A Numerical Study of the Stress Field at Forsmark Based on the Shearing Behaviour of Major Deformation Zones

A major element in the site investigations the Swedish Nuclear Fuel and Waste Management Co (SKB) conducts in order to select the most suitable area to site the final repository has been the measurement of the in situ stresses. In situ stress measurement, carried out inside a few boreholes, often very scarcely placed in a large area, cannot in detail reflect the spatial distribution of the in situ stresses at that area. Numerical simulations of the slip behaviour of major deformation zones, running through a site, create plausible distributions of the stress field at that site. The three dimensional program 3DEC was used to do such simulations for the Forsmark site. A large parallelepiped shaped mass of rock, including selected deformation zones, sized to represent the limits of the candidate site, was oriented such that two opposite sides ran parallel with the orientation of the regional crustal shortening. The model block, was then deformed at a very low rate in that direction while the other two vertical sides were set to prevent the transversal expansion. The continued deformation made a three dimensional stress field prevail within the model limits. By a number of subsequent comparisons with the in situ stress measurement results, a spatial distribution of the in situ stress field at the site was depicted.