Assessment of the transmissivity field in fractured rock: A case study in the tass tunnel

Paper in proceeding, 2009

Water Pressure Tests (WPTs) among other hydraulic tests were conducted in each borehole before and after the pre-excavation grouting of a test tunnel in the Äspö HRL (Hard Rock Laboratory) built in 2008 and 2009 “the TASS tunnel”. This paper shows how data from WPTs can give information on the connectivity and asses the transmissivity fields of fractured rock. The results showed that the connectivity range of the rock before grouting is approximately 3 m, which seems to be a common range in crystalline fractured rocks. Results clearly indicate that the most conductive parts of each fan were where the grout has penetrated the most during grouting. They also show that the reduction of the transmissivity in the rock by means of grouting is about four orders of magnitude within the studied rock volume of a fan. This sealing takes place easily on planar fractures where the transmissivity is high (a few conductive fractures that give the major contribution to the borehole transmissivity) and what is left unsealed is a highly channelized system. This channelized system will be extremely difficult to hit by future boreholes, which will make post-grouting a very complex task.