Stress control of hydraulic conductivity in fracture-saturated Swedish bedrock

Abstract

The granitoid bedrock exposed in the Äspö 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 spent nuclear fuel. Like most Precambrian rocks in the Fennoscandian shield, the rocks of Äspö are saturated by fractures with a large range of orientations after long and complicated deformation histories.The ca. 11,000 fractures documented throughout the Äspö HRL belong to six sets with distinctive orientations. Only about 8% of the total fracture population 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 change abruptly at a particular gently dipping fracture zone 240m deep in the laboratory. We correlate the numbers of wet fractures in particular sets to different regimes in the current stress field. Most wet fractures are subhorizontal in a stress regime prone to thrusting above a depth of 240m and subvertical with NW trends in an underlying stress regime prone to wrench faulting.Faults favourably orientated for slip or dilation in the ambient stress field have recently been shown to be the most active groundwater flow pathways near a plate boundary. Here we show a similar relationship for old fractures in a Precambrian shield embedded deep in the Eurasian plate. However, the stress regimes, and thus the anisotropy of transmissivity, are complicated by post-glacial uplift in the Fennoscandian shield and can change 90° within a few metres at depths and locations on the scale of tens and hundreds of metres.