Stress control of hydraulic conductivity in fracture-saturated Swedish bedrock

The granitoid bedrock exposed in the Aspo Hard Rock Laboratory is the best known rock mass in Sweden but is probably generally similar to most of the country in many characteristics relevant to the long term isolation of spen t nuclear fuel. Like most Precambrian rocks in the Fennoscandian shield, th e rocks of Aspo are saturated by fractures with a large range of orientatio ns after long and complicated deformation histories. The ca. 11,000 fractures documented throughout the Aspo HRL belong to six s ets with distinctive orientations. Only about 8% of the total fracture popu lation were wet when they were excavated. Although the wet fractures belong to the same six sets as the total population, their numbers in each set ch ange abruptly at a particular gently dipping fracture zone 240 m deep in th e laboratory. We correlate the numbers of wet fractures in particular sets to different regimes in the current stress field. Most wet fractures are su bhorizontal in a stress regime prone to thrusting above a depth of 240 ni a nd subvertical with NW trends in an underlying stress regime prone to wrenc h faulting. Faults favourably orientated for slip or dilation in the ambient stress fie ld have recently been shown to be the most active groundwater flow pathways near a plate boundary. Here we show a similar relationship for old fractur es in a Precambrian shield embedded deep in the Eurasian plate. However, th e stress regimes, and thus the anisotropy of transmissivity, are complicate d by post-glacial uplift in the Fennoscandian shield and can change 90'degr ees within a few metres at depths and locations on the scale of tens and hu ndreds of metres. (C) 2001 Elsevier Science B.V. All rights reserved.