International FEP List - Version 3
FEP 4.2.3 Mechanical processes [geosphere]

The mechanical processes that affect the host rock and other rock units, and the overall mechanical evolution of the geosphere. Included are the effects of mechanical loads caused by, among other processes, tectonic movements, glacial loading and unloading, removal of rock by weathering and erosion, or loading by sedimentation. Also included are loads imposed by repository components on the surrounding geosphere (e.g. the effects of swelling of bentonite buffer materials on the adjacent geosphere).

There is some overlap between the scope of FEP 4.2.3 (this FEP, Mechanical processes [geosphere] and FEP 4.2.1 (Thermal processes [geosphere]), 4.2.2 (Hydraulic processes [geosphere]), FEP 4.2.4 (Geochemical processes [geosphere]), FEP 4.2.5 (Biological processes [geosphere]) and FEP 4.2.6 (Radiological processes [geosphere]). FEPs 4.2.1 and 4.2.2, and FEPs 4.2.4 to 4.2.6 may all result in variations in mechanical loads on materials within the geosphere. Mechanical processes (this FEP, 4.2.3) covers the mechanical effects of the loads that are caused by the processes covered by these other FEPs.

Mechanical processes in the geosphere may influence the effectiveness of both the EBS and the geosphere barrier. These processes may be of short duration, for example fault movement during an earthquake, or of long duration, such as gradual loading and unloading during cycles of glaciation and deglaciation. During construction and operation of the repository, the stresses in the geosphere and consequent deformation will influence the characteristics of the disturbed zone that develops around the repository, which may influence migration of radionuclides and other contaminants after repository closure (if there are pathways through the EBS).

During operations and in the post-closure period, stresses in the geosphere around the repository will influence the stresses on the EBS components and potentially their deformation. Such deformation may affect the integrity of barrier materials and / or the volume and distribution of void space in the repository. The rate of repository resaturation may be affected, as might the development of gas pressures within the void space. Stresses in the geosphere will also influence the deformation of the geosphere. This may cause changes in porosity/porosity distribution and consequent changes in pressure head gradients. These effects may in turn drive movement of fluid (such as water, non-aqueous liquids or gases) through the geosphere. The change in porosity due to deformation, for example along faults, may influence the directions and characteristics of pathways along which radionuclides and other contaminants might migrate from the repository, through the geosphere. Certain mechanical processes, such as cataclasis along moving fault planes, could potentially change the ability of the rock to retard these radionuclides and other contaminants. Potentially, certain mechanical processes could lead to changes in the proportions of different fluid phases (such as water, non-aqueous liquids or gases) present. For example, in some locations sudden depressurisation, as might occur transiently in an earthquake, could lead to evolution of a separate gas phases, such as CO₂ or CH₄. Such effects could influence the partitioning of radionuclides and other contaminants among different phases.