Simulation of glacially-driven hydromechanical processes for safety assessment of geological disposal sites.
conference paper
The feasibility of a deep geological repository, located in clays of Tertiary age, is currently under study in the Netherlands. The clay represents a natural geological barrier, which must be able to effectively isolate the radioactive waste from the biosphere. A number of geological processes can affect the future stability of the clay, and accelerate the migration of radionuclides from the underground repository to the biosphere.
We studied the groundwater flow system under glacial conditions and the influence of a future glaciation on the stability of the clay hosting the repository. We extended a conceptual glacial groundwater flow model to incorporate the consolidation-driven flow in the clays, caused by the ice-loading. Then we developed a coupled flow-stress finite element model, the geometry of which was derived from a two-dimensional regional geological cross-section of the Netherlands. The finite element program DIANA was used to simulate a 20,000 year long glacial period. Several scenarios were run with various boundary conditions, loading conditions and geomechanical parameters for Tertiary clays.
We found that the process of consolidation has dominant effects on the pore water flow in the clay during glaciation. Loading by the ice leads to amplified outflow of groundwater from a clay body to the aquifers above and below the clay. The outflow rate reaches a maximum value of a few millimetres per year, which are three orders of magnitude higher than the present values. Hydraulic gradients, pore pressures and flow velocities in the aquifers generally increase at least one order of magnitude with regard to the present. The duration of the time spans with high outflow rates is very sensitive to the permeability of the clay and the dynamics of ice loading.
Simulation of radionuclide transport, based on the results of the hydromechanical modelling presented here, has shown that the natural isolation capacity of the clays does not deteriorate significantly under ice-loading conditions.
We studied the groundwater flow system under glacial conditions and the influence of a future glaciation on the stability of the clay hosting the repository. We extended a conceptual glacial groundwater flow model to incorporate the consolidation-driven flow in the clays, caused by the ice-loading. Then we developed a coupled flow-stress finite element model, the geometry of which was derived from a two-dimensional regional geological cross-section of the Netherlands. The finite element program DIANA was used to simulate a 20,000 year long glacial period. Several scenarios were run with various boundary conditions, loading conditions and geomechanical parameters for Tertiary clays.
We found that the process of consolidation has dominant effects on the pore water flow in the clay during glaciation. Loading by the ice leads to amplified outflow of groundwater from a clay body to the aquifers above and below the clay. The outflow rate reaches a maximum value of a few millimetres per year, which are three orders of magnitude higher than the present values. Hydraulic gradients, pore pressures and flow velocities in the aquifers generally increase at least one order of magnitude with regard to the present. The duration of the time spans with high outflow rates is very sensitive to the permeability of the clay and the dynamics of ice loading.
Simulation of radionuclide transport, based on the results of the hydromechanical modelling presented here, has shown that the natural isolation capacity of the clays does not deteriorate significantly under ice-loading conditions.
TNO Identifier
504610
Source title
Annual Conference of the International Association for Mathematical Geology (IAMG), Session M. Cancun.
Pages
12 p.
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