outstanding issue, lies with the challenge of characterization of the rock volumes that are required to be modelled, since the details of the fracture system geometry and their mechanical behaviour cannot be known before, or even after the tests. This challenge is understood as a general feature of fractured crystalline rocks in the international rock mechanics community, not a special issue limited only to Task B.
Besides the final reports of Task B, five papers were published in the Journal of Rock Mechanics and Geotechnical Engineering, as listed below.
S. Kwon, C. Lee, S. Jeon, H.-J. Choi, Thermo-mechanical coupling analysis of APSE using submodels and neural networks, Journal of Rock Mechanics and Geotechnical Engineering, Volume 5, Issue 1, February 2013, Pages 32-43
T. Koyama, M. Chijimatsu, H. Shimizu, S. Nakama, T. Fujita, A. Kobayashi, Y. Ohnishi, Numerical modeling for the coupled thermo-mechanical processes and spalling phenomena in Äspö Pillar Stability Experiment (APSE), Journal of Rock Mechanics and Geotechnical Engineering, Volume 5, Issue 1, February 2013, Pages 58-72
M. Rinne, B. Shen, T. Backers, Modelling fracture propagation and failure in a rock pillar under mechanical and thermal loadings, Journal of Rock Mechanics and Geotechnical Engineering, Volume 5, Issue 1, February 2013, Pages 73-83
R. Blaheta, P. Byczanski, M. Čermák, R. Hrtus, R. Kohut, A. Kolcun, J. Malík, S. Sysala, Analysis of Äspö Pillar Stability Experiment: Continuous thermo-mechanical model development and calibration, Journal of Rock Mechanics and Geotechnical Engineering, Volume 5, Issue 2, April 2013, Pages 124-135
P. Pan, X. Feng, Numerical study on coupled thermo-mechanical processes in Äspö Pillar Stability Experiment, Journal of Rock Mechanics and Geotechnical Engineering, Volume 5, Issue 2, April 2013, Pages 136-144