Geology and tectonics at Forsmark, Sweden

The complex rock mass at Forsmark is mainly composed of granitic rock types. The rock mass has been broken several times by different tectonic process with intermediate phases of sealing. The rock stress measurements indicate that there is no significant difference in stress field between different rock types (local scale) or between larger intrusions. Larger structures, such as the Singoe fault, seem to influence the direction of the principal stresses. There is a tendency for stress directions to be parallel to an adjacent structure. Minor structures, such as folds and geological irregulatities (such as increased joint frequency), influence the magnitude as well as the direction of the rock stresses. The theoretical shear stresses have been calculated from principal stresses. An increase in the ratio of maximum to minimum shear stresses seems to be in good agreement with more conductive parts of the rock mass, mainly because of widening of horizontal fractures. (orig./DG)