Task 3: BMT-3
Problems and research performed
The BMT3 (also as WP4 of the BENCHPAR) was a generic numerical exercise constructed from geological and hydrogeological characteristics of a northern hemisphere site subjected to a prescribed time sequence of climatically driven glaciation/deglaciation events. A generic spent-fuel repository was assumed to be located in a crystalline rock mass, which consists of low-permeability, low-porosity, sparsely fractured intact rock matrix (with 10-19 m2 or lower permeability), traversed by an interconnected 3-dimensional network of fracture zones. These conditions were representative of those that would be encountered in a shield setting. The objectives of BMT3 were: 1) to study, by analytical and/or numerical modelling the long-term evolution of a fractured rock mass in which a generic repository is located, as it undergoes a glaciation/deglaciation cycle in a time frame of 100 000 years (Fig. 8a); 2) to assess the impact of the coupled mechanical and hydraulic responses of the repository system to a glaciation/deglaciation cycle on its long-term performance in waste isolation; and, 3) to improve the scientific basis for supporting the safety case for a deep geological repository.
Simplified data from a specific Canadian Shield research area were utilized to make the simulations realistic. Site attributes were largely based on those of the Whiteshell Research Area (WRA) in eastern Manitoba, located on the western edge of the Canadian Shield at 255–290 masl (metres above mean sea level), Fig. 8b.
The numerical study was undertaken in three phases. Phase I aimed for enhancing numerical tools for simulations of the climate change, ice-sheet loading and basal thermal and hydrological regime, including permafrost phenomena. In Phase II, modelling covered the 3D ice-sheet/drainage process in order to provide boundary conditions for 2D and 3D modelling, and effects of groundwater salinity on the development of permafrost and perennially frozen ground. In Phase III, 3D ice-sheet/drainage simulations was conducted to generate spatially and temporally variable 2D mechanical and hydraulic boundary conditions to be used directly by other teams for coupled HM site-scale rock mass modelling in 2D and 3D, including studies on ice-sheet/drainage and thermodynamic permafrost models at the ice-bed interfaces.
Main achievements and outstanding issues
The transient coupled hydro-mechanical modelling in this study represents a major step forward in advancing the state of the science for modelling geosphere response to glaciation. Although models of glacier–groundwater, glacier–permafrost–groundwater and glacier–groundwater–shallow failure systems had been presented previously, this BMT was one of the first attempts to assess impacts at repository depths using site-specific (though simplified) data, and the study probably represents the first successful attempt at scaling down an ice-sheet/drainage model with 10 km resolution to a 200 m resolution to interface with site-scale sub-surface modelling.
The results provide valuable insights into the magnitude and rate of change of site-specific hydrogeologic and geomechanical response to external, transient climate forcing. They clearly demonstrate the importance of glaciation scenarios in performance assessments and the reality of effects that result from H-M coupling, and underline the need for transient analyses of these coupled phenomena. Although no enhancement to the computer codes was undertaken within the BMT3 study, it was no small achievement to demonstrate for the first time a capability to simulate the coupled H-M rock mass response to glaciation using realistic time-dependent glacial boundary conditions along with reasonably realistic site-specific hydraulic and mechanical attributes.
Results indicated the possibility of high hydraulic gradients, high flow velocities and flow reversal during deglaciation, high residual pore pressure long afterwards, and faster and deeper surface water recharge. Large hydraulic gradients could appear at the glacier terminus or near the sub-glacial channels (i.e. eskers). The extensive and thick ice-sheet could diminish the influence of topographic gradients, so that the groundwater flow regime beneath a continental ice-sheet could be controlled primarily by the glacially induced hydraulic and mechanical boundary conditions, the geometric structure of the fracture-zone network and the spatial distribution of hydraulic properties. Mechanical responses, though complicated, were relatively mild. No hydraulic jacking or hydraulic shearing was indicated at nominal repository horizons, although changes in orientation and the ratio of principal effective stresses were predicted. Coupled H-M modelling with the Mohr–Coulomb failure criterion suggested significantly lower factor-of-safety values than uncoupled modelling. Further work might be necessary to investigate whether the latter changes need to be taken into consideration in repository design. Other achievements include thermodynamic coupling between the ice-sheet and permafrost modeling, enhancement of the permafrost modeling code with regard to salinity transport and the demonstration of applicability to 2D (simplified) site-specific coupled T-H-M-salinity transport simulations of permafrost development and effects. Figure 9 shows the results of modelling the evolution of the hydraulic head during glaciation.
Publications
Besides the one report for BMT3 of Task 3, two papers produced from BMT3 research were published in the special issue of the International Journal of Rock Mechanics and Mining Sciences, Volume 42, Number 5–6:
- T. Chan, R. Christiansson, G.S. Boulton, L.O. Ericsson, J. Hartikainen, M.R. Jensen, D. Mas Ivars, F.W. Stanchell, P. Vistrand, T. Wallroth, DECOVALEX III BMT3/BENCHPAR WP4: The thermo-hydro-mechanical responses to a glacial cycle and their potential implications for deep geological disposal of nuclear fuel waste in a fractured crystalline rock mass, Int. J. Rock Mech. Min. Sci. 2005; 42(5–6): 805–827.
- T. Chan, F.W. Stanchell, Subsurface hydro-mechanical (HM) impacts of glaciation: Sensitivity to transient analysis, HM coupling, fracture zone connectivity and model dimensionality, Int. J. Rock Mech. Min. Sci. 2005; 42(5–6): 828–849.