Characterisation of the hydraulic properties of fractured rock from grouting data

Paper in proceeding, 2008

Pre-grouting of tunnels produces a large amount of data that so far has been of little use to analyse and characterize the properties of the rock. A fan of grouting boreholes is drilled ahead of the tunnel to excavate. In the boreholes water pressure tests (WPT) are made before grouting. After grouting control holes are drilled where new WPTs are performed. These data can give new information on the hydraulic structure of a fractured crystalline rock. The methods and findings are demonstrated for a test segment of a newly constructed railway tunnel, the Nygård tunnel. The paper shows also how the WPT data can be used to assess the characteristic size of the conductive fractures and channels from the WPTs before and after grouting. The interpretation is that there is a system of conductive fractures, which are relatively easy to fill with grout. They are connected by a network of less pervious channels, which make up low permeability connections between the conductive fractures. They are difficult to intersect by a borehole and are thus difficult to grout. Finally the findings support a conceptual model of the rock as a system of sparse conductive two-dimensional fractures connected by much less conductive one-dimensional channels.