The stress field is an important state variable and exerts strong control on the mechanical behaviour and stability of the rock mass. For a deep geological repository, the quantification of the undisturbed present-day stress field is relevant for both short-term (construction of underground and access structures) and the long-term (thermal stresses and gas migration) considerations.
Here it is assumed that the vertical stress Sv = g · ρ · z is a principal stress (g = gravitational acceleration, ρ= rock bulk density, z = depth). Based on this assumption, the maximum and minimum horizontal stresses, SHmax and SHmin, respectively, are also principal stresses. This simplified stress tensor has only four unknowns: the orientation of SHmax and the magnitudes of SHmin, SHmax and Sv (Nagra 2024o).
In Northern Switzerland, the assumption that the vertical stress Sv is a principal stress is reasonable and can be demonstrated by 3D geomechanical-numerical models (Nagra 2024o). The minimum principal stress (SHmin) was measured by means of microhydraulic fracturing (MHF) tests, and the maximum horizontal stress (SHmax) was derived from sleeve reopening (SR) tests. The detailed evaluation and assignment to stress magnitudes are explained in Nagra (2024o).
This section summarises the key results of the 3D stress tensor description in the various regions. It focuses first on stress data of the SHmax orientation (Section 4.4.1) and secondly on stress magnitude estimates (Section 4.4.2). Section 4.4.3 illustrates the results of geomechanical-numerical models that integrate all rock property and stress data for a continuous description of the stress state in 3D, before concluding with the main insights (Section 4.4.4). A detailed overview and evaluation of stress data and model results are provided in Nagra (2024o). The quantification of the ambient stress field is also of direct importance for other aspects covered in this report, in particular for the robust self-sealing of the Opalinus Clay (Section 5.7) and scenarios of future tectonic evolution (Section 6.2).