A sound knowledge of the in situ stress field is a necessary prerequisite for the analysis of long-term safety of a repository for spent nuclear fuel. The assumptions on the rock stress field have major impact on the analysis of the repository integrity, e.g. excavation damage zone, spalling and susceptibility to earthquakes. The geological structures, stress measurements and evidences that lead to the stress model for the Forsmark site are discussed in this study. The physical limits for possible stress states are determined by means of geomechanical considerations. This allows for the structural settings to be taken into account, and leads to an evidence-coherent stress model. The analyses in this study lead to an alternative stress model for the Forsmark site. The similarities and differences in the stress modelling assumptions by SKB and in this study are discussed and the implications concerning the repository layout and long-term behaviour are highlighted.

1 Introduction

One of the most important aspects in each geomechanical analysis is the appropriate understanding of the stress field, i.e. the in situ stresses including the pore pressure with their spatial and temporal variation. The stresses define the mechanical performance of the rock, the behaviour of fractures, fracture networks and faults. The virgin rock stresses also influence the hydraulic behaviour of the system. Any geomechanical or geohydraulic model used is generally bound directly or indirectly to the assumptions about the stress field. Hence, the knowledge of the in situ stress field, the pore pressure and their evolution over time is a necessary prerequisite for the analysis of the long term safety of a repository for spent nuclear fuel.

Substantial errors in the estimate of the initial stress field will influence the majority of mechanical interpretations of the repository performance, including safety during construction and operation, spalling during the thermal phase, fracturing in periods of increased fluid pressures during and at the end of glaciation cycles, and the impact of earthquakes on the existing faults and fractures.

This contribution consolidates all stress field information available about the Forsmark area and discusses its consistency. Based on that discussion an alternative stress model for the Forsmark area is derived and compared to existing models.

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